Characterization of dioxygenated cobalt(II)-carnosine complexes by Raman and IR spectroscopy
2002
Torreggiani, A. | Taddei, P. | Fini, G.
Raman and IR studies are carried out on carnosine (beta-alanyl-L-histidine, Carnos) and its complexes with cobalt(II) at different metal/ligand ratios and basic pH. Binuclear complexes that bind molecular oxygen are formed and information regarding the O-O bridge is obtained from the Raman spectra. When the Co(II)/Carnos ratio is less than or equal to 1, peroxobinuclear complexes are the predominant species and two forms (monobridged and dibridged) are identified by the presence of two Raman peaks in the vO-O region (750-850 cm(-1)). These peroxo complexes can be oxidized to yield a superoxo complex when the metal slightly exceeds the Carnos concentration, and there are consequent shifts of the vO-O band to higher wavenumbers. In addition, the chelated species in the 2:1 Co(II)/Carnos system is found to bind oxygen to a lesser degree. With respect to the coordination sites, each Co(II) ion of the binuclear dioxygenated complexes is bound to one oxygen atom and four nitrogen atoms: N(pi) and N(tau) of two Carnos molecules, the peptide, and the terminal amino nitrogen atoms. Conversely, when the metal/ligand ratio is 2, in addition to the dioxygenated complexes, a complex is formed in which the imidazole moiety of Carnos binds two different cobalt ions because the N(pi) and N(tau) nitrogens are both deprotonated.
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