Studies on the Protonation Constants and Solvation of α-Amino Acids in Dioxan–Water Mixtures

To gain more information about the effect of solvent on α-amino acids, the stoichiometric protonation constants of 10 α-amino acids (glycine, dl-alanine, dl-valine, l-leucine, l-isoleucine, dl-phenylalanine, l-serine, l-threonine, l-asparagine, and l-glutamine) in different dioxan–water mixtures have been determined potentiometrically using a combined pH electrode system calibrated in concentration units of hydrogen ion at 25°C with an ionic strength of 0.10 M. For all amino acids studied, it was observed that the log K1 values relating to the protonation equilibria of the anionic form are almost unaltered by the change in solvent composition. However, the log K2 values corresponding to the formation equilibria of cationic form increase with the increase in dioxan content. The variation of these constants is discussed on the basis of specific solute–solvent interactions and structural changes of amino acids from water to dioxan–water media. The zwitterionic to neutral form ratio of these acids in dioxan–water mixtures is also discussed.