Autoxidation of ascorbic acid catalyzed by the copper(II) bound to L-histidine oligopeptides, (His)(i)Gly and acetyl-(His)(i)Gly (i=9, 19, 29). Relationship between catalytic activity and coordination mode

Spectroscopic and kinetic studies on the autoxidation of ascorbic acid cata lyzed by copper complexes of histidine oligopeptides, (His)(i)Gly (i=4, 9, 19, 29), and their acetyl derivatives, Ac-(His)(i)Gly (i=9, 19) have been c arried out at pH 4.4 and 25 degrees C under dioxygen. The reaction,vas moni tored at 260 nm using a stopped-flow spectrophotometric technique, The reac tion fitted the "Michaelis-Menten" mechanism, and ascorbate was oxidized by the "Ping-Pong" mechanism. The Cu(II) complexed with the oligopeptide (i g reater than or equal to 9) enhanced the reaction approximately two-fold rel ative to the aqueous Cu(II), The catalytic activity depends an the molecula r weight which is related to the number of histidyl residues and on the coo rdination made of the copper-binding site. Results of circular dichroism (C D) experiments revealed the existence of two types of Cu(II). The catalytic ally active Cu(TI), which is accommodated in the imidazole clusters compose d of at least sis histidyl residues, exhibits d-d transition bands at 520 a nd 630 nm, and is easily dissociable, enhances the autoxidation; Ac-(His)(1 9)Gly 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, w here X and Y denote amino acids, inhibits the autoxidation, and exhibits ab sorption bands at 480 and 550 nm.