The Zn-2+ complex of 5-(1,4,7,10-tetraazacyclododecan-1-ylsulfonyl)-N,N-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 mu M). 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 Zn-2+, Fe-2+ and Co-2+ do not complex to the DSC macrocycle appreciably under the conditions of the experiment. The Cu-2+ 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 Zn-2+ ion, suggests that this system may be viable as a Zn-2+ 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 Zn-2+ 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