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

Abstract

Abstract The 17 O‐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 17 O‐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.