Twelve mono-and dimetallic complexes (the metals being Cu-II, Pt-II, and Zn-II) with terpyridine-based ligands have been prepared and fully characterized. The X-ray crystal structures of two of the complexes (monometallic Cu-II and Zn-II complexes with a morpholino-substituted terpyridine ligand) are reported. The affinities of the 12 complexes toward duplex and quadruplex (HTelo and c-myc) DNA have been investigated using a combination of techniques including fluorescent indicator displacement (FID) assay, UV-vis spectroscopy and circular dichroism (CD). These studies revealed that the dicopper and diplatinum complexes 11 and 12 bind very strongly to quadruplex DNA (up to K 7.97 x 10(6) M-1) and with good selectivity (up to 100-fold) over duplex DNA. In these dimetallic complexes, one of the metals is coordinated to a terpyridine moiety yielding square based pyramidal (with Cu-II) or square planar (with Pt-II) geometries. The second metal is coordinated to a dipicolyl amine linked to terpyridine by a three-atom spacer. We propose that these complexes bind to quadruplex DNA via a combination of interactions: pi-pi end-stacking between the metal-terpyridine fragment and the guanine quartet, and electrostatic/metal-phosphate interactions (between the metal-dipicolyl amine fragment and DNA's backbone).