title: Molecular flexibility and structural instabilities in crystalline l-methionine.
abstract: We have investigated the dynamics in polycrystalline samples of l-methionine related to the structural transition at about 307K 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 ~190K 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 130K in crystalline l-methionine. Large-amplitude motions observed at low frequencies are also activated above 275K, 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 275K, and such disorder can be linked to the flexibility of the molecular thiol-ether group.
authors.affiliation: Institute of Physics and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany; Helmholtz Zentrum Berlin für Materialien und Energie, Hahn Meitner Platz 1, 14109 Berlin, Germany.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Fischer
authors.forename: José A
authors.initials: JA
authors.lastname: Lima
authors.forename: Paulo T C
authors.initials: PT
authors.lastname: Freire
authors.forename: Francisco E A
authors.initials: FE
authors.lastname: Melo
authors.forename: Remco W A
authors.initials: RW
authors.lastname: Havenith
authors.forename: Josué Mendes
authors.initials: JM
authors.lastname: Filho
authors.forename: Ria
authors.initials: R
authors.lastname: Broer
authors.forename: Juergen
authors.initials: J
authors.lastname: Eckert
authors.forename: Heloisa N
authors.initials: HN
authors.lastname: Bordallo
mesh_terms: D003460:Crystallization; D057927:Hydrophobic and Hydrophilic Interactions; D008715:Methionine / Q000737:chemistry*; D033363:Neutron Diffraction; D044367:Phase Transition; D053838:Scattering, Small Angle; D013059:Spectrum Analysis, Raman; D044366:Transition Temperature; D014961:X-Ray Diffraction
chemical_list: D008715:Methionine
keywords: Amino acid; Computation; Dynamics; Flexibility; Spectroscopy; Structure