4-Alkylidene-2-imino-5-[4-alkylidene-5-oxo-1,3-imidazol-2-inyl] azamethylidene-1,3-imidazolidine - A Novel Colored Substructure in Melanoidins Formed by Maillard Reactions of Bound Arginine with Glyoxal and Furan-2-carboxaldehyde

When N^α-acetyl-L-arginine was heated with glyoxal in aqueous solution at pH 7.0 in the presence of furan-2-carboxaldehyde, an intense red-brown color developed. The compound mainly evoking this color was identified as (S,S)-1-[4-(acetylamino)-4-carboxy-1-butyl]-2-imino-4-[(Z)-(2-furyl)methylidene] -5-{2-[1-[4-(acetylamino)-4-carboxy-1-butyl]-4-[(E)-(2-furyl)methylidene]-5-oxo- 1,3-imidazol-2-inyl]}azamethylidene-1,3-imidazolidine (1, BISARG) by application of several one- and two-dimensional NMR experiments and, in addition, by LC/MSⁿ measurements and UV-vis spectroscopy. This is the first time that a chromophoric compound comprising four linked rings with two arginine moieties incorporated was identified in a Maillard reaction system. This novel type of chromophore indicates the possibility that food melanoidins might be generated by protein oligomerization via colored cross-linking structures, for example, between two arginine residues.