FREE-RADICAL TRANSIENTS IN PHOTOBLEACHING OF XANTHOPHYLLS AND CAROTENES

Carotenoids in chloroform and carbon tetrachloride photobleach upon na nosecond laser flash photolysis in two steps: instantaneously and in a second-order reaction. The rate constant for second-order reaction (f irst-order in a solvent derived radical and first-order in (excess) ca rotenoid) is largest for carotenes (9.8.10(8) M-1 s(-1) for beta-carot ene), intermediate for hydroxylated carotenoids, and smallest for carb onyl containing carotenoids (1.0 . 10(8) M-1 s(-1) for astaxanthin) in chloroform at 20 degrees C. Near infrared absorbing transients are fo rmed concomitant with photobleaching in chloroform (not detected in ca rbon tetrachloride). A species formed instantaneously is tentatively i dentified as either a carotenoid/solvent adduct or an ion-pair. A seco nd species is formed by decay of the instantaneously formed species an d is identified as the carotenoid radical cation. This species is form ed in a first-order reaction with a rate constant of approx. 5 . 10(4) s(-1) and absorbing at longer wavelength than the precursor. The life time (second-order decay) of the intermediates appears to be longest f or the carotenoids with the longest conjugated system. The results ind icate that carotenes are better antioxidants than xanthophylls as the carotenes, at least in the present lipophilic solvents, react faster w ith free radicals.