Effects of amine ligand bulk and hydrogen bonding on the rate of reaction of platinum(II) diamine complexes with key nucleotide and amino acid residues.

NMR spectroscopy has been used to observe the effects of the amine ligand on the rate of reaction of platinum diamine and triamine complexes with DNA and protein residues. Whereas [Pt(dien)Cl]Cl and Pt(dien)(D(2)O) have been known to react faster with thioether residues such as N-AcMet than with 5'-GMP, we found that Pt(Me(4)en)(D(2)O)(2) appeared to react faster with 5'-GMP. To quantitatively assess the factors influencing the rates of reaction, rate constants at pH 4 were determined for the reactions of Pt(en)(D(2)O)(2) [en = ethylenediamine] and Pt(Me(4)en)(D(2)O)(2) with N-AcMet, N-AcHis, 5'-GMP, and Guo (guanosine). In each case the less bulky complex (Pt(en)(D(2)O)(2)) reacts more quickly than does the bulkier Pt(Me(4)en)(D(2)O)(2), as expected. Both complexes reacted faster with 5'-GMP; however, analysis of the rate constants suggests that the Pt(en)(D(2)O)(2) complex favors reaction with 5'-GMP due to hydrogen bonding with the 5'-phosphate, whereas Pt(Me(4)en)(D(2)O)(2) disfavors reaction with N-AcMet due to steric clashes. Bulk had relatively little effect on the rate constant with N-AcHis, suggesting that peptides or proteins that coordinate via His residues would not have their reactivity affected by bulky diamine ligands.