The constants (K-s) and enthalpies (Delta H-s) for stacking interactions between purine nucleoside monophosphates were determined by calorimetry; the values thus obtained were guanosine as follows : K-s = 2.1 +/- 0.3 M(-1) and Delta H-s = -41.8 +/- 0.8 kJ/mol for adenosine 5’-monophosphate (5’AMP); K-s = 1.5 +/- 0.3 M(-1) and Delta H-s = -42.0 +/- 1.5 kJ/mol for guanosine 5’-monophosphate (5’GMP); and K-s = 1.0 +/- 0.2 M(-1) and Delta H-s = -42.3 +/- 1.1 kJ/mol for inosine 5’-monophosphate (5’IMP). The interaction of nickel(II) with purine nucleoside monophosphates was studied using potentiometric and calorimetric methods, with 0.1 M tetramethylammonium bromide as the background electrolyte, at 25 degrees C. The presence in solution of the complexes [Ni(5’GMP)(2)](2-) and [Ni(5’IMP)(2)](2-) was observed. The thermodynamic parameters obtained were log K-ML = 3.04 +/- 0.02, log K-ML2 = 2.33 +/- 0.02, Delta H-ML = -18.4 +/- 0.9 kJ/mol and Delta H-ML2 = -9.0 +/- 1.9 kJ/mol for 5’GMP; and log K-ML = 2.91 +/- 0.01, log K-ML2 = 1.92 +/- 0.01, Delta H-ML = -16.2 +/- 0.9 kJ/mol and Delta H-ML2 = -0.1 +/- 2.3 kJ/mol for 5’IMP. The relationships between complex enthalpies and the degree of macrochelation, as well as the stacking interaction between purine bases in the complexes are discussed in relation to previously reported calorimetric data.