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

Artificial metal-base tetrads composed of square-planar Cu(pyridine) complexes were covalently attached to both the 3' and 5' ends of tetramolecular DNA G-quadruplexes [Ld(G)LdT] (L = pyridine ligand) of different lengths. Owing to the planar four-point attachment of the metal complexes, the magnetic orbitals (d) of the d-configured Cu cations are placed in a coplanar fashion, separated by the stacked guanine tetrads. Pulsed EPR-derived Cu-Cu 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.