Selective oxidation of imidazole ring in histidine residues by the ascorbic acid-copper ion system

In connection with the physiological actions of active oxygen species on proteins, oxidative modification of histidine residues by the autoxidation of ascorbic acid was determined and the main oxidized compound was identified. Oxidation of imidazole ring by the ascorbic acid-copper ion system was considerably site-specific and assumed to be initiated by the addition of the hydroxyl radical (.0H) at C-2 position in the imidazole ring.