17O-NMR. of Enriched Acetic Acid,Glycine, Glutamic Acid and Aspartic Acid in Aqueous Solution. I. Chemical Shift Studies

The ¹⁷O-NMR. chemical shifts of the enriched amino acids glycine, aspartic acid and glutamic acid were measured in aqueous solution as a function of pH. High magnetic fields are necessary to resolve the α, β- and α, γ-carboxyl resonances of aspartic acid and glutamic acid, respectively. The chemical shifts of acetic acid were measured for comparative reasons. Ionization constants and titration shifts were obtained by nonlinear least-squares fits to one-proton titration curves. The average excitation energy approximation is discussed in terms of the observed changes in ¹⁷O-shielding on deprotonation. No intramolecular association between the α-amino group and the α-carboxyl group in the zwitterionic form is required to explain the high-frequency shift of the carboxylate ion. Also no indication of an intramolecular association between the α-amino group and the side-chain carboxyl groups of aspartic acid or glutamic acid was found.