Revisiting the phase transition sequence in L-methionine: Description of the disordering mechanism in an essential amino acid.

Abstract

Raman spectroscopy investigations on L-methionine (L-Met) performed in a large temperature range (170-420K) and in a wide spectral window (5-3600cm-1) have revealed an extended disordering mechanism triggered by thermally activated motions of the terminal side-chain atoms, from 250 up to 390K. This very progressive disordering process is characterized by two thermodynamic features, the first corresponding to a broad endotherm (250 → 310K) marking the beginning of the process, while the second ending the disordering transformation is a sharp endothermic peak at 390K. These thermodynamic events are correlated with the softening of lattice vibrations and the increase of the quasielastic scattering, considered as the signatures of displacive phase transitions. The amorphous-like band-shape of the low-frequency Raman spectrum collected above 390K, resulting from the strong anharmonicity of local motions, is contrasting with the detection of additional Bragg peaks above 390K by x-ray diffraction, consistent with the Cp jump accompanying the endothermic peak. These observations suggest that L-Met is progressively dynamically disordered adopting additional configurations in the crystalline lattice through rotations of CH3 and the side-chain flexibility not clearly detected by x-ray diffraction. These results should be crucial for considering the stability of dried proteins composed of methionine residues.