Abstract:

Enthalpy changes for the protonation of carboxyl group of four α amino acids(glycine, L –α-alanine, L-valine and L-serine) were measured in water ethanol mixtures (10－70wt％) at 298.15K using LKB-2277 Bioactivity Monitor. The corresponding entropy and Gibbs energy changes were also calculated. The results show that both enthalpy changes and entropy changes are favorable to the protonation of carboxyl groups of the investigated amino acids in water ethanol mixtures. However, the influence of the composition of ethanol in the mixed solvents on the enthalpy change and entropy changes is complicated . Both sσΔH1Θand sσΔS1Θ,the differences of enthalpy changes and entropy changes in mixed solvents and in pure water respectively, show a minimum approximately at xEtOH=0.1.The effects of side chains on the enthalpy change and entropy changes were also investigated using the proton transfer process between glycine and the other three amino acids. The results demonstrate that the proton transfer processes for alanine and valine are spontaneous but not for serine, which could be interpreted in terms of the electrostatic interaction between amino group and carboxyl group within the molecule and the interaction between carboxyl group and the solvent.

Key words: Enthalpy of protonation, Amino acids, Carboxyl groups, Mixed solvents