Interactions in solution of calcium(II) and copper(II) with nucleoside monophosphates: a calorimetric study.

The thermodynamic parameters enthalpy and entropy of the interaction between calcium(II) or copper(II) with 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) at 25 degrees C and pH 7 for Ca(II) or pH 3-5 for Cu(II). The experimental conditions were carefully selected to avoid polynuclear complex formation and nucleotide self-stacking. The calorimetric data confirm the tendency toward macrochelation which was indicated by Sigel after very precise potentiometric studies, and which follows the order Cu(II) > Ca(II) for the metal ions and GMP > IMP > AMP > CMP = UMP for the nucleotides. Macrochelate formation for these metal-nucleoside monophosphate complexes is energetically favorable and entropically unfavorable.