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(x)(2)-y(2)) of the d(9)-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.