Enthalpies of solution of thymine and uracil in water and dimethylsulfoxide

Enthalpies of solution of thymine and uracil in water and in dimethylsulfoxide (DMSO) were measured calorimetrically in the temperature range 25–40°C. H_s^o at 25°C for thymine and uracil in water were found to be 23.1±0.5 and 29.5±0.3 kJ-mol^–1, respectively. In DMSO, H_s^o were 7.9±0.1 and 10.2±0.1 kJ-mol^–1, respectively. In aqueous solution C_p^o for the two nucleic acid bases were relatively large and positive with C_p^o of thymine being larger. Both transfer quantities H_t^o and C_p,t^o for the proceses H₂ODMSO for the two nucleic acid bases were negative. It is proposed that, the differences in the values obtained for the two bases is due principally to increased order in the water adjacent to the methyl group in thymine.