Influence of pH on the decay of beta-carotene radical cation in aqueous Triton X-100: A laser flash photolysis study

The identification of the spectral information of carotenoid neutral radicals is essential for studying their reactivities towards O-2 and thereby evaluating their role in the antioxidant-prooxidant properties of the corresponding carotenoid. Recently, it was reported that beta-carotene neutral radical (beta-CAR(center dot)) has an absorption maximum at 750 nm. This contradicts the results of many reports that show carotenoid neutral radicals (CAR(center dot)) absorb in the same or near to the spectral region as their parent carotenoids. In this manuscript, the influence of pH on the decay of beta-carotene radical cation (beta-CAR-H center dot+), generated in an aqueous solution of 2% Triton X-100 (TX-100), was investigated, employing laser flash photolysis (LFP) coupled with kinetic absorption spectroscopy, to identify the absorption bands of the beta-carotene neutral radicals. By increasing the pH value of the solution, the decay of beta-CAR-H center dot+ is enhanced and this enhancement is not associated with the formation of any positive absorption bands over the range 550-900 nm. By comparing these results with the literature, it can be concluded that beta-carotene neutral radicals most probably absorb within the same spectral range as that of beta-carotene. The reaction pathways of the reaction of beta-CAR-H center dot+ with -OH have been discussed.