The Thermodynamics of Proton Dissociation of Adenine

The thermodynamic quantities associated with ionization of the N(1) and N(9) protons of adenine have been calorimetrically determined as a function of temperature. The Delta H values for proton dissociation of these groups, with pK values of 4.19 and 9.92, were found to be 5.1 and 9.1 kcal/mole, respectively, at 25 degrees C, mu = 0.025. The Delta C(p) values for proton dissociation of these groups were estimated to be -11 and -17 cal/mole-deg. These results indicate that the large heat capacity changes observed during conformational transitions of polynucleotides are not the result of ionization of the bases.