Protonation/deprotonation energetics of uracil, thymine, and cytosine in water from e.m.f./spectrophotometric measurements

The protonation/deprotonation constants for uracil (U) (pK 1 and pK 2 ), thymine (T) (pK 1 ) and cytosine (C) (pK 1 and pK 2 ) in water have been determined from emf measurements of Harned–Ehler type cells comprising H 2 and Ag–AgI electrodes at five equidistant temperatures ranging from 15–35 °C. The pK a values were fitted in the temperature equation pK a = AT −1 + B + CT by the method of least squares and the standard free energies (ΔG 0 ), entropies (ΔS 0 ), and enthalpies (ΔH 0 ) of protonation/deprotonation processes in water were evaluated using the values of the coefficients A, B, and C of the respective acids. The second step deprotonation constant for T was determined using precise spectrophotometric method. The results have been duly compared with the existing literature data and are also shown to derive important reflections on the sites of protonation/deprotonation in the light of pK values of some acids and bases of comparable functional groups and especially the entropies of protonation/deprotonation of the key DNA–RNA base molecules.