Beneath glycation, oxidation reactions may take place at cereal proteins during production of malt. The extent of oxidative chemical changes at malt proteins has not yet been studied. In the present short communication, malt protein was characterized by the determination of free thiol groups and degree of methionine oxidation as well as the sites that are reactive to covalent modification by 2,4-dinitrophenylhydrazine (DNPH, “protein carbonylation”). Protein carbonylation in pale malts was around 1.5 nmol/mg protein and increased with increasing malt colour. Investigations on the protein pellet isolated for determination of carbonylation revealed that solubility and colour may disturb the quantification of carbonyl sites in roasted malts. Free thiols decreased with increasing malt colour already in pale malts (EBC < 10). The formation of methionine sulfoxide (MetSO) was intensified with increasing malt colour. An amount of 7–20% of methionine was converted to MetSO in pale and dark malt, whereas nearly 60% of methionine was oxidized to MetSO in roasted malts. The formation of methionine sulfone was negligible. This study shows that malt proteins suffer from oxidation during kilning, and future studies will have to show whether this supports the pro- or antioxidant activity of malt.

Peptide-bound methionine may transfer oxidative damage from the thioether side chain to the peptide backbone, catalyzing decomposition in general and α-amidation in particular. In the present study, we focused on the reactivity and reaction pathways of peptides. We synthesized model peptides comprising methionine or not and investigated their overall tendency towards decomposition and formation of specific products under conditions mimicking the cooking process at 100°C in buffered solution (pH 6.0) in the presence of redox-active substances such as transition metal ions and reductones. Peptides containing methionine were more susceptible to α-amidation under all oxidative conditions, and the products of N-terminus-directed α-amidation were quantified. Exemplarily, after incubation in the presence of cupric sulfate, about 2.0 mol-% of the overall decomposition of Z-glycylmethionylglycine accounted for the formation of Z-glycinamide, whereas it was below 0.1 mol-% for Z-glycylalanylglycine. Surprisingly and different from previous observations, C-terminus-directed α-amidation was observed for the first time. From Z-glycylmethionylglycine, the respective products were formed in higher amounts than the N-terminus-directed α-amidation product Z-glycinamide under all applied oxidation conditions. The preference of electron transfer from the amino nitrogen bound in the peptide bond directed to the C-terminus may be ascribed to a sterically less demanding hexagonal 3-electron-2-center intermediate during methionine-catalyzed α-amidation.