Imidazole was found to be coordinated, as an exogenous ligand, to Fe3+ in a coordination complex formed between heme, the iron (III)-protoporphyrin IX complex, and a parallel G-quadruplexed DNA assembled from a single repeat sequence of the human telomere, d(TTAGGG), the stoichiometric ratio being 1:1 and the binding constant 19 M-1.