We have investigated the dynamics in polycrystalline samples of L-methionine related to the structural transition at about 307 K by incoherent inelastic and quasielastic neutron scattering, X-ray powder diffraction as well as ab-initio calculations. L-Methionine is a sulfur amino acid which can be considered a derivative of alanine with the alanine R-group CH3 exchanged by \CH3S\(CH2)2. Using X-ray powder diffraction we have observed at ~190 K an anomalous drop of the c-lattice parameter and an abrupt change of the β-monoclinic angle that could be correlated to the anomalies observed in previous specific heat measurements. Distinct changes in the quasielastic region of the neutron spectra are interpreted as being due to the onset and slowing-down of reorientational motions of the CH3-S group, are clearly distinguished above 130 K in crystalline L-methionine. Large-amplitude motions observed at low frequencies are also activated above 275 K, while other well-defined vibrations are damped. The ensemble of our results suggests that the crystalline structure of L-methionine is dynamically highly disordered above 275 K, and such disorder can be linked to the flexibility of the molecular thiol-ether group.
- Amino acid