Light-Induced Modulation of Chiral Functions in G-Quadruplex-Photochrome Systems

Abstract

The design of artificially engineered chiral structures has received much attention, but the implementation of dynamic functions to modulate the chiroptical response of the systems is less explored. Here, we present a light-responsive G-quadruplex (G4)-based assembly in which chirality enrichment is induced, tuned, and fueled by molecular switches. In particular, the mirror-image dependence on photoactivated azo molecules, undergoing trans-to-cis isomerization, shows chiral recognition effects on the inherent flexibility and conformational diversity of DNA G4s having distinct handedness (right- and left-handed). Through a detailed experimental and computational analysis, we bring compelling evidence on the binding mode of the photochromes on G4s, and we rationalize the origin of the chirality effect that is associated with the complexation event.

Figure 1

(A, B) Synthesized and investigated switches in trans form along with their optimized structures and corresponding electrostatic potential maps. (C) Quantification of the PSSs of Azo4F-LL-his (c_Azo4F-LL-his = 3 mM, in water at 25 °C) by ¹H NMR spectroscopy. The contents of trans and cis forms in solution were calculated from the intensity ratios of the integrals of the corresponding peaks.

Figure 2

Absorption spectra of (A) Azo-DD-his and (B) Azo4F-DD-his recorded in water solutions under excitation with different wavelengths of light (c_Azo-DD-his and c_Azo4F-DD-his = 30 μM). Inset in (B) shows the n → π* band of Azo4F-DD-his in the spectral region between 400 and 550 nm. Panels (C) and (D) show the computed energetic diagrams in the Franck–Condon region of the two lowest-lying excited states having π → π* and n → π* character. The corresponding n and π* orbitals for the latter are shown in yellow-red and purple-blue, respectively. The results of calculations were obtained at the ωB97xD/def2-TZVP level (see SI pp S33 for details).

Figure 3

(A, B) Induced thermal stabilization by Azo-DD/LL-his and Azo4F-DD/LL-his (trans and cis-rich PSS) on right-handed Bom17 or left-handed Z-G4. Results are presented as an average of three independent experiments. The trans–cis underlines the thermal stabilization difference between the trans isomer and the cis-rich PSS. The concentration of KCl in Tris-HCl buffer (10 mM) was 5 mM for Z-G4 and 15 mM for Bom17. (C) ¹H NMR spectra of the G-tetrad imino protons in the absence (0.0 equiv) and presence (0.25, 0.5, 0.75, and 1.0 equiv) of Azo4F-DD-histrans. (D) Representative binding site of Azo4F-DD-histrans during docking with the 3′-end of c-MYC Pu22 (cf. SI pp S37 for details).

Figure 4

Experimental ECD spectra resulting from the complexation of Azo-DD-histrans with (A) c-MYC Pu22 and (B) dsDNA. c_{Azo-DD-histrans} = 50 μM and c_{c-MYCPu22/dsDNA} = from 0 to 20 μM at 20 °C. The arrows aim to show the evolution of the binding profile and the appearance of isodichroic points. (C) The ground-state structure of the complex between the 3′-end of c-MYC Pu22 and Azo-DD-histrans obtained from metadynamics simulations as well as (D) its comparison with the ground-state structure of the free molecule. (E) Calculated circular dichroism spectra of Azo-DD-histrans in its free and bound state with c-MYC Pu22 using the ωB97xD/def2-TZVP/PCM method. (F) ECD photoswitching monitored at 450 nm of the c-MYC Pu22-Azo-DD-his complex (at 20 °C) by alternating cycles of UV (365 nm) and visible light (>485 nm).