Proton magnetic relaxation and molecular motion in polycrystalline amino acids

A proton magnetic resonance and relaxation study has been carried out on seven polycrystalline amino acids from 130 to 500 K at 60·2 MHz. These amino acids contain no methyl groups and all exhibit a single relaxation minimum. The measurements are well accounted for by the Kubo-Tomita theory of relaxation assuming a single correlation time which follows a simple activation law. In all cases the source of relaxation is provided by reorientation of the -NH3 groups in the zwitterion form of the molecules; other protons are relaxed by spin-exchange with those of the -NH3 group. The measured relaxation constants are consistent with inter-proton distances in the -NH3 groups recently measured by neutron diffraction.