Cross-linking of proteins by maillard processes: characterization and detection of lysine–arginine cross-links derived from glyoxal and methylglyoxal

α-Dicarbonyl compounds, such as glyoxal and methylglyoxal, are crucial intermediates in the browning and cross-linking of proteins by reducing sugars in the course of the Maillard reaction. The cross-linking units 2-ammonio-6-({2-[(4-ammonio-5-oxido-5-oxopentyl)amino]-4,5-dihydro-1H-imidazol-5-ylidene}amino)hexanoate (9) and 2-ammonio-6-({2-[(4-ammonio-5-oxido-5-oxopentyl)amino]-4-methyl-4,5-dihydro-1H-imidazol-5-ylidene}amino)hexanoate (10), designated as GODIC and MODIC, are identified and quantified from glyoxal/methylglyoxal-bovine serum albumin (BSA) incubations. Independent syntheses and unequivocal structural characterization are given for 9 and 10. A protocol was established for their determination by liquid chromatography–mass spectrometry (LC–MS) with electrospray ionization (ESI). BSA and the respective α-dicarbonyl compound were incubated at 37°C, pH 7.4 for 1 week, and the time-dependent formation of 9 and 10 was observed. The maximum value obtained from a solution containing 50g/L BSA and 2mM glyoxal or methylglyoxal after a 7-day incubation period corresponds to an arginine derivatization quota of 13.0±0.32mmol 9/mol Arg or 3.0±0.12 mmol 10/mol Arg. The cross-links 9 and 10 were also detected in a d-glucose–BSA incubation. From these results, it seems justified to assign an important role to 9 and 10 in the cross-linking of proteins in vivo as well as in foodstuffs. In an additional model study, formation of 9 and 10 was compared to that of the imidazolium cross-links GOLD 3 and MOLD 4.