Simultaneous binding of Pt(II) to three different sites (N7, N1, N3) of a guanine nucleobase

Reaction of ((NH sub(3)) sub(3)Pt(H sub(2)O)) super(2+) with 9-ethylguanine, GH, gives three products: the mononuclear complex (NH sub(3)) sub(3)Pt(GH)) super(2+) (1), which contains G platinated at N7, the dinuclear complex (NH sub(3)) sub(3)Pt(G)Pt(NH sub(3)) sub(3)) super(3+), which contains the deprotonated 9-ethylguanine ligand platinated at N7 and N1, and the trinuclear complex (((NH sub(3)) sub(3)Pt) sub(3)(G)) super(5+) (3), which has a deprotonated guanine ligand platinated at N7, N1, and N3. Formation of 3 occurs on addition of ((NH sub(3)) sub(3)Pt(H sub(2)O)) super(2+) to an aqueous solution of 1 via the intermediate 2 even if no base is added. Its formation is rationalized in terms of an electron-releasing effect of the Pt atom substiuting for the proton at N1. The pH dependence of the super(1)H NMR chemical shifts of all three complexes has been studied and the relevant acid/base equilibria established. The geometries of the triammineplatinum(II) moieties and of the tetrammineplatinum(II) cation are normal. As a consequence of threefold Pt coordination, there are significant changes in ring angles of the purine ligand which are associated with the atoms N9, C4, and C5.