The Influence of the Axial Ligands of a Series of Platinum(IV) Anti-Cancer Complexes on Their Reduction to Platinum(II) and Reaction With DNA

Abstract

The electrochemical reduction and DNA binding have been studied for a series of platinum(IV) complexes with Cl^-, OH^-, and carboxylate anions as the axial ligands ; [Pt(en)Cl₄], [Pt(en)Cl₂(OH)₂], and [Pt(en)Cl₂(OC(O)R)₂], R = CH₃, CH₂CH₃, CH₂CH₂CH₃. Cathodic reduction potentials vary by more than 650 mV with the tetrachloro complex reduced most readily and the dihydroxo least readily. The binding of the complexes correlates with the reduction potentials with the more readily reduced complexes binding more readily to DNA. The influence of the reducing agent glutathione on platinum binding to DNA was found to depend on whether it was added before or after Pt/DNA incubation. The results are consistent with octahedral platinum(IV) binding monofunctionally to DNA, and molecular modelling studies have been used to confirm that this is sterically feasible. The crystal structure of [Pt(en)Cl₂(OC(O)CH₃)₂] has been determined by X-ray diffraction methods and refined to R = 0.028 (977 F). The crystals are monoclinic, space group C2/c, a 15.569(6), b 8.104(1), c 13.188(1) Ǻ, β 136.38(2)°.