SPECTROPHOTOMETRIC STUDY OF THE REACTION BETWEEN COBALT(II)-DIPEPTIDECOMPLEXES AND MOLECULAR-OXYGEN

The influence of pH and ligand structure on the reaction of cobalt(II) complexes with various dipeptides and molecular oxygen was examined. The minimum pH value required for the formation of dioxygen adducts wa s found to be about 6.5. This value should be related to amidic deprot onation of dipeptides, which seems to occur in the examined systems at particularly low values. The location and steric hindrance of the sid e chains of the dipeptides have a strong influence on the reaction rat e. The presence of a substituent group on the N-terminal amino acid pr omotes oxygen coordination, while, when the substituent group is on th e C-terminal residual, a decrease of reaction rate is observed. A stab ilizing effect of the aromatic ring on the dioxygen adducts is found o nly when the substituent is in the C-terminal position, and seems to b e independent of the presence of additional coordinating groups. Some information regarding the mechanism of the irreversible decomposition of the cobalt(II) complexes has been obtained by studying the effect o f pH and ligand structure on the reaction rate.