Simultaneous Binding of Pt(II) to Three Different Sites (N7, N1, N3) of a Guanine Nucleobase

Article abstract of DOI:10.1021/ja00298a031

Reaction of <(NH3)3Pt(H2O)>²⁺ with 9-ethylguanine, GH, gives three products: the mononuclear complex <(NH3)3Pt(GH)>²⁺ (1), which contains G platinated at N7, the dinuclear complex <(NH3)3Pt(G)Pt(NH3)3>³⁺ (2), which contains the deprotonated 9-ethylguanine ligand platinated at N7 and N1, and the trinuclear complex <<(NH3)3Pt>3(G)>⁵⁺ (3), which has a deprotonated guanine ligand platinated at N7, N1, and N3.Formation of 3 occurs on addition of <(NH3)3Pt(H2O)>²⁺ to an aqueous solution of 1 via the intermediate 2 even if no base is added.Its information is rationalized in terms of an electron-releasing effect of the Pt atom substituting for the proton at N1, similar to the situation in complexes of 1-methyluracil and 1-methylthymine containing a Pt(II) coordinated to N3.The pH dependence of the 1H NMR chemical shifts of all three complexes has been studied and the relevant acid/base equilibria established. 1 and 3 have been isolated in crystalline form, and the crystal structure of a derivative of 3, containing half a molecule of (NO3)2 per trinuclear complex, is reported. <<(NH3)3Pt>3(G)>(NO3)5*0.5<(NH3)4Pt>(NO3)2*H2O (3a) crystallizes in the space group P1, with cell dimensions a = 16.393 (4) Angstroem, b = 13.889 (6) Angstroem, c = 8.437 (2) Angstroem, α = 106.34 (4) deg, β = 94.32 (3) deg, γ = 92.05 (3) deg.The structure was refined to R = 0.067 and R_w = 0.076 on the basis of 5811 reflections.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.