Characterisation of dioxygenated Co(II)-carnosine by Raman and IR spectroscopy

Raman and IR studies are carried out on carnosine (-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 OOO bridge is obtained from the Raman spectra. When the Co(II)/Carnos ratio is 1, peroxobinuclear complexes are the predominant species and two forms (monobridged and dibridged) are identified by the presence of two Raman peaks in the OOO region (750-850 cm1). 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 OOO band to higher wavenumbers. In addition, the chelated species in the 2 : 1 Co(II)/Carnos systemis 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 and N 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 and N nitrogens are both deprotonated.