Proton magnetic relaxation and molecular motion in polycrystalline amino acids

Abstract

A proton magnetic relaxation investigation has been carried out on a further six polycrystalline amino acids from 130 to 500 K at 60·2 MHz and in two cases at 20·0 MHz also. Asparagine and glutamine with amide side chains exhibit closely similar relaxation minima well accounted for in terms of the Kubo-Tomita relaxation theory by NH₃ group reorientation in the zwitterion form of the molecules. Cysteine and phenylalanine exhibit two relaxation mechanisms, one at higher temperatures due to NH₃ group reorientation and a weaker mechanism at lower temperatures ascribed to motion of their side chains. The unusual relaxation behaviour of arginine arises from the protonation of the guadinium side group, so generating three reorienting amino groups in the molecule. The exceptionally long relaxation times of proline stem from the rigidity of its ring structure. Relaxation constants, activation energies and time factors for all the amino acids studied in this series are tabulated and discussed. For 18 amino acids the relaxation constants for NH₃ group reorientation follow well an inverse dependence on the number of molecular protons being relaxed, and their absolute values agree well with those calculated using proton separations determined by neutron diffraction. Activation energies for NH₃ group reorientation range from 28 to 52 kJ/mole. Highest values are for amino acids with hydrocarbon side chains suggesting the formation of stronger hydrogen bonds by NH₃ groups in the crystal when in the absence of competition from polar interactions. Methyl rotors, unencumbered by hydrogen bonding, are characterized by lower activation energies in the range 7 to 22 kJ/mole.