Chiral metallohelices enantioselectively target hybrid human telomeric G-quadruplex DNA

Abstract

The design and synthesis of metal complexes that can specifically target DNA secondary structure has attracted considerable attention. Chiral metallosupramolecular complexes (e.g. helicates) in particular display unique DNA-binding behavior, however until recently few examples which are both water-compatible and enantiomerically pure have been reported. Herein we report that one metallohelix enantiomer Δ1a, available from a diastereoselective synthesis with no need for resolution, can enantioselectively stabilize human telomeric hybrid G-quadruplex and strongly inhibit telomerase activity with IC50 of 600 nM. In contrast, no such a preference is observed for the mirror image complex Λ1a. More intriguingly, neither of the two enantiomers binds specifically to human telomeric antiparallel G-quadruplex. To the best of our knowledge, this is the first example of one pair of enantiomers with contrasting selectivity for human telomeric hybrid G-quadruplex. Further studies show that Δ1a can discriminate human telomeric G-quadruplex from other telomeric G-quadruplexes.

Figure 1.

(A) Structure of Δ1a and Λ1a cations. UV melting profiles of the Tel22 in the absence and presence of Δ1a or Λ1a in Na⁺ buffer (B) and K⁺ buffer (C). Melting assays were measured in 10 mM Tris buffer containing 10 mM KCl or 100 mM NaCl, pH = 7.2. The concentration of Δ1a/Λ1a was equivalent with the concentration of Tel22 (1 μM in strand).

Figure 2.

CD titration of Tel22 with Δ1a (A) and Λ1a (B) in 10 mM KCl, 10 mM Tris buffer, pH = 7.2. Enantiomer was varied from 0 to 2 μM. CD titration of Tel22 with Δ1a (C) and Λ1a (D) in 10 mM Tris buffer, pH = 7.2. The concentration of the enantiomer was varied fom 0.5 to 4μM.

Figure 3.

Absorption spectra of Δ1a (A) and Λ1a (B) in the presence of Tel 22 DNA. Enantiomers was 5 μM, and Tel22 was varied from 0.5 to 10 μM. Arrow showed the absorption change of the enantiomer along with the addition of Tel22. Inset: plot of (ε_a-ε_f)/(ε_b-ε_f) versus concentration of Tel22. The assays were measured in 10 mM Tris buffer containing 10 mM KCl, pH = 7.2. (C) Schematic illustration of the individual 2-Ap position in Tel22 and the two types of Tel22 G-quadruplex conformations. (D) Plot of normalized fluorescence intensity at 370 nm of 2-Ap individually labeled Tel22 versus molar ratio of [Δ1a]/[DNA] in 10 mM KCl, 10 mM Tris buffer, pH = 7.2. 2-Ap labeled Tel22 was 0.5 μM in strand. (E) Job plot for complexation of Δ1a and Ap19-Tel22 in 10 mM KCl, 10 mM Tris buffer, pH = 7.2. [Δ1a]+[Ap19-Tel22] = 0.3 μM.

Figure 4.

(A) ¹H NMR spectra of Tel22 in the absence and presence of Δ1a/Λ1a. The assays were measured in 10 mM Tris–KCl buffer containing 10% D₂O. (B) Telomerase inhibition by Δ1a/Λ1a by using TRAP assay. Line 1: no enantiomer; Line 2–5: PCR products in the presence of 0.2, 0.5, 1, 2 μM Δ1a, respectively; Line 6–9: PCR products in the presence of 0.2, 0.5, 1, 2 μM Λ1a, respectively. (C) Schematic illustration of the chiral enantiomer selectively interacts with human telomeric hybrid G-quadruplex DNA.