A nuclear magnetic resonance study of molecular motion in solid l-glutamic acid

Abstract

Proton magnetic resonance absorption and spin-lattice relaxation time measurements have been carried out on solid l-glutamic acid, NH 3 +CH(CH 2) 2COOHCOO −, in the temperature range 77 to 400 K. The absorption line measurements show that the structure is rigid on the NMR time scale at the lowest temperatures studied, while at higher temperatures the amino group executes hindered rotation about the NC bond. The proton spin-lattice relaxation measurements in both the Zeeman and rotating frames of reference ( T 1 and T 1 ϱ ) reveal a single minimum in each case due to the dominant amino group motion. Analysis of the relaxation data yields an activational energy barrier of 31.1 ± 0.3 kJ mol − to the amino group motion. An additional motional process involving, the rotational oscillation of the methylene side chain has been invoked to explain the slight discrepancy in the second moment and the rotating-frame relaxation data. Both the absorption and the relaxation data are in accord with the fact that this amino acid exists as a zwitterion, wherein the α-amino group is protonated.