Selective Binding of Zn²⁺ Complexes to Human Telomeric G‑Quadruplex DNA

Abstract

The Zn²⁺ complex of 5-(1,4,7,10-tetraazacyclododecan-1-ylsulfonyl)-<i>N</i>,<i>N</i>-dimethylnaphthalen-1-amine, Zn­(DSC), binds selectively to the biologically relevant human telomeric (H-Telo) G-quadruplex. An increase in the Zn­(DSC) dansyl group fluorescence with a simultaneous shift in emission is consistent with the complex binding to H-Telo. The H-Telo G-quadruplex has two binding sites for Zn­(DSC) with binding constants in the low micromolar range (2.5 μM). Isothermal calorimetric titrations confirm low micromolar dissociation constants with a 2:1 stoichiometry. The interaction between H-Telo and Zn­(DSC) is highly pH-dependent, consistent with binding to the unpaired thymines in the G-quadruplex loops. As a result, Zn­(DSC) selectively binds to H-Telo over duplex DNA. In contrast to Zn²⁺, Fe²⁺ and Co²⁺ do not complex to the DSC macrocycle appreciably under the conditions of the experiment. The Cu²⁺ complex of DSC does not interact measurably with the H-Telo G-quadruplex. Interestingly, the H-Telo-Zn­(DSC) adduct self-assembles from its individual components at physiological pH and 100 mM KCl. The self-assembly feature, which is specific for the Zn²⁺ ion, suggests that this system may be viable as a Zn²⁺ sensor. Pentanucleotides were studied in order to better describe the binding of Zn­(DSC) to thymine sequences. NMR studies were consistent with the binding of Zn­(DSC) to thymine-containing oligonucleotides including CCTCC, CTTCC, and CTCTC. Studies showed that the dansyl group of Zn­(DSC) interacts with thymines in CTTCC. Fluorescence spectroscopy and ITC data indicate that Zn­(DSC) forms 2:1 adducts with thymines that are spaced (CTCTC) but not tandem thymines (CTTCC). These data are consistent with one Zn­(DSC) complex binding to two separate loops in the G-quadruplex. A second Zn²⁺ complex containing an acridine pendent, Zn­(ACR), binds tightly to pentanucleotides with both tandem and spaced thymines. Zn­(ACR) indiscriminately binds to both H-Telo and duplex DNA