Compact Conformation of Ba²⁺-Beauvericin Complex by Triple Cation‑π Interactions: Insight into Ion Transport Mechanism from Gas-Phase Structures

Abstract

Beauvericin (BEA), a natural cyclic hexadepsipeptide ionophore, can selectively transport metal ions through lipid bilayers. Although BEA can transport Ca²⁺ and Ba²⁺, previous studies have not yet revealed its ion-selective transport mechanism. In the present study, we identified a novel conformation of 1:1 BEA complexes selectively stabilized by triple cation-π interactions with Ba²⁺ among alkaline-earth and alkali metal ions, using electrospray ionization-ion mobility-mass spectrometry (ESI-IM-MS). The calculated IR spectrum of this conformation was well reproduced with the experimental IR photodissociation spectrum of the 1:1 BEA complex with Ba²⁺, which was not assigned previously. This conformation can maximize membrane permeability across lipid bilayers since its size (collision cross section) and polarity (solvent accessible 3D polar surface area) are very small and apolar, respectively. The gas phase structures provide insight into BEA’s structure–function relationship, in which the formation of a compact and apolar conformation facilitates ion-selective transport.

Ion transport for specific ions by ion channel proteins or ionophores plays a crucial role in biological actions. Ionophores are organic molecules that increase the membrane permeability of ions, and they exhibit antibiotic properties since they disrupt the concentration of essential ions that cells actively maintain on both sides of their membranes. In general ion transport mechanisms, carrier-ionophores bind to a specific ion in the hydrophilic exterior of lipid bilayers, shield it from the hydrophobic interior, transport it by diffusion, and release it on the other side of the hydrophilic exterior. , Considering not only the capture and release in the hydrophilic environment but also the diffusion across the hydrophobic environment is important in elucidating the ion-selective transport mechanism of carrier-ionophores at the molecular level. The size and polarity of the compounds, which depend on their conformations, are the key properties that determine the kinetics of the diffusion across the membrane. − In the pharmacological field, recent investigations have provided that conformational flexibility significantly improves membrane permeability of large molecules beyond the Lipinski’s rule of 5. − These compounds exhibit chameleonic behavior, undergoing conformational changes that shield or expose polar functionalities in response to changes in the surrounding environment. Based on these insights, it is crucial to carefully consider the conformations of large ionophore complexes with a metal ion, also in the case of ion transport.

Beauvericin (BEA), a cyclic hexadepsipeptide, is a natural large ionophore produced by various species of fungi in the and genera. , The structure of BEA is a macrocyclic with flexible side chains composed of alternating N-methyl-l-phenylalanine and d-hydroxyisovaleric acid residues, and it lacks hydrogen bond donors due to methylation of nitrogen atoms (Figure a). Consequently, BEA can adopt multiple conformations and form various complex stoichiometries. In addition, it possesses amphiphilic properties with both hydrophilic (three amide carbonyl and three ester carbonyl oxygens) and hydrophobic regions (three isopropyl and three benzyl groups), allowing for solubility and membrane permeability. BEA exhibits a variety of biological activities, such as cytotoxic, apoptotic, anticancer, anti-inflammatory, antimicrobial, insecticidal, and nematicidal activity, and is considered to have the potential to be developed as a medicine or a pesticide. − These biological activities are due to some unique active mechanisms, including ion transport. Therefore, understanding the ion-selective transport mechanism of BEA is essential for its further applications.

1.

(a) The skeletal formula of beauvericin (BEA). (b) X-ray structure of the BEA complex with barium picrate toluene solvate. Carbon, nitrogen, and oxygen are colored gray, blue, and red, respectively. Barium is shown in purple.

Although BEA has a high transport ability for Ca²⁺ and Ba²⁺ among alkaline-earth metal ions, , the ion-selective transport mechanism has not yet been rationalized. X-ray crystallography indicated that BEA forms a complex with Ba²⁺ in a 2:2 composition, where three picrate counteranions are strongly bound to Ba²⁺ (Figure b). , However, the 2:2 complex, which consists of seven chemical components, is likely stabilized by crystal packing forces with high symmetry. This high-component complex is unlikely to form stably in solution and also unlikely to contribute to the ion transport effectively. The ion selectivity of BEA in liposomes suggested that BEA has a high affinity for Ca²⁺, and would not transport Mg²⁺, Sr²⁺, and Ba²⁺ under the given experimental conditions. These condensed phase studies were inconsistent, as BEA might be sensitive to experimental conditions such as host/guest ratios, solvation, and counterion effects. It should be noted that the ionophore was employed at different concentrations with over 1000 times lower, equimolar, and five times higher than metal salts. Such differences in experimental conditions may lead to the involvement of complexes with different compositions, since BEA easily forms higher-order complexes with some metal ions, such as 2:1 and 3:2 (BEA: metal ion) compositions. , As these species take on different conformations, they may possess distinct ion affinities. Therefore, understanding ion-selective transport of BEA at the molecular level has been challenging. As a first step, it is necessary to reveal the conformations and compositions of BEA complexes with certain metal ions involved in ion transport under well-controlled experimental conditions. Gas phase studies have many advantages to address such experimental challenges. First, the composition of the complexes, solvation, and counterion effects can be well-controlled. Second, the gas phase allows for the investigation of conformations and reactions of compounds in lipid bilayer interiors while mimicking the low-dielectric environment. , Fujii and co-workers performed cryogenic infrared spectroscopy of mass-selected BEA complex ions and revealed the conformations of the 1:1 BEA complexes with alkaline-earth and alkali metal ions. , Comparisons between the infrared photodissociation (IRPD) spectra and the density functional theory (DFT) calculations indicated that BEA has higher binding affinities for Na⁺, Mg²⁺, and Ca²⁺ than for the other metal ions. These orders were rationalized by the size-matching model between the BEA cavity formed by six carbonyl oxygens and the metal ions. Furthermore, they investigated conformational changes of the 1:1 complexes with alkali metal ions upon hydration to reveal the ion release mechanism. Do and co-workers performed ion mobility-mass spectrometry (IM-MS) and investigated the conformations of the 1:1 complexes as well as 2:1 complexes with various metal ions. , However, these gas phase studies have not elucidated the selectivity of BEA for Ca²⁺ and Ba²⁺ based on the proposed conformations.

In the present study, we investigated the conformations of the 1:1 BEA complexes (BEA-M) with alkaline-earth (M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) and alkali metal ions (M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) by IM-MS in the gas phase and theoretical calculations. Conformations of the complex ion can be determined by comparing experimental and theoretical collision cross sections (CCSs) measured by IM-MS. We experimentally observed a highly compact conformation that is specifically formed in BEA-Ba²⁺. We examined the conformations and intermolecular interactions between BEA and the metal ions and discussed their impact on ion-selective transport. In particular, we focused on the membrane permeability, predicted from its size and polarity of conformations, as a critical factor in ion-selective transport.

Figure a shows CCS distributions of the BEA-M (M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) complexes measured with He buffer gas at room temperature in the drift tube. The CCS distributions were well fitted with one Gaussian function for the complexes of all alkali metal ions and the light alkaline-earth metal ions, Mg²⁺ and Ca²⁺. On the other hand, two Gaussian functions were necessary for the fittings of Sr²⁺ and Ba²⁺ complexes. Therefore, at least two conformers with different CCSs coexist for these complexes. The largest CCS bands, around 218 Ų, were strongly observed for all metal ions. The shoulder band (206.1 ± 0.4 Ų) was observed for BEA-Sr²⁺. The smallest CCS band (181.3 ± 0.2 Ų) was observed only for BEA-Ba²⁺. Namely, a highly compact conformer coexisted along with a bulky conformer commonly observed for all metal ions.

2.

(a) Collision cross section (CCS, ^DTCCS_He) distributions of the BEA-M (M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) complexes obtained at room temperature. Curves are Gaussian functions for fitting of the experimental data points (circles). Purple, blue, green, orange, and red lines indicate theoretical CCS ranges of the Bn0, Bn0′, Bn1, Bn2, and Bn3 conformers, respectively. (b) The conformer groups of the BEA-Ba²⁺ complexes were obtained from the conformational search by the CONFLEX 8 program with the MMFF94s force field. In Bn0, Bn0′, Bn1, Bn2, and Bn3, where zero, zero (one benzyl group bent to the other side to the cation), one, two, and three benzyl groups coordinate with the cation, respectively. (c) The calculated IR spectra (black line, IR peak half-width at half height with 5 cm^–1) of the most stable structures of Bn0, Bn0′, Bn1, Bn2, and Bn3, and the previous experimental IRPD spectrum (red line) measured by cryogenic infrared spectroscopy of BEA-Ba²⁺. Reprinted with permission from ref , Copyright 2023 American Chemical Society.

To assign the experimental CCSs to conformations, we calculated theoretical CCSs of stable conformers. We classified conformers based on the coordination numbers of the benzyl groups with the cation. Here, we define that the benzyl group coordinates with the cation if the distance between the cation and the center of a benzene ring is shorter than 5 Å. As shown in Figure b, conformers were classified into five groups: Bn0, Bn0′, Bn1, Bn2, and Bn3, where zero, zero (one benzyl group bent to the other side of the cation), one, two, and three benzyl groups coordinate with the cation, respectively. We optimized a total of 25 conformers for each metal cation in the DFT calculations, which were extracted from each conformer group by selecting 5 candidate structures with relatively low steric energy on the CONFLEX.

We assigned the conformations of the complexes by comparing the experimental and theoretical CCSs of each conformer group. The colored lines in Figure a represent the range of theoretical CCSs of each group. The largest CCS bands observed for the complexes of alkali metal ions, Mg²⁺, and Ca²⁺ were assigned to Bn0. These conformations were consistent with the results of previous studies. , The largest CCS bands observed for Sr²⁺ and Ba²⁺ were assigned to Bn0′. The shoulder band (206.1 ± 0.4 Ų) observed in BEA-Sr²⁺ was assigned to Bn1. The smallest CCS band (181.3 ± 0.2 Ų) observed in BEA-Ba²⁺ was assigned to Bn3. Experimental CCSs and relative intensities determined from these CCS distributions, as well as theoretical CCSs of the most stable structures in each conformer group, are summarized in Table S1.

Table shows relative zero-point corrected electronic energies (ΔE) and relative Gibbs free energies at 298 K (ΔG ₂₉₈) of the most stable structures of the BEA-M (M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) complexes in each conformer group. Comparisons between ΔE and ΔG ₂₉₈ demonstrated that thermodynamic effects significantly influenced the stability of the conformers. For the complexes with alkali metal ions, Bn0 was the most stable for both electronic energies and Gibbs free energies (Table S1). These theoretical results further supported the structural assignment of alkali metal ion complexes to Bn0 based on CCS comparisons. For the complexes with alkaline-earth metal ions, Bn3 was the most stable for BEA-Sr²⁺ and BEA-Ba²⁺, while Bn0 was the most stable for BEA-Mg²⁺. For BEA-Ca²⁺, Bn3 was the most stable in electronic energies, while Bn0 was the most stable in Gibbs free energies. These results are due to the fact that Bn3 is enthalpically favorable, while Bn0 is entropically favorable. Between the conformers with no benzyl group coordination, Bn0′ was more stable than Bn0 for BEA-Sr²⁺ and BEA-Ba²⁺. This stability of Bn0′ is caused by ion-dipole interactions with divalent ions since the coordination numbers of the carbonyl oxygens were three and five for Bn0 and Bn0′, respectively.

1. Relative Electronic Energies (ΔE) and Gibbs Free Energies (ΔG ₂₉₈) at 298 K of the Most Stable Structures of Each Conformer of the BEA-M (M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) Complexes Obtained at the B3LYP-D3­(BJ)/def2-SVP Level of Theory.

Complexes	Conformers	ΔE/kJ mol–1	ΔG 298/kJ mol–1
BEA-Mg2+	Bn0	0.0	0.0
	Bn0′	70.9	74.1
	Bn3	68.0	84.3
BEA-Ca2+	Bn0	20.4	0.0
	Bn0′	53.6	36.0
	Bn3	0.0	6.9
BEA-Sr2+	Bn0	76.8	51.6
	Bn0′	74.2	51.0
	Bn3	0.0	0.0
BEA-Ba2+	Bn0	118.3	82.2
	Bn0′	87.4	65.5
	Bn3	0.0	0.0

There are two discrepancies between the experimental and theoretical results. First, the relative ion intensities were inconsistent with the relative Gibbs free energies of the conformers of BEA-Sr²⁺ and BEA-Ba²⁺. This discrepancy may be due to the BEA-M system partially retaining Bn0 (or Bn0′) as stable conformers in solution, since metal ion is exposed to the solvent. Second, Bn3 of BEA-Sr²⁺ was not observed experimentally, despite being the most stable structure according to DFT calculations. This discrepancy may be attributed to the lower thermodynamic stability of Bn3 relative to Bn0 in BEA-Sr²⁺ compared to that in BEA-Ba²⁺. These results suggest that Bn3 can be formed in the gas phase via isomerization from Bn0 (or Bn0′), a stable conformer in solution, by utilizing retained and given energies.

We also compared the calculated IR spectra with the previous experimental IRPD spectrum measured by cryogenic infrared spectroscopy. Figure c shows the calculated IR spectra of the most stable structures of each conformer and the IRPD spectrum of BEA-Ba²⁺. The obtained harmonic vibrational frequencies were scaled by 0.9614 for comparison with the experimental IRPD spectrum. In the previous study by Fujii and co-workers, none of the calculated structures of BEA-Ba²⁺ accurately reproduced the intense bands in the IRPD spectrum, although they were tentatively assigned to Bn0. The calculated IR spectrum of Bn3 is in good agreement with the strongest bands at ∼ 1650 and 1750 cm^–1. Therefore, the selective formation of Bn3 in BEA-Ba²⁺ in the gas phase is supported by both CCS distributions and IRPD spectra.

Notably, Bn3 was observed specifically for BEA-Ba²⁺ among the complexes of alkaline-earth and alkali metal ions in IM-MS experiments. To reveal the factors behind the unique appearance of Bn3 for Ba²⁺ in the gas phase, we examined the conformations of Bn3. The most stable structures of Bn3 of BEA-M (M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) are shown in Figure a. For all conformations, the cation was encapsulated by three amide carbonyl oxygens and three benzyl groups. In other words, Bn3 is stabilized by both ion-dipole interactions and triple cation-π interactions. Considering ion-dipole interactions are more favorable for smaller ions than for larger ions because the formers match better in the BEA cavity, the stability of Bn3 should be dominated by the strength of triple cation-π interactions, which is a unique characteristic of Bn3.

3.

(a) Most stable structures of the Bn3 conformers of BEA-M (M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) complexes calculated at the B3LYP-D3­(BJ)/def2-SVP level. (b) Plot of the distances between the cation and the amide carbonyl oxygen, R(M–O), and (c) between the cation and the center of the benzene ring, R(M–Bn), against the metal ionic radii, r(M). As there are three pairs of R(M–O) and R(M–Bn), we distinguished them using three different markers. (d) Plot of the relative Gibbs free energies of Bn3 compared to Bn0 for alkali metal ions, Mg²⁺, and Ca²⁺, and those of Bn0′ for Sr²⁺ and Ba²⁺ (ΔΔG ₂₉₈) against r(M).

For quantitative investigation, the distances between the cation and the amide carbonyl oxygens, R(M–O), as well as between the cation and the center of the benzene ring of a benzyl group, R(M–Bn), are plotted against the metal ionic radii, r(M), in Figure b,c, respectively. As the cation is coordinated by three amide carbonyl oxygens and three benzyl groups in Bn3, we represented each distance with three different markers. For R(M–O), all markers overlapped for all metal ions, and the distance increased linearly with increasing r(M). These results indicate that three amide carbonyl oxygens coordinate at equal distances, and the larger ions coordinate further above the BEA cavity. On the other hand, R(M–Bn) had the minimum point at Ba²⁺ (∼3.39 Å). R(M–Bn) increased with smaller and larger r(M) compared to Ba²⁺, and three benzyl groups coordinated at equal distance (all markers overlapped) except for Mg²⁺. Figure S1 shows schematically the dependence of R(M–O) and R(M–Bn) on r(M). These results suggest that triple cation-π interactions were maximized at Ba²⁺ due to the minimum R(M–Bn), while for smaller and larger ions, the interactions were gradually weakened.

The thermodynamic stability was strongly correlated with R(M–Bn). Relative Gibbs free energies of Bn3 compared to Bn0 for the complexes of alkali metal ions, Mg²⁺, and Ca²⁺ (ΔΔG ₂₉₈) are plotted against r(M), along with those of Bn0′ for Sr²⁺ and Ba²⁺ complexes in Figure d. The major component of cation-π interactions is electrostatic interactions, where the strength depends on the cation-benzene distance and charge density. − In this figure, ΔΔG ₂₉₈ had a minimum similar to the dependence of R(M-Bn). Additionally, divalent ions were extremely stabilized compared to monovalent ions, reflecting their high charge. These results indicate that Bn3 of BEA-Ba²⁺ is significantly stabilized by triple cation-π interactions due to the minimum R(M-Bn) and the high charge. Therefore, Bn3 was specifically formed for BEA-Ba²⁺ in the gas phase due to the maximum triple cation-π interactions. Although K⁺ was matched to the encapsulation space due to the close ionic radius to Ba²⁺, the Bn3 conformer is unstable than Bn0 as shown in because triple cation-π interactions with K⁺ are weaker than those with Ba²⁺ due to its lower charge. This is consistent with the fact that Bn3 of BEA-K⁺ was not observed in IM-MS experiments.

Finally, the contribution of the Bn3 conformation to ion-selective transport is discussed based on the size (CCS) and polarity (solvent accessible 3D polar surface area, SA 3D PSA) of the conformers. The CCS of Bn3 was very small, which is advantageous for membrane permeability. The SA 3D PSA of the Bn3 conformer of BEA-Ba²⁺ (14.1 Ų) was much smaller than that of Bn0′ (45.6 Ų), as shown in Table S2. This small SA 3D PSA is consistent with the conformation where hydrophobic side chains completely point outward (Figure a). The unique conformation of Bn3 efficiently shields the cation and makes the molecular surface apolar. Consequently, Bn3 can maximize membrane permeability and diffusion rate in the hydrophobic lipid bilayer interior due to the minimum CCS and SA 3D PSA among the possible conformers. We suggest that BEA can dynamically change its conformation from the hydrophilic exterior to the hydrophobic interior of lipid bilayers to achieve ion-selective transport. The origin of the ion-selective transport of BEA for Ba²⁺ may be due to the specific formation of the Bn3 conformation with Ba²⁺.

In conclusion, we investigated the conformations of the beauvericin (BEA) 1:1 complexes with alkaline-earth (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) and alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) by electrospray ionization-ion mobility-mass spectrometry (ESI-IM-MS) and theoretical calculations. In the distributions of collision cross sections (CCSs) between an ion and a He atom at room temperature, two types of conformers were clearly separated in BEA-Ba²⁺. By comparing the experimental and theoretical CCSs, the conformers were assigned to Bn0′ and Bn3, where the coordination numbers of the benzyl groups with the cation were zero and three, respectively. Notably, Bn3 was experimentally observed by IM-MS for the first time and was specifically formed for Ba²⁺, for which BEA has a high transport ability. The density functional theory (DFT) calculations indicated that Bn3 is enthalpically favorable while Bn0′ is entropically favorable. The novel conformation of Bn3 is significantly stabilized by triple cation-π interactions in the gas phase. Selective interactions of Bn3 are achieved by the size-matching between the encapsulation space of Bn3 and the metal ion. Due to the minimum cation-benzene distance and the high charge, triple cation-π interactions are maximized for the Ba²⁺ complex among those with alkaline-earth and alkali metal ions, resulting in the energetic stability of Bn3 exceeding that of Bn0′ in the gas phase. Moreover, the cation is encapsulated by BEA, and the hydrophobic side chains completely point outward. The CCSs and solvent accessible 3D polar surface area (SA 3D PSA) indicated that the size and polarity of Bn3 are the smallest among the possible conformers. Therefore, Bn3 can maximize membrane permeability and diffusion rate in the hydrophobic interior of lipid bilayers. We suggest that ion-selective transport of BEA for Ba²⁺ is achieved by chameleonic behavior via dynamic conformational change from the Bn0′ conformation in the hydrophilic exterior to the Bn3 conformation in the hydrophobic interior. Although BEA also selectively transports Ca²⁺ in biological studies, Bn3 of BEA-Ca²⁺ was not observed in the present IM-MS experiments and was energetically unfavorable in the quantum chemical calculations. Our study also suggests that ion-selective transport of BEA for Ca²⁺ is influenced by additional factors such as the coexistence of higher-order complexes, solvation, and counterion effects. Among the considerable factors, BEA could form a 2:1 complex dominantly with Ca²⁺ in chloroform. The studies of 2:1 BEA complexes with metal ions are now undergoing, and the current results indicate that the most compact conformation is formed in the 2:1 complex with Ca²⁺, which may also be advantageous for ion transport. We believe that these results can promote the understanding of the physicochemical principles underlying the ion-selective transport of large ionophores as well as inspire the rational molecular design of ion recognition, transport, and separation systems in supramolecular chemistry. Details of research results of the 2:1 BEA complexes with metal ions will be reported separately.

Methods

The detailed experimental configuration has been described in earlier studies. − BEA-M ions were formed via electrospray ionization (ESI) from methanol solutions containing 0.1 mM of BEA along with alkaline-earth and alkali metal chlorides (LiCl, NaCl, KCl, RbCl, CsCl, MgCl₂, CaCl₂, SrCl₂, or BaCl₂). The prepared solution was delivered to a metal capillary at a flow rate of 2.0 μL min^–1 by a syringe pump. A potential difference of +2.5 kV was maintained between the metal capillary and a heated desolvation capillary (∼350 K). The ions existing the heated desolvation capillary were introduced into an ion funnel situated near its outlet and were focused into a quadrupole ion trap (QIT). In the QIT, the ions were accumulated for 40 ms, with helium gas flowing at 16 sccm to assist efficient trapping. Subsequently, the pulsed ions were introduced into an ion drift tube using a pulsed electric field at a given time (t = t ₀). Within the drift tube, helium buffer gas was filled at 295.2 ± 1.6 K and 1.100 ± 0.003 Torr. The applied static electric field (E) was 6.1 V cm^–1, resulting in an E/N value of 17 Td (N is the number density of the buffer gas, 1 Td = 10^–17 V cm). After traversing the drift tube, the ions were transferred into the acceleration region of the time-of-flight mass spectrometer (TOF-MS) and accelerated to ∼4 keV by pulsed electric fields at a given time from the pulse for the injection into the drift tube: t = t ₀ + Δt. Finally, the ions were detected by a dual microchannel plate placed at the end of the reflectron TOF-MS.

The delay time between the injection and acceleration pulses, Δt, was defined as “arrival time”, from which the drift velocity of the ions in the drift tube, v _d, was numerically calculated. An ion mobility K, a proportional constant between v _d and E, , depends on N and thus usually reduces to K ₀, which is defined as K ₀ = K·(N/N ₀), where N ₀ denotes Loschmidt’s number. From the Mason-Schamp equation, the reduced mobility K ₀ in the drift tube was given as

$$K_0=\frac{3q}{16N_0}{\left(\frac{2\pi }{k_{\mathrm{B}}\mu T_{\mathrm{eff}}}\right)}^{1/2}\frac{1}{\mathrm{\Omega }}$$ 1

where q is the charge of the ion, k _B is the Boltzmann constant, μ is the reduced mass of the ion and the buffer gas atom, T _eff is the effective temperature of the ions, and Ω is a collision cross section (CCS). , Additionally, T _eff was given as

$$T_{\mathrm{eff}}=T_{\mathrm{BG}}+\frac{m_{\mathrm{BG}}{v_{\mathrm{d}}}^2}{3k_{\mathrm{B}}}$$ 2

where T _BG and m _BG are the temperature and mass of buffer gas, respectively. Consequently, the CCS of the ion is calculated by measuring the arrival time. In the IM-MS experiment, a set of TOF mass spectra were obtained sequentially by scanning the arrival time. As a result, a two-dimensional (2D) plot of m/z vs arrival time was obtained. Subsequently, we obtained a plot of arrival time distribution (ATD) by cutting out the ion intensity of a certain m/z peak. Finally, the CCS distributions were obtained by converting the ATDs. We analyzed the CCS distributions by fitting Gaussian functions with a width based on CCS resolution of the current experimental apparatus.

The conformational search and DFT calculations were carried out to assign the conformations of the BEA-M complex ions. The geometry of BEA was taken from its reported crystal structure. The metal ions were subsequently positioned at the center of mass of BEA. The conformational search was conducted using the CONFLEX 8 program, employing the MMFF94s force field with a search limit of 10 kcal mol^–1. , Because the force field could not be applied to Rb⁺, Cs⁺, Sr²⁺, and Ba²⁺, the initial structures for DFT calculations were generated by replacing the metal ions in the conformers of BEA-K⁺ and BEA-Ca²⁺ for monovalent and divalent ions, respectively. Top five stable conformers in each conformer group were subsequently optimized at the B3LYP-D3­(BJ)/def2-SVP level of theory using the Gaussian 16 program. The vibrational analysis, zero-point energy correction, and natural population analysis were performed at the same level of theory. The theoretical CCSs of optimized candidate structures were calculated by the trajectory method using the IMoS program. We used the Lennard-Jones parameters embedded in the IMoS program. The polarity of the conformer was evaluated by solvent accessible 3D polar surface areas (SA 3D PSA). SA 3D PSA was calculated in Pymol 3.1, referring to a previous literature.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.5c01573.

• Experimental CCSs, relative intensities, theoretical CCSs, relative electronic energies, relative Gibbs free energies of 1:1 BEA complexes with alkaline-earth and alkali metal ions, interpretation for the minimum point of the distance between the cation and the center of the benzene ring of a benzyl group at Ba²⁺, geometrical coordinates of conformers in Figures b and Figure a, and solvent accessible 3D polar surface area of the most stable structures of conformers of the 1:1 BEA complexes with alkaline-earth metal ions (PDF)

• Transparent Peer Review report available (PDF)

The authors declare no competing financial interest.