Isolation and Characterization of Glyoxal−Arginine Modifications

5-(4,5-Dihydroxy-2-imino-1-imidazolidinyl)norvaline (1) was identified as the only product of the early reaction of arginine with glyoxal, which was slowly degraded to N 5-[[(carboxymethyl)amino](imino)methyl]ornithine (3, N 7-carboxymethylarginine). No other structures could be detected within a range of pH 4−8 and 20−50 °C in reaction conditions. The rates of formation for both products increased with pH and temperature. In equilibrium, the vincinal diol groups of 1 were 86% trans configured. The formation of 1 was reversible, as could be shown by cis-trans isomerization of the separated isomers and by regeneration of arginine in the presence of the α-dicarbonyl trapping reagents, o-phenylenediamine and aminoguanidine. Both 1 and 3 were converted to 5-(2-imino-5-oxo-1-imidazolidinyl)norvaline (2) only under strong acidic conditions. Keywords: Glyoxal−arginine modifications; Maillard reaction; imidazoles; N7 -carboxymethylarginine; advanced glycation endproduct