Metallo-responsive switching between hexadecameric and octameric supramolecular G-quadruplexes

Abstract

We report the metallo-responsive high fidelity switching between hexadecameric and octameric supramolecular G-quadruplexes triggered by a change in the metal cation promoter from potassium to strontium, respectively.

Further advances in the development of molecular machinery require the construction of components that alternate their configuration between, at least, two distinct states. Most current examples of artificial supramolecular machinery rely on interconversions between specific configurations of mechanically linked systems (e.g., catennanes, rotaxanes) or within contorted structures (e.g., cis-trans isomers, foldamers).^1-3 A complementary, and comparatively underdeveloped, strategy will take advantage of changes in the oligomeric state (molecularity) of self-assembled supramolecules. In biological systems, fluctuations in the oligomeric state of assemblies provide an efficient way to alter their structure and, thus, their function. For example, changes in the oligomeric state for the protective antigen of the anthrax toxin (heptamer-octamer) provide a means of controlling cytotoxicity.⁴ Analogous changes in RuvA enzymes (tetramer-octamer) enable control of the dynamics of processing DNA in Holliday junctions.⁵

Most responsive multimeric assemblies rely on an all or none strategy where the two available states are assembled or fully disassembled.⁶ An additional challenge is the development of systems where two well-defined oligomeric states (in particular those with molecularities larger than trimers or tetramers) interconvert with high fidelity.^{7, 8} The interconversion can be achieved by tuning intrinsic parameters via covalent modification of the monomeric building blocks with responsive functional groups (e.g., photo-, redox- or pH-active) which is a commonly used approach. Alternatively, such tuning can be achieved through the modulation of extrinsic parameters by altering the surroundings of the supramolecule such as the solvent,^9-11 temperature,¹² or metal cations.^13,14 Herein, we present a metallo-responsive supramolecular G-quadruplex (GQ) that switches between a hexadecameric and octameric states in processes triggered by changes in the metal cation from potassium to strontium, respectively (Fig. 1).¹⁵

Fig. 1.

Schematic representation of the cycle of the metallo-responsive supramolecules formed by 1, a tetrad of which is represented in the center. Side view depictions of the octamer 1₈•Sr2⁺ (top, blue) and the hexadecamer 1₁₆•3K⁺ (bottom, red). Strontium and potassium cations are represented by green and orange spheres, respectively.

Supramolecular GQs are formed by the self-assembly of guanine, or related derivatives, that form stacks of planar hydrogen-bonded tetramers in the presence of cations of appropriate size (e.g., Na⁺, K⁺, NH₄⁺).^16,17 In recent years we have studied the self-assembly of 8-aryl-2′-deoxyguanosine derivatives (8ArGs) and surveyed the effects of modulating intrinsic and extrinsic parameters, on the structure and properties of their corresponding assemblies.^18,19 Properties such as their selectivity for binding²⁰ and transporting metal cations²¹ can also be modulated by such parameters. Specifically, the 8-(m-acetylphenyl)-2’-deoxyguanosine (mAG) scaffold induces the reliable formation of hexadecamers, in both organic¹⁹ and aqueous media,²² even when containing large, and sterically demanding groups such as dendrons.^23,24 Furthermore, modulation of these parameters has enabled the development of solvo-⁹ and thermo-¹² responsive systems, paving the way to more elaborate supramolecular machinery.

We have shown that titrating a solution of 1 in CD₃CN with KI, initially leads to the formation of the D₄-symmetric octamer 1₈•K⁺ (from 0.0313 to 0.125 equiv of KI), leading to the formation of the hexadecamer 1₁₆•3K⁺ after the addition of 0.5 equiv of KI.⁹ Similarly, the addition of SrI₂ (0.125 equiv) to a solution of 1 (30 mM) in CD₃CN reveals a single set of signals corresponding to the octamer 1₈•Sr²⁺. In contrast, higher concentration of Sr²⁺ decreases the fidelity and appears to destabilize the octamer (as previously reported for other GQs²⁵), but does not induce the formation of a hexadecamer. DOSY experiments in CD₃CN are consistent with the formation of 1₈•Sr²⁺, giving a D = (7.0 ± 0.2) × 10^-10 m² s^-1 at 298 K, which corresponds to a hydrodynamic radius (R_H) of 9.2 Å. The constitution of 1₈•Sr²⁺, is further supported by vapour pressure osmometry (VPO) and HR-ESI MS (CD₃CN with 0.125 equiv of SrI₂). The former provides a molecular weight in solution of 4,566 Da (ESI^†, Table S2), whereas the latter reveals a clean spectrum with a base peak at m/z = 2145.7058 amu corresponding to 1₈•Sr²⁺ (ESI^†, Fig. S3).

Surprisingly, in CD₃CN with 0.5 equiv of KI, increasing proportions of SrI₂ shifts the equilibrium from the hexadecamer 1₁₆•3K⁺ towards 1₈•Sr²⁺ (Fig. 2). The octamer 1₈ is detectable after the addition of just 0.03 equiv of SrI₂ (Fig. 2) and it becomes the main species after the addition of 0.125 equiv of such salt. Although the cations of potassium and strontium have similar ionic radii (1.51 and 1.26 Å, respectively),²⁶ in acetonitrile, their distinct charge densities seem to dictate the resulting molecularity of the assemblies of 1. Whereas, a hexadecamer can accommodate three consecutive potassium cations in its internal channel, the corresponding arrangement with divalent strontium cations is untenable due to the increased electrostatic repulsion.^27,28

Fig. 2.

¹H NMR (500 MHz, 298.2 K) spectra for 1 (30 mM) in (a) CD₃CN with 0.5 equiv of KI, 97 % of hexadecamer (H) 1₁₆•3K⁺; (b) 0.0313 equiv of SrI₂, 79% H and 19% octamer (O) 1₈•Sr^2+; (c) 0.0625 equiv of SrI₂ 60% H and 39 % O; (d) 0.125 equiv of SrI₂ 100% O.

The addition of KI to 1₈•Sr²⁺ does not lead to the formation of 1₁₆•3K⁺ (ESI^†, Fig. S7), and the persistence of the former, even after adding an excess of KI, is consistent with its higher thermal stability (Tm ≥ 333.2 K). This stabilization effect of strontium cations have been attributed to a stronger coordination between C6-carbonyl groups.^25,29 In order to achieve a reversible metallo-responsive behaviour, and revert the cycle from an octamer to a hexadecamer, we decided to use sulphate as a counter anion for potassium capable of sequestering the strontium cations from 1₈•Sr²⁺. The addition of 0.125 equiv of K₂SO₄ to the octamer 1₈•Sr1²⁺ induces it to switch back to the hexadecamer 1₆•3K⁺ (ESI^†, Fig. S8). A precipitate of SrSO₄ is observed due to the strong ionic pair formation. Further addition of SrI₂ reverts once again the cycle inducing the switch of the hexadecamer to the octamer (ESI^†, Fig. S9). Three full cycles were performed, as illustrated in Fig. 3 with no obvious signs of fatigue in the system.

Fig. 3.

Changes in the molecularity of the assemblies of 1 induced by alternating the metal cation between potassium (red hexagons) and strontium (blue circles).

Direct addition of K₂SO₄ to a solution of 1 in acetonitrile leads to the formation of 1₈•K^+; (ESI^†, Fig. S10), but even an excess of this salt does not promote the formation of 1₁₆•3K⁺. Potassium sulfate enables the interconversion of 1₈•SrI₂ to 1₁₆•3KI by increasing the chemical potential and eventual net outward flow of strontium cations from within the octamer to the bulk solution (where it precipitates as SrSO₄). This process, however, needs the presence of a soft counter anion like iodide to increase the activity of the cation in solution leading to the high fidelity self-assembly of GQs.³⁰ Precipitation of SrSO₄ enables completion of the cycle by leaving potassium as the only cation available to promote the formation of the hexadecamer 1₁₆•3K⁺.

Conclusions

Here we have shown for the first time how alternating the metal cation promoter enables the control, and reversible switching, between two well-defined supramolecular GQs. This metallo-responsive system represents our latest addition to a toolbox of active supramolecular GQs that already contains solvo-⁴ and thermo-⁷ responsive systems. We expect these to be valuable in the development of versatile supramolecular machinery.