Combined Raman and IR spectroscopic study on the radical-based modifications of methionine

Among damages reported to occur on proteins, radical-based changes of methionine (Met) residues are one of the most important convalent post-translational modifications. The combined application of Raman and infrared (IR) spectroscopies for the characterisation of the radical-induced modifications of Met is described here. Gamma-irradiation was used to simulate the endogenous formation of reactive species such as hydrogen atoms (^•H), hydroxyl radicals (^•OH) and hydrogen peroxide (H₂O₂). These spectroscopic techniques coupled to mass experiments are suitable tools in detecting almost all the main radical-induced degradation products of Met that depend on the nature of the reactive species. In particular, Raman spectroscopy is useful in revealing the radical-induced modifications in the sulphur-containing moiety, whereas the IR spectra allow decarboxylation and deamination processes to be detected, as well as the formation of other degradation products. Thus, some band patterns useful for building a library of spectra–structure correlation for radical-based degradation of Met were identified. In particular, the bands due to the formation of methionine sulfoxide, the main oxidation product of Met, have been identified. All together, these results combine to produce a set of spectroscopic markers of the main processes occurring as a consequence of radical stress exposure, which can be used in a spectroscopic protocol for providing a first assessment of Met modifications in more complex systems such as peptides and proteins, and monitoring their impact on protein structure.