DISSOCIATION-CONSTANTS OF ADENINE IN VARIOUS WATER PLUS ORGANIC-SOLVENT MIXTURES

The dissociation constant values of adenine (6-aminopurine) were deter mined at 25.0 +/- 0.1-degrees-C by potentiometric pH titration in pure water and different mixed solvent mixtures (methanol, ethanol, N,N-di methylformamide [DMF], dimethylsulfoxide [DMSO], acetone and dioxane). The ionization of adenine depends on both the proportion and the natu re of the organic solvent used. It is observed that pKa1 and pKa2 are slightly influenced as the solvent is enriched in methanol and ethanol , and remains practically constant in the presence of different amount s of DMF and DMSO. A pronounced change in the pKa2 value is observed a s the solvent is enriched in acetone or dioxane. These results are dis cussed in terms of average macroscopic properties of the mixed solvent and the possible variation in microheterogeneity of solvation shells around the solute. It is concluded that the electrostatic effect has o nly a relatively small influence on the dissociation equilibrium of bo th N1H+ and N9H groups of the adenine moiety. Other solvent effects su ch as the solvent basicity and the different stabilization of the conj ugate acid free base by hydrogen-bonding interactions in aquo-organic solvent media as well as proton-solvent interaction play an important role in the acid dissociation equilibrium.