Binding of platinum-diaminonitroxyl complexes to animal DNA

Reaction of pt(II)(DAPO)X-2 complexes (where DAPO is trans-3,4-diamino-2,2,6,6-tetramethylpiperidine-1-oxyl, X-2 = (NO3)(2), oxalato (Ox) or 1,1-cyclobutanedicarboxylato (Cbdca)) with a bovine spleen DNA in 0.01 M NaHCO3 at 37 degrees 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 greater than or equal to 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 Delta 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 greater than or equal to 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 similar to 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, Delta 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 (similar to 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; Delta T, melting range width; H, degree of hyperchromicity; R, degree of renaturation; AAS, atomic absorption spectroscopy; HPLC, high performance liquid chromatography. (C) 1998 Elsevier Science Inc. All rights reserved.