Hydroxyl radical scavenging by carnosine and Cu(II)-carnosine complexes: a pulse-radiolysis and spectroscopic study.
PURPOSE: To obtain a wider insight into the general properties of carnosine and to provide support to its anti-oxidative role. This property is hypothesized to be linked to various mechanisms including free-radical scavenging and metal chelation (i.e. Cu(II)). METHODS: Pulse-radiolysis experiments were performed by a 12 MeV electron linear accelerator (LINAC) on carnosine/copper(II) (2:1) and carnosine aqueous solutions at different pH. Raman spectra of solid samples were obtained by a Bruker IFS 66 spectrometer. RESULTS: As well as for free carnosine, in the presence of copper ions the interaction of carnosine with *OH radicals involves the imidazole group of the molecule. The oxidation of copper (II)-carnosine system by *OH radicals is related to the pH-dependent structure of the copper(II)-carnosine complex. Raman spectra indicate that at alkaline pH the formation of a dimeric species containing two carnosine molecules complexed to two Cu2+ ions takes place. This structure can address the *OH attack more selectively than carnosine itself to different sites of the imidazole ring. The formation of at least two different *OH-radical adducts occurs and positions C(2) and C(5) of the imidazole ring are the preferential sites for the *OH attack, as the heterocyclic ring is mainly present as its N(1)-protonated tautomeric form. CONCLUSION: These studies provide further evidence about the formation of carnosine-copper complexes and the predominance of a dimeric structure at slightly basic pH. The chelation of Cu(II) is not detrimental to the scavenging ability of carnosine. Raman spectra are helpful in identifying the structure of the copper(II)-carnosine complexes and in predicting the preferential sites for the *OH attack to the carnosine-copper system.