INTERACTIONS OF NICKEL(II) WITH ADENOSINE, URIDINE AND CYTIDINE MONOPHOSPHATES. A CALORIMETRIC STUDY

Abstract The thermodynamic parameters of the interaction between nickel(II) and 3′AMP, 2′AMP, 5′UMP and 5′CMP in aqueous solution were determined calorimetrically (ionic strength adjusted to 0.1 with tetramethylammonium bromide, 25°C at physiological pH). The experimental conditions were carefully selected to avoid polynuclear complex formation. The calorimetric data confirm the existence of the [(Ni(NMP)) 2 ] 2− complexes for 3′AMP and 2′AMP, the latter previously studied by Frey and Stuehr by potentiometry. The [(Ni(NMP) 2 ] 2− complex formation constant (log K 2 = 1.96 and 2.16 for the 3′AMP and 2′AMP derivatives, respectively) and complex formation enthalpies ( ΔH 1 = + 4.5 kJ mol −1 and ΔH 2 = −10.0 kJ mol −1 for the 3′AMP complexes, and ΔH 1 = +2.4 kJ mol −1 and ΔH 2 = −22.3 kJ mol −1 for the 2′AMP complexes) were determined. The values of ΔH 1 for the complexes Ni(5′UMP) and Ni(5′CMP) are, respectively, 8.3 and 8.7 kJ mol −1 . The degree of macrochelation indicated by Sigel after very careful potentiometric studies is confirmed by the calorimetric data, and follows the order: [Ni(5′AMP)] cl > [Ni(2′AMP)] cd > [Ni(3′AMP)] cl . These results confirm the formation of the ML 2 complex in solution and agree very well with the hypothesis of ‘stacking’ between the purine bases promoted for metal ions.