Thermochemistry of aqueous solutions of alkylated nucleic acid bases. VIII. Enthalpies of hydration of 6-alkyluracils

Abstract

Enthalpies of solution in water ΔH ^o_sol and enthalpies of sublimation ΔH ^o_sub were determined for a number of crystalline derivatives of uracil: 1,6-dimethyluracil (m ^1,6₂ Ura), 1,3,6-trimethyluracil (m ^1,3,6₃ Ura), 6-ethyl-1,3-dimethyluracil (e⁶m ^1,3₂ Ura), 6-propyl-1,3-dimethyluracil (pr⁶m ^1,3₂ Ura) and 6-butyl-1,3-dimethyluracil (but⁶m ^1,3₂ Ura). Standard enthalpies of hydration ΔH ^o_hydr and standard enthalpies of interaction ΔH ^o_int of the solutes with their hydration shells were calculated. The data obtained show that dependence of ΔH ^o_int on the number of-CH₂- groups of n-alkyl chain added upon substitution of diketopyrimidine ring is nonlinear. This finding is discussed in connection with results of X-ray diffraction structure determinations for the crystalline compounds.