[The role of labile complexes with oxygen in antioxidant activity of carotenoids]

The present research work was done with the main purpose to study early stages of interaction of carotenoids (Car) with molecular oxygen and clarify their role in the mechanism responsible for Car radiochemical stability and carotenoid ability to decrease concentration of the most active oxygen transients like superoxide anion radicals (O2.-). Alcoholic and phosphate buffer (pH 7.5) solutions of carotenoid fucoxanthin (Fx) were used for investigation of the oxygen effect on the absorption spectra in the UV-Visible range. Special analysis of time dependent reversible shifts of absorption bands of evacuated Fx solution after contact with O2 indicated existence of equilibrium between two distinct forms of Car: Fx and the labile charge transfer complex (Fx+delta...O2-delta). The velocity of the achievement of equilibrium state and a degree of reversibility depend on chemical structure of the carotenoid, oxygen content and the solvent nature. Radiation-chemical methods were used to confirm the important role of primary Car oxocomplexes in different redox processes. It appeared that the yield of radiation-chemical bleaching of Fx, G-Fx, is 0.02-0.05 molecule/100 eV in the presence of oxygen, which in hundred times less the yield achieved in anaerobic conditions. The obtained results provide the evidence of Fx high level of stability under radiation, and demonstrate the supreme importance of reversible oxocomplex (Fx+delta...O2-delta) in stabilizing carotenoids in aerobic medium. The pulse radiolysis method with spectrophotometric registration of transients was used for generation and studying of mechanism O2.- interaction with different carotenoids. Introduction of any carotenoids containing oxygen (10(-5) M) in phosphate buffer solutions (pH 7.5) caused a red-shift of absorption maximum (from 5 to 15 nm) and difference in kinetics of O2.- decay. These results prove that radiation generated esolv- are directly accepted by (Car...O2) with consequent formation of superoxocomplexes (Car...O2.-) instead of O2.-. On the base of detecting the following transformation of superoxocomplexes the peroxocomplex (Car+...O2(2-)) was identified. In case of Fx a peroxocomplex (Fx+...O2(2-)) had absorption band with lambda max at approximately 360 nm. It is very important to mention that beta-carotene does not cause the similar effect and gets easily oxydized when exposed to the air.