Autoxidation of Ascorbic Acid Catalyzed by the Copper(II) Bound to L-Histidine Oligopeptides, (His)iGly and Acetyl-(His)iGly (i=9, 19, 29). Relationship between Catalytic Activity and Coordination Mode.

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Spectroscopic and kinetic studies on the autoxidation of ascorbic acid catalyzed by copper complexes of histidine oligopeptides, (His)iGly (i=4, 9, 19, 29), and their acetyl derivatives, Ac-(His)iGly (i=9, 19) have been carried out at pH 4.4 and 25°C under dioxygen. The reaction was monitored at 260 nm using a stopped-flow spectrophotometric technique. The reaction fitted the "Michaelis-Menten" mechanism, and ascorbate was oxidized by the "Ping-Pong" mechanism. The Cu(II) complexed with the oligopeptide (i≥9) enhanced the reaction approximately two-fold relative to the aqueous Cu(II). The catalytic activity depends on the molecular weight which is related to the number of histidyl residues and on the coordination mode of the copper-binding site. Results of circular dichroism (CD) experiments revealed the existence of two types of Cu(II). The catalytically active Cu(II), which is accommodated in the imidazole clusters composed of at least six histidyl residues, exhibits d-d transition bands at 520 and 630 nm, and is easily dissociable, enhances the autoxidation; Ac-(His)19Gly is likely to accommodate approximately three active Cu(II) ions. The Cu(II), which is complexed tightly with the terminal H2N-X-Y-His- moiety, where X and Y denote amino acids, inhibits the autoxidation, and exhibits absorption bands at 480 and 550 nm.

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