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

Artificial metal-base tetrads composed of square-planar Cu^II(pyridine)₄ complexes were covalently attached to both the 3′ and 5′ ends of tetramolecular DNA G-quadruplexes [Ld(G_3–5)LdT]₄ (L = pyridine ligand) of different lengths. Owing to the planar four-point attachment of the metal complexes, the magnetic orbitals (d_x²−y²) of the d⁹-configured Cu^II cations are placed in a coplanar fashion, separated by the stacked guanine tetrads. Pulsed EPR-derived Cu^II–Cu^II 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.