Interactions in solution of Zn(II) and Cd(II) with nucleoside monophosphates. A calorimetric study

The thermodynamic parameters enthalpy and entropy of the interaction between zinc(II) or cadmium(II) and 5′UMP, 5′CMP, 5′AMP, 5′GMP or 5′IMP in aqueous solution were determined calorimetrically (ionic strength adjusted to 0.1 with tetramethylammonium bromide, 25 °C and pH 7 for Cd(II) and 5.5 for Zn(II)). The experimental conditions were carefully selected to avoid polynuclear complex formation and nucleotide selfstacking. The calorimetric data are in agreement with the trend to macrochelation indicated by Sigel after very careful potentiometric studies, and follow the order GMP>IMP>AMP>CMP=UMP for the nucleotides, being very alike for these metal ions. Macrochelate formation for these metal–nucleoside monophosphate complexes is energetically favorable and entropically unfavorable.