Binding of platinum-diaminonitroxyl complexes to animal DNA

Abstract

Reaction of Pt II(DAPO)X 2 complexes (where DAPO is trans-3,4-diamino-2,2,6,6-tetramethylpiperidine-1-oxyl, X 2=(NO 3) 2, oxalato (Ox) or 1,1-cyclobutanedicarboxylato (Cbdca)) with a bovine spleen DNA in 0.01 M NaHCO 3 at 37°C for 24 h gives rise to formation of platinated DNA. The [bound Pt II(DAPO)]/[nucleotide] ratio ( r) depends on the initial ratio of the reagents and on the nature of leaving ligands X. Nitroxyl–nitroxyl distances in platinated DNA determined by the ESR suggest that at r⩾0.1 Pt II(DAPO) fragments are uniformly attached to DNA. But at lower r, the thermal characteristics of modified DNA (melting temperature T m, melting range width Δ T) and the guanine-to-adenine platination degree ratios G Pt/A Pt imply that the nature of leaving ligands X affect the selectivity of DNA platination. At r⩾0.1, nitroxyl groups can approach each other so close that, in an acidic medium, the electron transfer from one nitroxyl group to another becomes possible, and the nitroxyls readily disproportionate to diamagnetic products. Correlation time of nitroxyl rotation in Pt II(DAPO)–DNA adducts is ∼10 −8 s, which is related to predominantly bifunctional bonding of Pt II(DAPO) with DNA. Platination-induced distortion of DNA was evidenced by changes in T m, Δ T and degree of hyperchromicity H. The major part of adducts form the intrastrand cross-links which destabilize the structure of DNA duplex. The interstrand Pt II(DAPO) cross-linking (∼1% of the adducts) facilitates renaturation of despiralized DNA molecules upon cooling. Two types of Pt II(DAPO)–DNA adducts are revealed, which differ substantially in their rates of deplatination with NaCN. ESR, electron spin resonance; r, degree of modification; cisplatin, cis-diamminedichloroplatinum(II); T m, melting temperature; Δ T, melting range width; H, degree of hyperchromicity; R, degree of renaturation; AAS, atomic absorption spectroscopy; HPLC, high performance liquid chromatography.