Model studies on the metal-catalyzed protein oxidation: structure of a possible His-Lys cross-link

Redox active transition metals such as copper and iron contribute to biomacromolecular damage that occurs during oxidative stress in a number of degenerative disorders and results in protein cross-linking. On the basis of suggestive evidence for an oxyradical-induced cross-linking between His and Lys side chains, we investigated the Cu(II)-catalyzed oxidation of 4-alkylimidazoles in the presence of amines, as surrogates for these amino acid side chains, using ascorbic acid as a continual source of reducing equivalents. A model His-Lys cross-link was isolated and structurally characterized as a 5-alkyl-5-hydroxy-4-(alkylamino)-1,5-dihydroimidazol-2-one by NMR and mass spectrometry. Evidence that the 2-imidazolone, the principal oxidation product found in the absence of amine, is an intermediate in the formation of the imidazole-amine adduct was that higher yields of the cross-link adduct were obtained starting with the 2-imidazolone. Possible mechanisms for formation of the cross-link and other observed products are discussed.