From glycine to glutamic acid: analysis of the proton-binding isotherm of glutamic acid

The process of the analysis of the protonation of glycine is extended to the three-site molecule of glutamic acid with its amino and two carboxyl groups. Detailed data on the binding of protons to glutamic acid are available not only for protonation of the three groups simultaneously but also for derivatives in which the alpha and beta carboxyl groups are esterified. These data plus data on the protonation of glutaric acid provide the necessary information for a complete description of the protonation process with a limited number of reasonable assumptions. The assumptions lead to the conclusion that stabilization of the molecule of glutamic acid occurs on all steps of the protonation with the predominant stabilization occurring in the early steps of the reaction. An Appendix is included showing that the experimental data for both glycine and glutamic acid can be generated with hypothetical molecules. For glycine, identical experimental isotherms can result from protonation of two different nitrogen groups as well as two different negative groups. With glutamic acid three hypothetical molecules are capable of generating the identical experimental isotherms. They are (i) three nitrogen groups, (ii) three negative groups, and (iii) two nitrogen groups combined with one negative group. Interpretation of binding data requires explicit assumptions defining both the interactions and the nature of the binding sites.