Di-copper(ii) DNA G-quadruplexes as EPR distance rulers.

Artificial metal-base tetrads composed of square-planar CuII(pyridine)4 complexes were covalently attached to both the 3' and 5' ends of tetramolecular DNA G-quadruplexes [Ld(G3-5)LdT]4 (L = pyridine ligand) of different lengths. Owing to the planar four-point attachment of the metal complexes, the magnetic orbitals (dx2-y2) of the d9-configured CuII cations are placed in a coplanar fashion, separated by the stacked guanine tetrads. Pulsed EPR-derived CuII-CuII distances and angles were found to be in agreement with those obtained from molecular dynamics simulations. DNA-confined transition metal spin labels open new ways to study oligonucleotide structure and DNA-protein complexes.