Skip to main content
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2021 Nov 8.
Published in final edited form as: Nat Chem. 2019 Aug 23;11(9):768–778. doi: 10.1038/s41557-019-0314-x

The mechanisms of boronate ester formation and fluorescent turn-on in ortho-aminomethylphenylboronic acids

Xiaolong Sun 1,2, Brette M Chapin 2, Pedro Metola 2, Byron Collins 2, Binghe Wang 3,*, Tony D James 4,*, Eric V Anslyn 2,*
PMCID: PMC8573735  NIHMSID: NIHMS1749950  PMID: 31444486

Abstract

ortho-Aminomethylphenylboronic acids are used in receptors for carbohydrates and various other compounds containing vicinal diols. The presence of the o-aminomethyl group enhances the affinity towards diols at neutral pH, and the manner in which this group plays this role has been a topic of debate. Further, the aminomethyl group is believed to be involved in the turn-on of the emission properties of appended fluorophores upon diol binding. In this treatise, a uniform picture emerges for the role of this group: it primarily acts as an electron-withdrawing group that lowers the pKa of the neighbouring boronic acid thereby facilitating diol binding at neutral pH. The amine appears to play no role in the modulation of the fluorescence of appended fluorophores in the protic-solvent-inserted form of the boronic acid/boronate ester. Instead, fluorescence turn-on can be consistently tied to vibrational-coupled excited-state relaxation (a loose-bolt effect). Overall, this Review unifies and discusses the existing data as of 2019 whilst also highlighting why o-aminomethyl groups are so widely used, and the role they play in carbohydrate sensing using phenylboronic acids.


Physical organic chemistry is a discipline in which experimental and theoretical approaches are used to delineate reaction mechanisms, uncovering mother nature’s chemical steps, physical phenomena and reactivity1. Many postulates, and sometimes heated debates, have been investigated and settled using the tools of this discipline. For example, the classic debate surrounding the norbornyl carbocation has only recently been settled with a low-temperature (40 K) crystal structure2. Another is the controversy surrounding interpretation of data concerning a self-replicating system that occurred between Rebek and Menger, which was settled with extensive kinetic analysis by Reinhoudt3. As a last example, the theoretical and experimental examination of low-barrier hydrogen bonds has far-reaching implications from supramolecular chemistry to enzyme catalysis4,5. In the realm of chemosensors, how receptors based on o-aminomethylphenylboronic acids turn-on emission and whether their structures possess N—B bonds when binding vicinal diols have seen a variety of postulates. Once again, the studies described below demonstrate how exploration using physical organic chemistry ultimately results in a clear picture of mechanisms.

Boronic acids, both alkyl and aryl, have been accessible for around 150 years, since 18606. The interaction of boronic acids with diols or anions has been intensively investigated610 and boronic acids have been exploited in a range of applications as diverse as NMR shift reagents11,12, functional polymers13, molecular self-assembled materials14,15, tissue histology16 and fluorescence imaging of carbohydrates17. Further, they are among a small set of reactions that involve dynamic covalent bonding7. In addition, sensors for reactive oxygen and reactive nitrogen species have been developed based on oxidative removal of the boronic acid group1821.

Significantly, boronic acids are extensively utilized in synthetic receptors for the molecular recognition and sensing of carbohydrates, as well as various other vicinal-diol-containing compounds, and various anions6,2236. The detection of carbohydrates is commonly pursued in disease diagnostics, such as diabetes16,3741. Carbohydrates present a challenge for molecular recognition because of their high solvation energies in water4244 and their great structural diversity45. Importantly, boronic acids are able to overcome this solvation limitation because the binding event does not replace the solvent, as with non-covalent binding, but rather interchanges covalent bonds. Thus, in the field of host–guest chemistry, boronate ester formation is recognized as a very unique reaction (Fig. 1a)4652. Importantly, the incorporation of a boronic acid into almost any scaffold reliably imparts high-to-low millimolar binding affinities to vicinal diols, catechols and α-hydroxycarboxylate-containing guests in competitive media without further structural manipulation. However, phenylboronic acid itself is limited in its utility because it only shows significant binding a few pH units above physiological pH5355. This is a structural feature that is often overlooked.

Fig. 1 |. Chemical reactions and primary data relevant to the topics discussed in this Review.

Fig. 1 |

a, Reversible binding motifs for boronate ester formation. b, Boronate ester formation and N—B bonding mechanism in carbohydrate fluorescence sensing. c, pH titrations of 1 (1 μM) in 2:1 H2O:CH3OH and 50 mM NaCl alone (yellow circle) and in the presence of 50 mM of each of the following carbohydrates: d-mannose (blue diamond), d-galactose (red square), lsorbose (orange triangle), d-glucose (green cross), inositol (brown plus) and d-fructose (purple dash); data taken from ref. 111. The fluorescence responses are all normalized to 1 at the initial values. d, The ‘pKa switch’ or ‘hydrolysis/solvolysis’ mechanism in carbohydrate fluorescence sensing.

Early studies by Wulff55,56 as well as Lorand57 delineated substituent effects for phenylboronic acids binding various diol species. The most significant was placement of an aminomethyl group on the ortho position to the boronic acid, which is a structural motif that led to significantly improved binding at neutral pH58. Since the inception of this exploration, many groups have incorporated o-aminomethylphenylboronic acids into a variety of chemical receptors and/or sensors to improve affinity, tune selectivity, change quantum yield and/or modulate the wavelength of emission13,41,5966. By exploiting o-aminomethylphenylboronic acids appended with different fluorophores, a series of turn-on fluorescence probes have been created for the sensing and imaging of carbohydrate and glycoproteins on cell surfaces17,61. Most significant for practical applications, a modular sensory system of this type has been developed by Glysure into a fully functional fibre-optic sensor for the continuous monitoring of glucose with patients in intensive care units67.

Summarizing the debates

The recognition of saccharides via boronic acids often relies on an interaction between a Lewis acidic boronic acid and a proximal amine. However, the true nature of the nitrogen—boron interaction (especially in an aqueous environment) has been debated. Different mechanisms have been proposed for the emission turn-on of o-aminomethylphenylboronic acids appended with fluorophores in response to saccharide binding in aqueous media (Table 1). Shinkai and James originally postulated a photo-induced electron transfer (PET) mechanism for saccharide sensing based upon a change of N—B bonding strength upon sugar binding62,63. However, as an alternative mechanism to the N—B dative bond, Wang put forth a pKa switch theory involving the breaking of an N—B bond and insertion of solvent upon sugar binding64,65. With the question ongoing, Anslyn and Larkin explored an unanticipated aggregation and disaggregation mechanism66. However, none of the hypotheses were consistent with all the data for o-aminomethylphenylboronic-acid-based sensors. Thus, in 2018, Anslyn and James revealed that modulating the internal conversion induced by —B(OH)2 (an example of a ‘loose-bolt’ effect) explains how potentially all o-aminomethylphenylboronic-acid-based fluorescence sensors signal the presence of sugars68.

Table 1 |.

Different proposed mechanisms for emission turn-on

graphic file with name nihms-1749950-t0007.jpg

Thus, the goal of this treatise is to put forth a unifying theory that explains all the data and addresses several issues: the structural manner in which the boron and nitrogen interact, the role of the o-aminomethyl group in both thermodynamics and kinetics, assignment of pKa values, the mechanism of boronate ester formation and the manner in which an o-aminomethyl group influences the fluorescence. In order to do so, we give a historical synopsis of the use of an aminomethyl group in phenylboronic acids; the mechanistic postulates are compared side by side; structural studies and their mechanistic implications are reanalysed; various computational studies are highlighted and finally the most up-to-date understanding of the fluorescence response is discussed.

Boronate ester formation and N—B bonding

In a series of landmark studies by Shinkai and James, anthracene-based receptors such as 1 were used to signal the presence of various carbohydrates in neutral aqueous media via a fluorescence turn-on response (Fig. 1b)62,63,69. It was clear that the o-aminomethyl group in 1 improved the thermodynamics of binding at neutral pH, but it was also proposed to be involved in modulating the fluorescence of the receptor in the presence and absence of sugars. Their postulate involved a weak dative bond to the boron atom from the amine nitrogen, resulting in near-sp3 hybridization of the boron atom at neutral pH (dashed N—B line shown in 1, Fig. 1b)70. By contrast, the boron in phenylboronic acid, PhB(OH)2, is sp2 at neutral pH (a fact that has been established using 11B NMR)71. Upon boronate ester formation in either structure 1 or phenylboronic acid, a five-membered ring involving the boron atom is formed. Shinkai and James reasoned this ring would be more strained if the boron atom was sp2-hybridized than if it was sp3-hybridized72. Hence, the N—B bond was postulated to stabilize the boronate adduct because pyramidalization of the boron would be induced by the Lewis acid–base coordination with the neighbouring nitrogen atom. In essence, the two interactions reinforce each other: N—B bonds involving a pyramidalized boron atom are stronger, while boronate ester formation is stabilized by N—B bonding23,72. For these reasons, the strength of the N—B bond in the ester was proposed to increase relative to the acid (solid N—B line in Fig. 1b).

The postulated modulation of the N—B bond strength was derived because it nicely explained the turn-on of fluorescence found for structure 1 and its analogues62,63,69. PET73,74 from the nitrogen atom’s lone-pair of electrons in 1 was proposed to quench the emission of anthracene because this lone pair was not strongly coordinated to boron. However, upon boronate ester formation, the energy of the nitrogen donor’s electron pair drops due to stronger coordination, therefore lowering the extent of PET. Hence, PET quenching is decreased upon ester formation, which results in a turn-on of fluorescence (Fig. 1c). This was a very logical postulate in the 1990s because other pioneers in the chemosensing field, Czarnik75 and DeSilva76, were reporting the use of sensors in which nitrogen lone pairs were involved in PET quenching. Irrespective of the actual role of the o-aminomethyl group, the discovery of the general class of compounds analogous to compound 1, and the associated proposed mechanism, remains a landmark study in the history of chemosensors.7779

An alternative explanation

Wang put forth an alternative mechanism to the N—B dative bond theory64,65. He noted that crystal structures of boronic acids actually have shorter N—B bond lengths than those in the corresponding boronate esters, meaning that the bond is stronger in the acid than the ester (this is the opposite of what is shown in Fig. 1b). He and Franzen used density functional theory (DFT) for computational modelling and found that the relative changes in the N—B bond strength upon ester formation were not significant enough to produce the large changes in PET quenching necessary to explain the behaviour of 1 upon binding sugars. In this paper, they noted that protonation of the amine was a more likely explanation for the arresting of the PET quenching found with sugars, citing the fact that an N—H bond is far stronger than an N—B bond. In a subsequent analysis, Wang proposed what has come to be known as the ‘pKa switch’ or ‘hydrolysis/solvolysis’ mechanism (Fig. 1d)65. The notion is that N—B bonding exists in the boronic acid, while the boronate ester chelates a solvent between the boron and the amine (called ‘hydrolysis’), thereby protonating the amine. For the purposes of this treatise, we note that the product in Fig. 1d has a solvent molecule inserted between the amine nitrogen and boron, which has turned out to be an important structural postulate first introduced by Wang.

Wang also pointed out other issues that were inconsistent with the N—B bond strength modulation postulate. First, the emission intensity of 1 is independent of the pKa of the boronate ester79, even though the pKa varies as a function of which sugar is used. It is well documented35,57 with phenylboronic acid that lowering of the pKa of the boronic acid depends upon which sugar is added to the solution. Since the strength of the N—B bond should modulate the extent of PET, and because each sugar has a different electron-withdrawing ability, the electrophilicity of different boronate esters should vary, and therefore, the fluorescence of 1 should also vary with different sugars. However, the earlier work by James and Cooper showed that the change in fluorescence is not dependent on the sugar79, and Wang reconfirmed this experimentally with four different carbohydrates65. Wang further noted that sugars which form trivalent binding of the boron should not modulate the fluorescence if an N—B bond occurs, since the third valency would break the N—B bond, while the hydrolysis mechanism would still show a fluorescence change, because the third valency would simply take the place of the hydroxyl group in the boronate ester. Experimentally, trivalent sugars give very large fluorescence changes65.

pH titration evidence for different perspectives

A large amount of the experimental data revealing emission turn-on in the presence of sugars for boronic acids such as 1 comes from pH titrations (as seen in Fig. 1c). These titrations can be rationalized with either the N—B bond or hydrolysis mechanisms, and thus these experiments cannot differentiate between these mechanisms64,65. There are two types of pH titrations often performed: fluorescent and 11B NMR. In fluorescence titrations of 1, upon changing the pH from low to high, large emission decreases between pHs of 6 to 7 are observed, and another smaller decrease above pH 11 occurs. By contrast, in the presence of a sugar, the first fluorescence decrease is minor, while a large decrease occurs at the higher pH63.

Let us examine how to interpret the pH titrations in light of the two mechanistic postulates, and which chemical species the pKa steps interconvert. First, with the notion of an N—B bond formation in mind, it is logical to assign pKa1 (6–7) to the deprotonation of the ammonium ion, which is lowered in comparison to the pKa of a standard alkyl ammonium (9–11) due to the resulting coordination with boron (Fig. 2a). The lowering of the pKa is analogous to that observed for ammonium ions in the presence of metals due to metal–amine coordination80. Therefore, pKa2 would correspond to hydroxylation of the boron with subsequent breaking of the N—B bond. This pKa is in the standard range of that for boronic acids (9–10). Note that hydroxylation of a boronic acid is accompanied by the release of a proton (not shown), and hence is a Brønsted acid dissociation reaction with an associated pKa value (Fig. 2a).

Fig. 2 |. The assignment of pKa values that would be associated with the three mechanistic postulates described in this Review.

Fig. 2 |

Each scheme represents the structures involved with o-aminomethylphenylboronic acids as a function of pH (low to high pH given from left to right). In each case, the red arrows depict the dominant structures formed when raising the pH in the presence of a sugar. a,b, The N—B bonding postulate in the absence (a) and presence (b) of a diol, such as a sugar, hydroxycarboxylate or catechol. c,d, The pKa switch postulate (that is, hydrolysis or solvent-insertion) in the absence (c) and presence (d) of a diol, such as a sugar, hydroxycarboxylate or catechol. e,f, The loose-bolt postulate in the absence (e) and presence (f) of a diol, such as a sugar, hydroxycarboxylate or catechol. Given the discussion presented in this Review, this last postulate shows the proper assignments of the pKa values.

Continuing with the postulate of N—B bonding, deprotonation of the ammonium group leads to liberation of the nitrogen lone-pair (pKa1), which is then weakly coordinated to the boron atom (Fig. 2b), and would therefore be predicted to quench the anthracene emission. Hydroxylation of the boron in the boronic acid only slightly increases quenching (pKa2). On the other hand, for this theory to operate, addition of a sugar leads to boronate ester formation (Fig. 2b) and deprotonation of the ammonium (pKa1) replaces an N—H bond with a strong dative N—B bond, essentially not affecting the ability of the nitrogen to be involved in PET (intermediate pH range of Fig. 1c). Hydroxylation of the boron in the boronate ester, however, does free up the amine’s lone-pair leading to quenching of the anthracene emission (high pH range of Fig. 1c). Figure 2a,b shows equilibrium arrows connecting individual species, while the red arrows depict the major structures formed when raising the pH in the presence of a sugar. Note that the structures on the upper right and lower left are presumed not to be present to a significant extent under neutral pH conditions.

Let us now examine the pH titrations with the assumption that the pKa switch mechanism is operable (Fig. 2c,d). At low pH the amine is protonated, and in the presence of a diol there is little binding. As the pH is raised in the absence of a sugar, pKa1 would correspond to deprotonation of the amine to make an N—B bond, while pKa2 is hydroxylation of the boron. This is exactly the same as Fig. 2a. However, in the presence of a sugar, the proposal is very different. As the pH is raised, the sugar binds and the N—B bond is hydrolysed. Logically, it must follow that the second pKa would be deprotonation of the ammonium ion, with a slightly raised value relative to a normal ammonium because of its proximity to the negative boron centre. Note that in the absence of the diol, the first and second pKas are deprotonation of the ammonium ion and hydroxylation of the N—B bond, respectively, while in the presence of a diol it is reversed; the first and second pKas are N—B bond hydroxylation and ammonium ion deprotonation, respectively. This mechanistic postulate is therefore called a ‘pKa switch’.

One last piece of evidence that Wang used to support the hydrolysis mechanism (Fig. 2c,d) was stated as, “there are ample literature precedents proving that the first pKa is the deprotonation of the [ammonium] with concomitant formation of a N—B bond.” This literature precedent was work done by Anslyn81. These experiments were the pH titrations of compounds 2 and 3 (Fig. 3), which were followed by 11B NMR spectroscopy, which is very sensitive to the hybridization of the boron atom71. The goal of the Anslyn experiments was to determine if the pKas of secondary and tertiary amines proximal to the boronic acids were similar or not, due to the possibility that with a secondary amine deprotonation could lead to a standard covalent N—B bond rather than a dative N—B bond interaction81. He found nearly identical pKa values for o-aminomethylphenylboronic acids involving tertiary and secondary amines, which thereby rules out a standard covalent N—B bond.

Fig. 3 |.

Fig. 3 |

The molecular structures of the key compounds discussed in this Review.

To explore if the pKas will switch between boronic acids and boronate esters, Anslyn examined the 11B NMR spectra of 4 in the presence of saturating amounts of different diols, and found at most one-half of a pKa unit difference between 4 and boronate esters formed from various diols for both the first and second pKa values62. Further, he found that the pKa of 4 does not change significantly with different sugars. These observations suggest that the acid/base reactions of the o-aminomethylphenylboronate esters are not significantly different from those of their corresponding boronic acids. Such results are different from literature publications for simple phenylboronic acids82, suggesting a role of the neighbouring amino group in affecting the pKa of the boron species through electrostatic effect due to the 1,5-relationship.

With the clarity of hindsight, it is obvious that there is a problem with Anslyn’s interpretation of his 11B NMR experiments81. The data really only reveal whether the boron atom is trigonal planar versus tetrahedral, and therefore the experiments cannot distinguish N—B bonds from other structures containing an sp3-hybridized boron species. Anslyn had interpreted the deprotonation associated with the first pKa to result in N—B bond formation upon pyramidilization of the boron because N—B bonding was the prevailing picture81. But there is now a different interpretation that involves solvent insertion in both the boronic acids and boronate esters, as discussed immediately below.

Structural and 11B NMR evidence for different perspectives

To distinguish between the N—B bond and pKa switch mechanisms, Anslyn’s group performed a series of X-ray crystallographic and 11B NMR studies. The 11B NMR spectra of titrations of 4 (Fig. 3) with catechol, hydrobenzoin and α-hydroxyisobutyric acid were examined62. The chemical shifts of the products formed during titration were correlated with those found for purified boronate esters for which crystal structures were obtained. A trigonal planar boron has a resonance in the range of 28–30 ppm, while signals for N—B bonds appear around 14–15 ppm, and solvent insertion resonances are found between 8 and 10 ppm. The conclusion was that solvent insertion is observable in protic media for boronic acid 4 and all the boronate esters created. There was one exception—a small extent of N—B bond formation (estimated as 5%) was found when using catechol, meaning that N—B bonds and solvent insertion can co-exist in equilibrium.

Computational results from Larkin and James led to a similar conclusion83. They found that the solvent-inserted species are lower in energy than N—B dative bonded species. Importantly, all evidence points to solvent insertion being the dominant species in protic media for both boronic acids and boronate esters. This being the case, neither the N—B bond mechanism nor the pKa switch mechanism can be operative. Thus, while logically sound, the postulates illustrated in Fig. 2ad were shown to be incorrect in protic media.

Further support for solvent insertion

Solvent insertion is also supported by crystal structures of boronic acids with sugars that can act as trivalent ligands, supplying three oxygen atoms to the boron83,84. In these cases, the inserted solvent is replaced by an OH group from the sugar itself. This is well accepted for fructose83, forming structures such as 5 (Fig. 3). In fact, Norrild found that glucose rearranges to its furanose form when binding with a bisboronic acid receptor 6 (Fig. 3), as revealed by a series of coupling constant measurements84. The postulate is that the driving force is to exploit the energetically favourable trivalent interaction with glucose. The most recent computational studies by Larkin confirm the lower energy of trivalent sugar geometries85.

The o-aminomethyl group in the assignment of the pKas

As discussed above, the first pKa of o-aminomethylphenylboronic acids and esters is between 5 and 7 and leads directly to solvent insertion. Thus, we must now conclude that the first pKa corresponds to hydroxylation of the boronic acid/ester giving directly a solvent-inserted structure (Fig. 2e,f, vide infra why this is called ‘loose bolt’). Hence, the proximity of the positively charged ammonium group depresses the pKa of the boronic acid to around 5–7 from the common values of 9–10. This occurs irrespective of whether the boron atom is part of a boronic acid or a boronate ester, because the first pKa values do not significantly vary between these two species (discussed above). With this conclusion in mind, the second pKa value of o-aminomethylphenylboronic acids or boronate esters must correspond to the deprotonation of the ammonium ion, occurring at pH values of around 11–12. This second pKa is raised 1–2 units above that of normal ammonium groups due to the proximity of the negatively charged boronate group. Thus, the first and second pKas are now clearly assigned.

The o-aminomethyl group in boronate ester formation

If the ammonium group perturbs the pKas of the boronic acids and boronate esters, it is logical that it could also play a key role in the mechanism of boronate ester formation. For example, consider what steps need to be involved when starting with a solvent-inserted boronic acid and transitioning to a solvent-inserted boronate ester (Fig. 4a). The inserted solvent first needs to be expelled, then replaced by an alcohol of the diol or saccharide, followed by bond rotations and further stepwise replacements of inserted solvent(s), leading to a fully bound guest. Because the first replacement of the inserted solvent by the guest is intermolecular, it is likely slow relative to the subsequent steps that are intramolecular and chelate the diol or saccharide to the boron atom.

Fig. 4 |. Mechanistic considerations for the role of the o-aminomethyl group.

Fig. 4 |

a, The proposed mechanism for boronate ester formation at pHs between the first and second pKas, based upon kinetics, crystal structures and isotope effects. The red equilibrium arrows show the dominant pathway, involving general acid-catalysed expulsion of an inserted solvent with a general base-catalysed delivery of the guest. b, The possibility of losing the inserted solvent in a single step if the solvent is not highly ionized between the N and B.

The release of an inserted solvent would lead to species 7 in Fig. 4. This could happen in two steps: ammonium deprotonation and loss of hydroxide/alkoxide from the boron centre, in any order. In either case, this would require loss of a very poor leaving group at neutral pH. Alternatively, the solvent could be lost in one step by simple decomplexation from the boron (see the ‘general acid-catalysis’ pathway). This would involve forming an o-aminomethylphenylboronic acid in a high-energy state (7) because it does not possess the preferred boronic acid and amine protonation states at the operating pH. Interestingly, the question of stepwise or single-step loss of the inserted solvent is dependent on the ionization state of the inserted solvent. If the solvent is fully deprotonated when inserted, thereby forming a zwitterionic boronate anion and ammonium cation, its single step departure as a neutral species requires a proton transfer to occur simultaneously with departure (as shown in the pathway that is shown with red arrows in Fig. 4a). If the solvent is not ionized when inserted, it can simply depart with no proton transfer (Fig. 4b). The former possibility, concerted protonation and leaving group departure, is an example of intramolecular general acid-catalysis and can be analysed by classic experiments such as isotope effects.

Any of the alternatives for loss of solvent all generate high-energy intermediates with which the diol-containing guest subsequently reacts. This means there is a step that forms an intermediate prior to reaction with the guest, which is a mechanism that should show saturation kinetics, as with an SN1 mechanism86. Alternatively, a mechanism in which the solvent-inserted o-aminomethylphenylboronic acid reacts directly with the guest would consistently show second-order kinetics (as with an SN2 reaction).

Anslyn studied the kinetics of the reaction of 1 with fructose, both at low and high fructose concentrations78,87. At low concentrations of fructose, the kinetics appeared second-order, and the y-intercept of the kinetic plots revealed ratios of k1 and k−1. Hence, the reaction appeared analogous to an SN2 mechanism, except for the fact that there was a non-zero y-intercept, which is indicative of equilibrium kinetics87. Yet, at high fructose concentrations saturation kinetics were found (that is, zero-order in fructose). The kinetic data show that a mechanism involving a rate-determining step prior to reaction with the guest is operative. It was proposed that this first step is loss of the inserted solvent to generate 7 (Fig. 4a). Such a mechanism is analogous to an SN1 reaction, in which leaving group departure leads to a reactive intermediate that takes on a nucleophile. However, unlike SN1 chemistry, which loses kinetic dependence at low concentrations of nucleophile, the boronic acid mechanism requires hundreds of equivalents of fructose to reach saturation. This makes perfect sense, given that the reverse step that competes with the first insertion of guest is insertion of a solvent molecule. This competing re-insertion of solvent is analogous to the common ion effect in SN1 mechanisms87, but with boronic acids the ‘common ion’ is the solvent. By fitting the kinetic data, Anslyn estimated that fructose is around 1,000 times better as a nucleophile when adding to 7 than the solvent.

The Anslyn group also addressed the issue of whether the inserted solvent is significantly ionized or retains substantial O—H bonding (8, Fig. 4b). General acid-catalysed loss of the solvent involves the movement of a proton, and hence should have an isotope effect, while the other possibilities would have almost no isotope effect. A crystal structure of solvent-inserted species 8 (Fig. 4) with the proper resolution to find the position of the hydrogen between the O and N atoms shows a shorter H—N bond than O—H bond, supporting a significant extent of ionization of the inserted solvent1. Consistent with this finding, Anslyn uncovered an isotope effect of 1.42 for the reaction of 1 with fructose87. This value is smaller than what may be expected but is clearly a substantial effect. The isotope effect is evidence that the general acid-catalysed pathway for expulsion of the inserted solvent occurs. It should be noted that if the expulsion is general acid-catalysed, then the reverse reaction, solvent insertion, would be general base-catalysed (as noted in Fig. 4a).

In summary, the o-aminomethyl group plays the roles of: (1) increasing the thermodynamics of binding diol species at neutral pH by lowering the pKa of the boronic acids to the physiological range and (2) speeding up the sugar binding by an intramolecular general acid-catalysis of leaving group departure from the boronic acid.

Aggregation and disaggregation

Having settled the assignment of pKa values, whether solvent insertion or N—B bonding occurs in protic media, and how the o-aminomethyl group affects the thermodynamics and kinetics, the only major remaining issue concerns the role of the o-aminomethyl group on the fluorescence modulation upon sugar binding. We have already covered that the original PET postulate is not consistent with structural data. Further, the structural evidence also disagrees with the pKa switch notion because solvent insertion is dominant for both boronic acids and boronate esters. Therefore—what is the mechanism of the fluorescence turn-on?

To probe the role(s) of the o-aminomethyl group in modulating the emission of boronic-acid-based sensors that incorporate this group, Anslyn initiated a detailed photophysical study of compound 167. The original pH titration data from Shinkai and James62 revealed quite a large emission turn-on (around 30-fold) in a pH titration with fructose in a solution of 2:1 water/methanol with 50 mM NaCl. Anslyn found that the emission would gradually turn on with sonication without addition of fructose, or just by sitting in the cuvette with continued irradiation. These surprising results led his group to analyse the emission spectra in more detail. First, they confirmed by 11B NMR spectroscopy in a water/methanol mixture67 and X-ray crystallography, that solvent insertion dominates. Second, when increasing the wavelength of the emission spectra of 1 out to 600 nm, a broad structureless emission from 460 to 600 nm was found. Admittedly, this peak could sometimes be quite small and easily missed, or with other preparations, this peak could be very large. Upon irradiation or sonication, this peak diminished even without the addition of a sugar, while the highly structured emission of anthracene would increase at the same time. Upon delving into the literature, it became clear that this emission peak was associated with a known excimer of anthracene8890. Thus, it seemed that 1 was aggregated in the condition used for the original pH titrations (2:1 water/methanol with 50 mM NaCl; Fig. 5a).

Fig. 5 |. Reactions and data relevant to the discussion of photophysics of the systems described in this Review.

Fig. 5 |

a, Aggregation and disaggregation before and after binding with diols. b, Fluorescence spectra of a saturated solution of 1 in 2:1 water/methanol with 50 mM NaCl. Emission scan with λex = 368 nm (blue). Excitation scan with λem = 417 nm (red). Excitation scan with λem = 520 nm (green). Emission scan with λex = 408 nm (purple). Adapted from ref. 66, American Chemical Society (b).

To confirm the presence of an excimer, Anslyn performed the classic91 analysis of measuring an excitation spectrum at a wavelength that the excimer absorbs, but that the anthracene monomer does not. This gave an excitation spectrum in the region of the anthracene monomer that was also broad and structureless (Fig. 5b). This confirmed that the broad emission at long wavelengths was indeed indicative of an aggregated species of 1 . Apparently, the aggregate breaks up with sonication and/or irradiation, but also upon addition of fructose.

As just stated, the emission turn-on of 1 is complicated by a disaggregation phenomenon, but potentially the emission turn-on was additionally due to some electronic factors resulting from fructose binding. To decipher the extent that disaggregation and fructose binding influence the emission, compound 1 was titrated with fructose in pure methanol. Albeit 11B NMR spectroscopy confirmed the binding of fructose, there was no emission turn-on in this solvent, and no excimer was observed in the absence of sugar. In addition, a compound lacking a boronic acid (9, Fig. 3) turned on fluorescence upon the addition of fructose in a near-identical manner as 1 in the NaCl methanol/water solution. This revealed that a vast majority of the emission turn-on was not related to the binding of fructose, but rather a disaggregation phenomenon that occurs as a result of the addition of fructose into the solution. Molecular dynamics from the Larkin group showed that, indeed, fructose causes a solvent effect that breaks up aggregates of 1, supporting this hypothesis67.

So far, different mechanisms for the emission turn-on of o-aminomethylphenylboronic acids with appended fluorophores in response to saccharide binding in aqueous media have been postulated, such as PET, ‘pKa switch’ and disaggregation. However, none of the hypotheses are consistent with all the data for boronic-acid-based sensors. For example, there was a curious feature to the Anslyn studies. In the NaCl methanol/water mixture there was consistently an additional twofold to threefold increase in fluorescence of 1 upon fructose binding that was never achieved by sonication or irradiation of 1 alone. This meant that of the nearly 30-fold increase, about a 10- to 15-fold increase was due to disaggregation, but the additional 2- to 3-fold increase was, indeed, due to fructose binding.

Loose bolt internal conversion

To reveal the mechanism of this additional emission turn-on, the Anslyn and James groups joined forces68. They noted that most all o-aminomethylphenylboronic acid sensors for sugars typically show an emission turn-on in the range of twofold to fivefold61,9296. Thus, they set out to study a series of sensors with this functionality, but using those that are freely soluble in water and methanol (10, 11, and 12, Fig. 3), thereby removing any complications from aggregation. This study allowed them to focus on the roles of the boronic acid, boronate ester and o-aminomethyl group on the photophysics, revealing the mechanism for the fluorescence turn-on. This study primarily focused on the use of 10, which James had shown could be used as a sensor for peroxynitrite after binding fructose97.

As originally done by Shinkai and James with 1, the first step was to analyse the pH titration of 10 with and without fructose and assign the structures of the o-aminomethylphenylboronic acid and fructose ester thereof in different pH ranges (Fig. 6a). In the pH range between 7 and 10, there was approximately a threefold (within a twofold to fivefold range) fluorescence turn-on response in the presence of fructose, which confirms fructose binding. As the first pKa of the boronic acid (that is, hydroxylation of the boron) is approached by raising the pH, an anionic boronate (R—B(OH3)) is formed without fructose (recall Fig. 2e). In the presence of fructose, a 10–fructose complex is formed within the pH range 6–10. Above pH 10, the fluorescence of both 10 and the 10–fructose complex drop, owing to PET from an amine lone-pair through deprotonation of the ammonium (pKa2). 11B NMR spectroscopy revealed that compound 10 was solvent-inserted in protic media with and without fructose binding. An analogous compound to 10 without the boronic acid group did not show any response towards fructose in aqueous conditions. Further, no excimer was observed. These data supported the design criteria that there was no aggregation of this boronic acid species.

Fig. 6 |. Further data and reactions related to the photophysics of the systems described in this Review.

Fig. 6 |

a, Fluorescence pH titration for boronic acid probe 10 (4 μM; circles) and 10 with fructose complex (10: 4 μM; fructose: 100 mM; squares) in water. Ex = 450 nm, slit/slit: 2 nm/2 nm; data taken from ref. 68. b,c, Fluorescence changes in the replacement of B–(OH)2 to B–OR groups in water (b) and B–OMe to B–OR groups in methanol (c). Arrow denotes internal conversion quenching, which is blocked with any form of a boronate ester. d, Isotopic effect for fluorescence changes when the –B(OH)2 groups are converted to –B(OD)2.

To further probe the emission turn-on with water-soluble entities, compounds 11 and 12 were prepared. Compound 11 is meant to mimic all the features of 1 except with a solubilizing ammonium group, while 12 carries a pyrene fluorophore. Compounds 10, 11 and 12 showed a twofold to fivefold turn-on of emission in water upon binding fructose, but no turn-on of emission when binding fructose in methanol. Yet 11B NMR spectroscopy showed fructose was binding. In other words, upon converting –B(OMe)2 groups to esters with a sugar, there is no emission response—but upon replacing –B(OH)2 groups with esters of either methanol or a sugar, the emission does turn on.

These results prompted Anslyn and James to ask a simple question: what is fundamentally different about the structure of such groups in water or methanol?68

The answer was obvious: the –B(OH)2 groups in water possess B–OH bonds, which are converted to B–OR bonds in either methanol or when binding a sugar (Fig. 6b,c). The fact that the replacement of B–OMe groups with B–OR groups does not change the fluorescence, yet the replacement of B–OH groups with B–OR groups in water does turn on the emission, is key to the puzzle. It must be that the –B(OH)2 groups quench the fluorescence of the anthracene, and upon conversion to any form of a boronate ester, the fluorescence increases.

It is well known that the O–H bond vibrations in water quench the fluorescence of fluorophores by accepting the electronic excitation energy into excited vibrational states of the water98. Further, intramolecularly attached alcohols, or carboxylic acids, can also take up this excited electronic state energy99,100. This is a form of internal conversion that lowers the fluorescence quantum yield. The classic98,101,102 test for this is to convert the –OH groups to –OD, which, due to their lower frequencies, are less efficient energy acceptors. The Anslyn–James study revealed that in D2O, in which all the –B(OH)2 groups are converted to –B(OD)2 groups, the emissions of the sensors alone are just as high as the emission in methanol. Further, upon addition of fructose, no additional emission turn-on was found (Fig. 6d).

Such an internal conversion mechanism is commonly referred to as the loose-bolt effect103107. Just as a loose bolt can absorb energy from a running motor via vibrating and further loosening (or tightening), a high-frequency rotor in resonance with an electronic excited state can absorb energy108. In our case, the proposal is that the loose bolt enhances internal conversion because electronic energy ‘leaks out’ through –B(OH)3 vibrations109. In methanol the B–OH groups are B–OMe groups, and the quenching from the B(OH)3 vibrations is arrested, thereby turning on fluorescence, and the same occurs when a sugar binds. Even studies that postulate PET from the boronate anion could alternatively quench from this loose-bolt mechanism110.

Conclusions

This treatise has delineated a historical account of the ideas and concepts put forth by Shinkai–James and Wang relating to the role of the o-aminomethyl group in phenylboronic acids upon sugar binding, covering both the N—B bond and the pKa switch postulates. Both concepts were able to explain how the emission properties of boronic acids and boronate esters would change. However, with evidence from 11B NMR spectroscopy from Anslyn, arguments concerning pKa values, as well as crystal structures from Norrild, it was clear that alternative explanations were needed for the mechanism of fluorescence turn-on.

In terms of chemical structures, solvent-inserted species consistently dominant for both boronic acids and boronate esters in protic media. In turn, the o-aminomethyl group lowers the pKa of the proximal boronic acid due to its electron-withdrawing nature and a field effect from the ammonium ion. Consequently, the pyramidalization of the boron atom required for boronate ester formation is facilitated at a lower pH. Further, the intramolecular hydrogen bond between the ammonium cation and the boronate anion formed upon solvent insertion facilitates leaving group departure because the ammonium group acts as a general acid-catalyst for liberating the inserted solvent. This conclusion is supported by both kinetics and isotope effect studies. Thus, the o-aminomethyl group both improves the thermodynamics of boronate ester formation, but also improves the kinetics of exchange at the boron centre.

As to the mechanism of fluorescence changes upon sugar binding, the evidence points to the o-aminomethyl group having no role. Neither a PET nor a pKa switch mechanism is operative. Instead, with poorly soluble sensors, there is the possibility of disaggregation of the hosts upon addition of sugar, and this can turn on the fluorescence. However, emission enhancement seems to generally occur for boronic acids by arresting a form of internal conversion, commonly referred to as a loose bolt effect. Importantly, by converting –B(OH)3 groups to –B(OR)3 groups (or even –B(OD)3 groups), emission turns on commonly by factors of twofold to fivefold.

Thus, after 25 years since the 1994 landmark study from Shinkai and James that spawned an explosion of work into the use of boronic acids as sugar sensors and receptors, and of excitement and enthusiasm in the chemosensing community, a series of studies from various researchers can be combined to paint a consistent picture of the role that o-aminomethyl groups play in the molecular recognition properties of phenylboronic acids. This body of work reveals how consistent and coordinated physical organic studies from numerous groups can lead to a unified mechanistic picture, even when the mechanisms only differ by subtle changes in positions of protons, or even just vibrational changes between –OH, –OD and–OR groups.

Acknowledgements

The authors gratefully acknowledge financial support for the several years of their work in the area of boronic acids. E.V.A. is grateful to the NSF (CHE-0716049, CHE-1212971), the Welch Foundation (F-1151) and the Welch Regents Chair (F-0046). B.W. acknowledges the Georgia Research Alliance, Georgia Cancer Coalition and the NIH throughout the years for their support of the boronic acid-relate projects (GM084933, CA159567, DK55062, CA88343, NO1-CO-27184, CA113917, CA123329 and GM086925). T.D.J. wishes to thank the Royal Society for a Wolfson Research Merit Award.

Footnotes

Competing interests

The authors declare no competing interests.

References

  • 1.Anslyn EV & Dougherty DA Modern Physical Organic Chemistry (University Science Books, 2006). [Google Scholar]
  • 2.Scholz F et al. Crystal structure determination of the nonclassical 2-norbornyl cation. Science 341, 62–64 (2013). [DOI] [PubMed] [Google Scholar]
  • 3.Reinhoudt DN, Rudkevich DM & de Jong F Kinetic Analysis of the Rebek self-replicating system: is there a controversy? J. Am. Chem. Soc 118, 6880–6889 (1996). [Google Scholar]
  • 4.Cleland W Low-barrier hydrogen bonds and enzymatic catalysis. Arch. Biochem. Biophys 382, 1–5 (2000). [DOI] [PubMed] [Google Scholar]
  • 5.Schutz CN & Warshel A The low barrier hydrogen bond (LBHB) proposal revisited: the case of the Asp⋯His pair in serine proteases. Proteins 55, 711–723 (2004). [DOI] [PubMed] [Google Scholar]
  • 6.Hall DG Boronic Acids: Preparation, Applications in Organic Synthesis and Medicine (John Wiley and Sons, 2006). [Google Scholar]
  • 7.Bull SD et al. Exploiting the reversible covalent bonding of boronic acids: recognition, sensing, and assembly. Acc. Chem. Res 46, 312–326 (2013). [DOI] [PubMed] [Google Scholar]
  • 8.Guo Z, Shin I & Yoon J Recognition and sensing of various species using boronic acid derivatives. Chem. Commun 48, 5956–5967 (2012). [DOI] [PubMed] [Google Scholar]
  • 9.Fang H, Kaur G & Wang B Progress in boronic acid-based fluorescent glucose sensors. J. Fluoresc 14, 481–489 (2004). [DOI] [PubMed] [Google Scholar]
  • 10.Heagy MD & Meka RK in Comprehensive Supramolecular Chemistry II (ed. Atwood JL) Ch. 4.19, 615–647 (Elsevier, 2017). [Google Scholar]
  • 11.Kelly AM, Perez-Fuertes Y, Arimori S, Bull SD & James TD Simple protocol for NMR analysis of the enantiomeric purity of diols. Org. Lett 8, 1971–1974 (2006). [DOI] [PubMed] [Google Scholar]
  • 12.Pérez-Fuertes Y et al. Simple protocol for NMR analysis of the enantiomeric purity of primary amines. Org. Lett 8, 609–612 (2006). [DOI] [PubMed] [Google Scholar]
  • 13.Cambre JN & Sumerlin BS Biomedical applications of boronic acid polymers. Polymer 52, 4631–4643 (2011). [Google Scholar]
  • 14.Fujita N, Shinkai S & James TD Boronic acids in molecular self-assembly. Chem. Eur. J 3, 1076–1091 (2008). [DOI] [PubMed] [Google Scholar]
  • 15.Nishiyabu R, Kubo Y, James TD & Fossey JS Boronic acid building blocks: tools for self assembly. Chem. Commun 47, 1124–1150 (2011). [DOI] [PubMed] [Google Scholar]
  • 16.Dai C et al. Using boronolectin in MALDI-MS imaging for the histological analysis of cancer tissue expressing the sialyl Lewis X antigen. Chem. Commun 47, 10338–10340 (2011). [DOI] [PubMed] [Google Scholar]
  • 17.Sun X, Zhai W, Fossey JS & James TD Boronic acids for fluorescence imaging of carbohydrates. Chem. Commun 52, 3456–3469 (2016). [DOI] [PubMed] [Google Scholar]
  • 18.Sun X et al. Reaction-based Indicator displacement Assay (RIA) for the selective colorimetric and fluorometric detection of peroxynitrite. Chem. Sci 6, 2963–2967 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19.Sun X et al. “Integrated” and “insulated” boronate-based fluorescent probes for the detection of hydrogen peroxide. Chem. Commun 49, 8311–8313 (2013). [DOI] [PubMed] [Google Scholar]
  • 20.Chan J, Dodani SC & Chang CJ Reaction-based small-molecule fluorescent probes for chemoselective bioimaging. Nat. Chem 4, 973–984 (2012). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 21.Lippert AR, Van de Bittner GC & Chang CJ Boronate oxidation as a bioorthogonal reaction approach for studying the chemistry of hydrogen peroxide in living systems. Acc. Chem. Res 44, 793–804 (2011). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.James T & Shinkai S in Topics in Current Chemistry (ed. Penadés S) Vol. 218, Ch. 6, 159–200 (Springer, 2002). [Google Scholar]
  • 23.James TD, Phillips MD & Shinkai S Boronic Acids in Saccharide Recognition (Royal Society of Chemistry, 2006). [Google Scholar]
  • 24.James TD, Sandanayake KRAS & Shinkai S Saccharide sensing with molecular receptors based on boronic acid. Angew. Chem. Int. Ed. Engl 35, 1910–1922 (1996). [Google Scholar]
  • 25.Jin S, Cheng Y, Reid S, Li M & Wang B Carbohydrate recognition by boronolectins, small molecules, and lectins. Med. Res. Rev 30, 171–257 (2010). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Wright AT et al. Differential receptors create patterns that distinguish various proteins. Angew. Chem. Int. Ed 44, 6375–6378 (2005). [DOI] [PubMed] [Google Scholar]
  • 27.Bicker KL et al. Synthetic lectin arrays for the detection and discrimination of cancer associated glycans and cell lines. Chem. Sci 3, 1147–1156 (2012). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Gray CW & Houston TA Boronic acid receptors for α-hydroxycarboxylates: high affinity of Shinkai’s glucose receptor for tartrate. J. Org. Chem 67, 5426–5428 (2002). [DOI] [PubMed] [Google Scholar]
  • 29.Wang W, Gao X & Wang B Boronic acid-based sensors. Curr. Org. Chem 6, 1285–1317 (2002). [Google Scholar]
  • 30.Lee JW, Lee J-S & Chang Y-T Colorimetric Identification of carbohydrates by a pH indicator/pH change inducer ensemble. Angew. Chem. Int. Ed 45, 6485–6487 (2006). [DOI] [PubMed] [Google Scholar]
  • 31.Schiller A, Wessling RA & Singaram B A Fluorescent sensor array for saccharides based on boronic acid appended bipyridinium salts. Angew. Chem. Int. Ed 46, 6457–6459 (2007). [DOI] [PubMed] [Google Scholar]
  • 32.Edwards NY, Sager TW, McDevitt JT & Anslyn EV Boronic acid based peptidic receptors for pattern-based saccharide sensing in neutral aqueous media, an application in real-life samples. J. Am. Chem. Soc 129, 13575–13583 (2007). [DOI] [PubMed] [Google Scholar]
  • 33.Musto CJ, Lim SH & Suslick KS Colorimetric detection and identification of natural and artificial sweeteners. Anal. Chem 81, 6526–6533 (2009). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 34.Zhang X, You L, Anslyn EV & Qian X Discrimination and classification of ginsenosides and ginsengs using bis-boronic acid receptors in dynamic multicomponent indicator displacement sensor arrays. Chem. Eur. J 18, 1102–1110 (2012). [DOI] [PubMed] [Google Scholar]
  • 35.Springsteen G & Wang B A detailed examination of boronic acid–diol complexation. Tetrahedron 58, 5291–5300 (2002). [Google Scholar]
  • 36.Zaubitzer F, Buryak A & Severin K Cp*Rh-based indicator-displacement assays for the identification of amino sugars and aminoglycosides. Chem. Eur. J 12, 3928–3934 (2006). [DOI] [PubMed] [Google Scholar]
  • 37.Yasuda H, Kurokáwa T, Fujii Y, Yamashita A & Ishibashi S Decreased d-glucose transport across renal brush-border membrane vesicles from streptozotocin-induced diabetic rats. Biochim. Biophys. Acta Biomembr 1021, 114–118 (1990). [DOI] [PubMed] [Google Scholar]
  • 38.Fedorak RN, Gershon MD & Field M Induction of intestinal glucose carriers in streptozocin-treated chronically diabetic rats. Gastroenterology 96, 37–44 (1989). [DOI] [PubMed] [Google Scholar]
  • 39.Mallia AK, Hermanson GT, Krohn RI, Fujimoto EK & Smith PK Preparation and use of a boronic acid affinity support for separation and quantitation of glycosylated hemoglobins. Anal. Lett 14, 649–661 (1981). [Google Scholar]
  • 40.Yang W et al. Diboronic acids as fluorescent probes for cells expressing sialyl Lewis X. Biorg. Med. Chem. Lett 12, 2175–2177 (2002). [DOI] [PubMed] [Google Scholar]
  • 41.Sun X & James TD Glucose sensing in supramolecular chemistry. Chem. Rev 115, 8001–8037 (2015). [DOI] [PubMed] [Google Scholar]
  • 42.Goldberg RN & Tewari YB Thermodynamic and transport properties of carbohydrates and their monophosphates: the pentoses and hexoses. J. Phys. Chem. Ref. Data 18, 809–880 (1989). [Google Scholar]
  • 43.Cesàro A in Thermodynamic Data for Biochemistry and Biotechnology 177–207 (Springer, 1986). [Google Scholar]
  • 44.Franks F Physical chemistry of small carbohydrates-equilibrium solution properties. Pure Appl. Chem 59, 1189–1202 (1987). [Google Scholar]
  • 45.Essentials of Glycobiology 2nd edn (eds. Varki A et al. ) 23–36 (Cold Spring Harbor Laboratory Press, 2009). [PubMed] [Google Scholar]
  • 46.Moulin E, Cormos G & Giuseppone N Dynamic combinatorial chemistry as a tool for the design of functional materials and devices. Chem. Soc. Rev 41, 1031–1049 (2012). [DOI] [PubMed] [Google Scholar]
  • 47.Lehn J-M From supramolecular chemistry towards constitutional dynamic chemistry and adaptive chemistry. Chem. Soc. Rev 36, 151–160 (2007). [DOI] [PubMed] [Google Scholar]
  • 48.Lehn J-M Supramolecular Chemistry: Concepts and Perspectives (Vch, 1995). [Google Scholar]
  • 49.Rowan SJ, Cantrill SJ, Cousins GR, Sanders JK & Stoddart JF Dynamic covalent chemistry. Angew. Chem. Int. Ed 41, 898–952 (2002). [DOI] [PubMed] [Google Scholar]
  • 50.Corbett PT et al. Dynamic combinatorial chemistry. Chem. Rev 106, 3652–3711 (2006). [DOI] [PubMed] [Google Scholar]
  • 51.Cougnon FBL & Sanders JKM Evolution of dynamic combinatorial chemistry. Acc. Chem. Res 45, 2211–2221 (2012). [DOI] [PubMed] [Google Scholar]
  • 52.Wilson A, Gasparini G & Matile S Functional systems with orthogonal dynamic covalent bonds. Chem. Soc. Rev 43, 1948–1962 (2014). [DOI] [PubMed] [Google Scholar]
  • 53.Weith HL, Wiebers JL & Gilham PT Synthesis of cellulose derivatives containing the dihydroxyboryl group and a study of their capacity to form specific complexes with sugars and nucleic acid components. Biochemistry 9, 4396–4401 (1970). [DOI] [PubMed] [Google Scholar]
  • 54.Hirata O, Kubo Y, Takeuchi M & Shinkai S Mono- and oligosaccharide sensing by phenylboronic acid-appended 5,15-bis(diarylethynyl)porphyrin complexes. Tetrahedron 60, 11211–11218 (2004). [Google Scholar]
  • 55.Wulff G Selective binding to polymers via covalent bonds. The construction of chiral cavities as specific receptor sites. Pure Appl. Chem 54, 2093–2102 (1982). [Google Scholar]
  • 56.Wulff G, Dederichs W, Grotstollen R & Jupe C in Affinity Chromatography and Related Techniques 207–216 (Elsevier, 1982). [Google Scholar]
  • 57.Lorand JP & Edwards JO Polyol complexes and structure of the benzeneboronate ion. J. Org. Chem 24, 769–774 (1959). [Google Scholar]
  • 58.Wulff G, Lauer M & Bohnke H Rapid proton transfer as cause of an unusually large neighboring group effect. Angew. Chem. Int. Ed. Engl 23, 741–742 (1984). [Google Scholar]
  • 59.Jin S, Wang J, Li M & Wang B Synthesis, evaluation, and computational studies of naphthalimide-based long-wavelength fluorescent boronic acid reporters. Chem. Eur. J 14, 2795–2804 (2008). [DOI] [PubMed] [Google Scholar]
  • 60.Zhu L et al. A structural investigation of the N—B Interaction in an o-(N,N-dialkylaminomethyl)arylboronate system. J. Am. Chem. Soc 128, 1222–1232 (2006). [DOI] [PubMed] [Google Scholar]
  • 61.Zhai W, Sun X, James TD & Fossey JS Boronic acid-based carbohydrate sensing. Chem. Asian J 10, 1836–1848 (2015). [DOI] [PubMed] [Google Scholar]
  • 62.James TD, Sandanayake KRAS & Shinkai S Novel photoinduced electron-transfer sensor for saccharides based on the interaction of boronic acid and amine. J. Chem. Soc. Chem. Commun 1994, 477–478 (1994). [Google Scholar]
  • 63.James TD, Sandanayake KRAS & Shinkai S A glucose-selective molecular fluorescence sensor. Angew. Chem. Int. Ed. Engl 33, 2207–2209 (1994). [Google Scholar]
  • 64.Franzen S, Ni W & Wang B Study of the mechanism of electron-transfer quenching by boron—nitrogen adducts in fluorescent sensors. J. Phys. Chem. B 107, 12942–12948 (2003). [Google Scholar]
  • 65.Ni W, Kaur G, Springsteen G, Wang B & Franzen S Regulating the fluorescence intensity of an anthracene boronic acid system: a B–N bond or a hydrolysis mechanism? Bioorg. Chem 32, 571–581 (2004). [DOI] [PubMed] [Google Scholar]
  • 66.Chapin BM et al. Disaggregation is a mechanism for emission turn-on of ortho-aminomethylphenylboronic acid-based saccharide sensors. J. Am. Chem. Soc 139, 5568–5578 (2017). [DOI] [PubMed] [Google Scholar]
  • 67.Crane BC et al. The development of a continuous intravascular glucose monitoring sensor. J. Diabetes Sci. Technol 9, 751–761 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 68.Sun X, James TD & Anslyn EV Arresting “loose bolt” internal conversion from —B(OH)2 groups is the mechanism for emission turn-on in ortho-aminomethylphenylboronic acid-based saccharide sensors. J. Am. Chem. Soc 140, 2348–2354 (2018). [DOI] [PubMed] [Google Scholar]
  • 69.James TD, Samankumara Sandanayake KRA & Shinkai S Chiral discrimination of monosaccharides using a fluorescent molecular sensor. Nature 374, 345–347 (1995). [Google Scholar]
  • 70.James TD, Linnane P & Shinkai S Fluorescent saccharide receptors: a sweet solution to the design, assembly and evaluation of boronic acid derived PET sensors. Chem. Commun 1996, 281–288 (1996). [Google Scholar]
  • 71.Nöth H & Wrackmeyer B in Nuclear Magnetic Resonance Spectroscopy of Boron Compounds (eds Diehl P, Fluck E & Kosfeld R) (NMR Basic Principles and Progress 14, Springer, 1978). [Google Scholar]
  • 72.James TD in Creative Chemical Sensor Systems 107–152 (Springer, 2007). [Google Scholar]
  • 73.Silva AP Recent evolution of luminescent photoinduced electron transfer sensors. A review. Analyst 121, 1759–1762 (1996). [Google Scholar]
  • 74.Bissell RA et al. in Photoinduced Electron Transfer V 223–264 (Springer, 1993). [Google Scholar]
  • 75.Beeson JC, Huston ME, Pollard DA, Venkatachalam TK & Czarnik AW Structural requirements for efficient photoinduced electron transfer (PET) quenching in 9-aminoalkylanthracenes. J. Fluoresc 3, 65–68 (1993). [DOI] [PubMed] [Google Scholar]
  • 76.De Silva AP et al. Signaling recognition events with fluorescent sensors and switches. Chem. Rev 97, 1515–1566 (1997). [DOI] [PubMed] [Google Scholar]
  • 77.Lehn JM Supramolecular chemistry—scope and perspectives molecules, supermolecules, and molecular devices (Nobel Lecture). Angew. Chem. Int. Ed. Engl 27, 89–112 (1988). [Google Scholar]
  • 78.Matsumura T, Iwatsuki S & Ishihara K Direct kinetic measurements for the fast interconversion process between trigonal boronic acid and tetragonal boronate ion at low temperatures. Inorg. Chem. Commun 8, 713–716 (2005). [Google Scholar]
  • 79.Cooper CR & James TD Selective fluorescence signalling of saccharides in their furanose form. Chem. Lett 27, 883–884 (1998). [Google Scholar]
  • 80.Chaberek S, Courtney RC & Martell AE Stability of metal chelates. II. β-Hydroxyethyliminodiacetic acid. J. Am. Chem. Soc 74, 5057–5060 (1952). [Google Scholar]
  • 81.Wiskur SL et al. pKa values and geometries of secondary and tertiary amines complexed to boronic acids—implications for sensor design. Org. Lett 3, 1311–1314 (2001). [DOI] [PubMed] [Google Scholar]
  • 82.Yoon J & Czarnik AW Fluorescent chemosensors of carbohydrates. A means of chemically communicating the binding of polyols in water based on chelation-enhanced quenching. J. Am. Chem. Soc 114, 5874–5875 (1992). [Google Scholar]
  • 83.Larkin JD, Fossey JS, James TD, Brooks BR & Bock CW A computational investigation of the nitrogen—boron interaction in o-(N,N-dialkylaminomethyl)arylboronate systems. J. Phys. Chem. A 114, 12531–12539 (2010). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 84.Norrild JC & Eggert H Boronic acids as fructose sensors. Structure determination of the complexes involved using 1JCC coupling constants. J. Chem. Soc. Perkin Trans. 2 1996, 2583–2588 (1996). [Google Scholar]
  • 85.Kearns FL et al. Modeling boronic acid based fluorescent saccharide sensors: computational investigation of d-fructose binding to dimethylaminomethylphenylboronic acid. J. Chem. Inf. Model 59, 2150–2158 (2019). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 86.Norrild J A fluorescent glucose sensor binding covalently to all five hydroxy groups of α-D-glucofuranose. A reinvestigation. J. Chem. Soc., Perk. Trans. 2 1999, 449–456 (1999). [Google Scholar]
  • 87.Collins BE, Metola P & Anslyn EV On the rate of boronate ester formation in ortho-aminomethyl-functionalised phenyl boronic acids. Supramol. Chem 25, 79–86 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 88.Chandross EA Photolytic dissociation of dianthracene. J. Chem. Phys 43, 4175–4176 (1965). [Google Scholar]
  • 89.Chandross EA, Ferguson J & McRae E Absorption and emission spectra of anthracene dimers. J. Chem. Phys 45, 3546–3553 (1966). [Google Scholar]
  • 90.McVey JK, Shold DM & Yang N Direct observation and characterization of anthracene excimer in solution. J. Chem. Phys 65, 3375–3376 (1976). [Google Scholar]
  • 91.Momiji I, Yoza C & Matsui K Fluorescence spectra of 9-anthracenecarboxylic acid in heterogeneous environments. J. Phy. Chem. B 104, 1552–1555 (2000). [Google Scholar]
  • 92.Arimori S, Bell ML, Oh CS, Frimat KA & James TD Modular fluorescence sensors for saccharides. J. Chem. Soc. Perkin Trans. 1 2001, 803–808 (2002). [PubMed] [Google Scholar]
  • 93.Camara JN, Suri JT, Cappuccio FE, Wessling RA & Singaram B Boronic acid substituted viologen based optical sugar sensors: modulated quenching with viologen as a method for monosaccharide detection. Tetrahedron Lett. 43, 1139–1141 (2002). [Google Scholar]
  • 94.Arimori S, Phillips MD & James TD Probing disaccharide selectivity with modular fluorescent sensors. Tetrahedron Lett. 45, 1539–1542 (2004). [Google Scholar]
  • 95.Phillips M & James T Boronic acid based modular fluorescent sensors for glucose. J. Fluoresc 14, 549–559 (2004). [DOI] [PubMed] [Google Scholar]
  • 96.Xing Z et al. Selective saccharide recognition using modular diboronic acid fluorescent sensors. Eur. J. Org. Chem 2012, 1223–1229 (2012). [Google Scholar]
  • 97.Sun X et al. A water-soluble boronate-based fluorescent probe for the selective detection of peroxynitrite and imaging in living cells. Chem. Sci 5, 3368–3373 (2014). [Google Scholar]
  • 98.Dereka B & Vauthey E Direct local solvent probing by transient infrared spectroscopy reveals the mechanism of hydrogen-bond induced nonradiative deactivation. Chem. Sci 8, 5057–5066 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 99.Harris CM & Selinger BK Proton-induced fluorescence quenching of 2-naphthol. J. Phy. Chem 84, 891–898 (1980). [Google Scholar]
  • 100.Sun Z-N, Liu F-Q, Chen Y, Tam PKH & Yang D A highly specific BODIPY-based fluorescent probe for the detection of hypochlorous acid. Org. Lett 10, 2171–2174 (2008). [DOI] [PubMed] [Google Scholar]
  • 101.Mirbach MJ, Mirbach MF, Cherry WR, Turro NJ & Engel P Solvent isotope effect on the fluorescence of azoalkanes. Chem. Phys. Lett 53, 266–269 (1978). [Google Scholar]
  • 102.Shizuka H & Tobita S Proton-induced quenching and hydrogen-deuterium isotope-exchange reactions of methoxynaphthalenes. J. Am. Chem. Soc 104, 6919–6927 (1982). [Google Scholar]
  • 103.Lewis GN, Magel TT & Lipkin D The absorption and re-emission of light by cis- and trans-stilbenes and the efficiency of their photochemical isomerization. J. Am. Chem. Soc 62, 2973–2980 (1940). [Google Scholar]
  • 104.Kortüm G & Dreesen G ÜQber die Konstitutionsabhängigkeit der Schwingungsstruktur im Absorptionsspektrum von aromatischen Kohlenwasserstoffen. Chem. Ber 84, 182–203 (1951). [Google Scholar]
  • 105.Guesten H, Mintas M & Klasinc L Deactivation of the fluorescent state of 9-tert-butylanthracene. 9-tert-butyl-9,10(Dewar anthracene). J. Am. Chem. Soc 102, 7936–7937 (1980). [Google Scholar]
  • 106.Lewis GN & Calvin M The color of organic substances. Chem. Rev 25, 273–328 (1939). [Google Scholar]
  • 107.Hofer LJE, Grabenstetter RJ & Wiig EO The fluorescence of cyanine and related dyes in the monomeric state 1. J. Am. Chem. Soc 72, 203–209 (1950). [Google Scholar]
  • 108.Rurack K, Dekhtyar ML, Bricks JL, Resch-Genger U & Rettig W Quantum yield switching of fluorescence by selectively bridging single and double bonds in chalcones: involvement of two different types of conical intersections. J. Phy. Chem. A 103, 9626–9635 (1999). [Google Scholar]
  • 109.Ogunsipe A, Chen J-Y & Nyokong T Photophysical and photochemical studies of zinc(ii) phthalocyanine derivatives-effects of substituents and solvents. New J. Chem 28, 822–827 (2004). [Google Scholar]
  • 110.DiCesare N, Adhikari DP, Heynekamp JJ, Heagy MD & Lakowicz JR Spectroscopic and photophysical characterization of fluorescent chemosensors for monosaccharides based on N-phenylboronic acid derivatives of 1,8-naphthalimide. J. Fluoresc 12, 147–154 (2002). [PMC free article] [PubMed] [Google Scholar]
  • 111.Collins BE A Kinetic Investigation of Boronic Acid/Diol Interactions and Pattern-Based α-Chiral Carboxylate Recognition. PhD Thesis, Univ. Texas at Austin; (2005). [Google Scholar]

RESOURCES