Free radical transients in photobleaching of xanthophylls and carotenes

Abstract

Carotenoids in chloroform and carbon tetrachloride photobleach upon nanosecond laser flash photolysis in two steps: instantaneously and in a second-order reaction. The rate constant for second-order reaction (first-order in a solvent derived radical and first-order in (excess) carotenoid) is largest for carotenes (9.8.10(8) M-1 for beta-carotene), intermediate for hydroxylated carotenoids, and smallest for carbonyl containing carotenoids (1.0.10(8) M-1 S-1 for astaxanthin) in chloroform at 20 degrees C. Near infrared absorbing transients are formed concomitant with photobleaching in chloroform (not detected in carbon tetrachloride). A species formed instantaneously is tentatively identified as either a carotenoid/solvent adduct or an ion-pair. A second species is formed by decay of instantaneously formed species and is identified as the carotenoid radical action. This species is formed 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 lifetime (second-order decay) of the intermediates appears to be longest for the carotenoids with the longest conjugated system. The results indicate that carotenes are better antioxidants than xanthophylls as the carotenes, at least in the present lipophilic solvents, react faster with free radicals.