Interactions of nickel(II) with adenosine, uridine and cytidine monophosphates. A calorimetric study

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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.