Solid-state proton relaxation of biomolecular components

Abstract

A proton magnetic relaxation study has been carried out on a series of biomolecular components in the solid state in the range 50 to 500 K. In the family of twenty amino acids encountered in proteins, reorientation of the NH 3 group in the zwitterion form of the molecules provides an effective source of relaxation in all except arginine and proline. Reorientation of methyl groups, where present in the side chains, provides a second source of relaxation resolved at lower temperatures. Additional motions are encountered in cysteine and phenylalanine. Reorientation of the NH 3 group provides an effective relaxation mechanism in diglycine and triglycine, as in glycine itself, although the reorienting group has progressively more protons to relax until in polyglycine its effect is weak. Well-resolved relaxation minima in glycyl- l-alanine and l-alanylglycine are attributed to independent reorientation of the NH 3 and CH 3 groups in these dipeptides. Activation energies, relaxation constants, and other parameters characterizing the motions are derived and compared. First results are presented of solid-state proton relaxation in the enzyme lysozyme, where reorientation of methyl groups along the popypeptide chain is the probable chief source of relaxation. A distribution of correlation times characterizing the motions is evident. The five nucleic acid bases have been examined. In solid thymine methyl group reorientation generates a well-defined relaxation minimum. The absence of methyl groups in the three other DNA bases-adenine, cytosine, and guanine-and in uracil, leads to much longer relaxation times. The amino groups are evidently strongly hindered. Preliminary measurements on calf thymus DNA suggest the importance of the thymine methyl groups as sources of relaxation.