Kinetics of the displacement of aqua ligands from cis-diaqua(ethylenediamine)-platinum(II)perchlorate by adenosine 5′-monophosphate in aqueous medium

The kinetics of the interaction of adenosine 5′-monophosphate (5′-AMP) with cis-[Pt(en)(H2O)2]2+ have been studied spectrophotometrically as a function of [Pt(en)(H2O)2]2+, [5′-AMP] and temperature at pH 4.0, where the substrate complex exists predominantly as the diaqua species. Both N1 and N7 donor sites of 5′-AMP are active for coordination to Pt at this pH. Base stacking and metal-induced macrochelate formation of 5′-AMP plays a vital role in determining the concentration limit of 5′-AMP during kinetics. Substitution occurs in two consecutive steps; both dependent on the 5′-AMP concentration. Activation parameters for both steps have been calculated. The low ΔH1‡ (42.76 ± 1.64 kJ mol−1) and large negative values of ΔS1‡ (−112.1 ± 5.1 J K−1 mol−1) as well as ΔH2‡ (58.1 ± 1.4 kJ mol−1) and ΔS2‡ (−84.2 ± 4.4 J K−1 mol−1) indicate associative modes of activation for both ligand substitution processes in the two consecutive steps.