Protonation of purines and related compounds in dimethylsulfoxide and water

Abstract

The ionization constants of the conjugated acids BH+ of purine, adenine, 9-methyladenine, adenosine, guanine, and hypoxanthine, and of the related compounds imidazole, N-methylimidazole, benzimidazole, and 4-amino-2,6-dimethylpyrimidine have been determined potentiometrically in Me2SO. The heats of solution and the heats of protonation of most of these bases have been obtained by calorimetry. Some additional determinations were carried out in water so that thermodynamic data in Me2SO and in water could be compared. Our discussion of the solvent effect on these data is largely based on a consideration of the enthalpies of transfer of B and BH+ and emphasizes the contrasting H-bonding properties of the solvents and of the functional groups on the solutes B and BH+.