Proton magnetic relaxation and molecular motion in polycrystalline amino acids

Abstract

A proton magnetic relaxation study has been carried out on a further seven polycrystalline amino acids from 130 to 500 K at 60·2 MHz, supplemented by measurements of the spectrum down to 4 K on five of them. These amino acids all have one or two methyl groups in their side chain, and exhibit two relaxation minima. Clear evidence is given that the relaxation minima at lower temperatures are attributable to reorientation of the methyl groups, while those at higher temperatures are attributable to reorientation of the -NH₃ groups in the zwitterion configuration of the molecules. Values of the relaxation constants, activation energies and time factors which best characterize the kinetics of both motions are tabulated. Effects of methyl group tunnelling are found for methionine and valine.