Thermochemistry of aqueous solutions of alkylated nucleic acid bases. II. Apparent molar relative enthalpies of dilution of solutions containing uracil and thymine derivatives.

Apparent molar relative enthalpies of dilution phi(L)of aqueous solutions of a series of alkylated diketopyrimidines: m(1)Ura, m(1,3)(2)Ura, m(1,3)(2)Thy, m(1,3,6)(3)Ura and e(1,3)(2)Thy were measured as a function of concentration of the solutes at three temperatures 298.15, 308.15 and 318.15 K. Dilution proved to be an endothermic process over the whole range of molalities m and temperatures studied for all compounds except the e(1,3)(2)Thy solution, the dilution of which, with the exception of the lowest concentrations (m > 0.2-0.3) was an exothermic process. Partial molar relative enthalpies of dilution -L(2)(m) derived from phi(L)(m) functions were analysed as if they were composed of two additive contributions: an endothermic one -L(2), (m(1)) and an exothermic one 1.2, (m(as)0), owing to the presence in the solutions of a free monomer m(1) or associated species m(as), respectively. Partial molar heat capacities of the solutes, evaluated by differentiation of -L(2)(m) functions in respect to temperature, decreased with the rise of concentration in the order of the tendency of the solutes to stacking association. Changes in heat content and in heat capacity of solutions upon their dilution are interpreted in terms of involvement of hydrophobic hydration and association of the solutes.