We have investigated the interaction of the intramolecular human telomeric DNA G-quadruplex with a hemicyanine-peptide ligand, by studying the rate of quadruplex opening with a complementary DNA oligonucleotide. By employing a minimal kinetic model, the relationship between the observed rate of quadruplex opening and the ligand concentration has enabled estimation of the dissociation constant. A van't Hoff analysis revealed the enthalpy and entropy changes of binding to be -77 +/- 22 kJ mol(-1) and -163 +/- 75 J mol(-1) K(-1), respectively. Arrhenius analyses of the rate constants of opening free and bound quadruplex gave activation energies of 118 +/- 2 and 98 +/- 10 kJ mol(-1), respectively. These results indicate that the presence of the ligand has only a small effect on the activation energy, suggesting that the unbinding of the ligand occurs after the transition state for quadruplex unfolding.