Article,
Molecular flexibility and structural instabilities in crystalline L-methionine
BIOPHYSICAL CHEMISTRY, (2013)
DOI: 10.1016/j.bpc.2013.06.011
Abstract
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 similar to 190 K an anomalous drop of
the c-lattice parameter and an abrupt change of the beta-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. (C) 2013
Elsevier B.V. All rights reserved.
