Formation of the peptide-specific imidazolidin-4-one moiety in alanine containing Maillard reaction mixtures

Due to the facile formation of amide linkages during Maillard reaction and their possible conversion into imidazolidin-4-one moiety through interaction with aldehydes, such structures were sought and identified in simple Maillard model systems such as alanine, glucose/alanine, and glyoxal/alanine to minimise the complexity of the possible imidazolidin-4-one structures and to facilitate the interpretation of label incorporation data. To confirm the formation of imidazolidin-4-ones in these model systems a useful strategy based on Py-GC/MS analysis was developed using isotope labelling technique to identify products incorporating two nitrogen atoms, three C-3′, three C-2′ and one C-1′ atom from alanine. Products simultaneously possessing these atom configurations and having a molecular weight of 142 amu were targeted using specifically labelled precursors such as [15N]alanine, [13C-1]alanine, [13C-2]alanine, [13C-3]alanine and [U-13C6]glucose. Based on isotope labelling studies, spiking with known precursors and comparison with established mass spectral fragmentation patterns, the formation of isomeric 3-ethyl-2,5-dimethylimidazolidin-4-ones was proposed.