Studies on the protonation constants and solvation of alpha-amino acids indioxan-water mixtures

To gain more information about the effect of solvent on alpha-amino acids, the stoichiometric protonation constants of 10 alpha-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 mixt ures have been determined potentiometrically using a combined pH electrode system calibrated in concentration units of hydrogen ion at 25 degrees C wi th an ionic strength of 0.10 M. For all amino acids studied, it was observe d that the log K-1 values relating to the protonation equilibria of the ani onic form are almost unaltered by the change in solvent composition, Howeve r, the log K-2 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. Th e zwitterionic to neutral form ratio of these acids in dioxan-water mixture s is also discussed. (C) 2000 Academic Press.