A Pyrazolato‐Bridged Dinuclear Platinum(II) Complex Induces Only Minor Distortions upon DNA‐Binding

The cytotoxic, pyrazolato-bridged dinuclear platinum(II) complex [(cis-{Pt(NH 3 ) 2 }) 2 (μ-OH)(μ-pz)] 2+ (pz = pyrazolate) has been found to cross-link two adjacent guanines of a double-stranded DNA decamer without destabilizing the duplex and without changing the directionality of the helix axis. A 1 H NMR study of the oligonucleotide d(CTCTG*G*TCTC)-d(GAGACCAGAG), cross-linked at the two G* guanines by [(cis-{Pt-(NH 3 ) 2 }) 2 (μ-pz)] 3+ , and molecular dynamics simulations of the explicitly solvated duplex were performed to characterize the structural details of the adduct. The dinuclear platinum cross-link unwinds the helix by approximately 15°, that is, to a similar extent as the widely used antitumor drug cisplatin, but, in contrast to the latter, induces no significant bend in the helix axis. The Watson-Crick base-pairing remains intact, and the melting temperature of the duplex is unaffected by the cross-link. The helical twist is considerably reduced between the two platinated bases, as becomes manifest in an unusually short sequential H1'-H1' distance. This unwinding also affects the sugar ring of the guanosine in the 3'-position to the cross-link, which presents an N⇄S equilibrium. This is the first cytotoxic platinum complex that has been successfully designed by envisioning the structural consequences of its binding to DNA.