Thermochemistry of aqueous solutions of alkylated nucleic acid bases V. Enthalpies of hydration of N-methylated adenines

Abstract

Enthalpies of solution in water, ΔH ^o _sol , and of sublimation, ΔH ^o _subl , were determined experimentally for a number of crystalline N-methyl adenines: m⁶Ade, m ^6,6₂ Ade, m⁹Ade, m ^6,9₂ Ade, and m ^6,6,9₃ Ade. Derived standard enthalpies of hydration ΔH ^o _hydr , were corrected for the calculated cavity terms ΔH ^o _cav to yield enthalpies of interaction ΔH ^o _int of the solutes with their hydration shells. The increments of ΔH ^o _int per unit area of the water-accessible molecular surface S _B , δΔH ^o _int (CH₃)/δS _B (CH₃), for the particular methyl groups: is considered to be the net effect of the gain in the energy resulting from van der Waals' interactions and of the loss in the energy due to polar interactions upon methyl substitution. It proved to vary somewhat numerically in agreement with the theoretically predicted hydration schemes of adenine. Comparison of ΔH ^o _int /S _B value for adenine with those previously determined for uracil and thymine indicates that the aminopurine moiety is less hydrated than the diketopyrimidine ring.