Solvent-induced high fidelity switching between two discrete supramolecules

Abstract

Here we show the reversible high fidelity switching between two discrete self-assembled supramolecules made from a lipophilic 8-phenyl-2'-deoxyguanosine derivative induced by an indirect solvent effect. A hexadecameric supramolecule containing four stacked tetramers is formed in acetonitrile aided by higher potassium concentrations. When the amount of weakly solvated potassium decreases, due the lower activity of potassium iodide in chloroform, an octamer is formed after the dissociation of the two outer tetramers in the hexadecamer. The switching process results from an unprecedented subtle interplay between the activity of potassium iodide and the steric crowding within the self-assembled structure. Besides the possible applications in nanoconstruction, this phenomenon sheds light into the mechanism of formation of self-assembled supramolecules made from guanosine derivatives.

Figure 1

(a, Top view) Chemical structure of the mECGi and mAGi tetramers and (b, side view) cartoon depiction of the same tetramers. The blue and red triangles represent the differences in steric bulk in mECGi relative to mAGi respectively.

Figure 2

¹H NMR (500 MHz, 298.2 K) spectra of five samples of mECGi (30 mM) in (a) CD₃CN with 0.125 eq. of KI, and with 0.5 eq. of KI using variable CDCl₃:CD₃CN solvent ratios (b) 0:1, (c) 1:1, (d) 4:1, (e) 1:0. The supramolecule exists primarily as (mECGi)₁₆•3K⁺(H, b–d) or (mECGi)₈•K⁺ (O, a, e). For a full view of all the spectra see Figures S5, S7.

Figure 3

(a) Dose-response curve for the titration of mECGi (blue) and mAGi (red), 15 mM in CD₃CN. (b) Melting profiles for (mAGi)₁₆ & (mECGi)₁₆ (5 mM, 0.5 eq. KI) in CD₃CN as determined by ¹H NMR (500 MHz). The T_m is the inflexion point in the melting profiles as determined by the minimum in the respective first derivative plots.

Figure 4

Partial ¹H NMR spectra (500 MHz, 298.2 K) of mECGi and mAGi in (a) CD₃CN and (b) CDCl₃. (c) The same as (a) but with 1 eq. of [2.2.2]-cryptand. The peaks in the region of 11–13 ppm correspond to the N¹H and the broad peak at around 10.8 ppm corresponds to the N²H on the Watson-Crick edge of the guanine moiety. The hexadecamers show a signature of doubled set of resonances due to the two sets of tetrads that are in different chemical environments. (a–c, right) Diagrams illustrating the relative chemical potentials (μ) for the hexadecamers (μ_H), putative partially-filled hexadecamer (μ_H*) and octamers (μ_O). The dotted arrows indicate that the decay of H is spontaneous under the conditions shown on the left.

Scheme 1

Representation of the solvent-induced switching between a hexadecamer and an octamer. The potassium cations are represented by green spheres in which the more tightly bound potassium cations are darker (inner binding site) and the more weakly bound cations are lighter (outer binding site). H* represents a metastable hexadecamer with empty cation binding sites between the outer and inner tetrads.