Thermodynamic study of the formation of adenine nucleotide · manganese complexes: II. Calorimetric results

Enthalpic variations in the formation of adenine nucleotide · manganese complexes, as measured by microcalorimetry, are reported. All the results are obtained in the temperature range 6–30°C at I = 0.2 and pH values 7.00 or 7.50. All the reactions are endothermic and the ΔH values increase with the length of the phosphate chain and with temperature.The ΔH values are compared with those previously obtained for adenine nucleotide · magnesium complexes.The comparison between calarimetric and potentiometric ΔH values is made. The divergence observed between these results at low temperature leads us to assume the formation of nucleotide aggregates induced by the presence of manganese ions. This hypothesis is confirmed by differential ultraviolet spectra.