Quenching mechanisms and kinetics of carotenoids in riboflavin photosensitized singlet oxygen oxidation of vitamin D-2

The oxidation of vitamin D-2 in a solution of 12% water, 88% acetone and 15 ppm riboflavin under light or dark was studied by measuring the headspace oxygen. Riboflavin accelerated the oxidation of vitamin D-2 by singlet oxyg en under light, but did not affect vitamin D-2 oxidation under dark. Quench ing mechanisms and kinetics within the range of 0-20 ppm beta -carotene or fucoxanthin and with 0-80 ppm retinyl acetate or retinol on 15 ppm riboflav in photosensitized singlet oxygen oxidation of vitamin D-2 were also studie d. The rate of singlet oxygen formation by 15 ppm riboflavin was 1.78 mu mo le oxygen/mL headspace-hour. The reaction rate constant of vitamin D-2 with singlet oxygen was 2.2 x 10(7) M-1 s(-1). The carotenoids minimized the ox idation of vitamin D-2 by quenching singlet oxygen. The total quenching rat e constants of retinol, retinyl acetate, fucoxanthin and beta -carotene wer e in the order of 1.22 x 10(8), 5.98 x 10(8), 1.78 x 10(9) and 5.00 x 10(9) M-1. s(-1) respectively, which suggests that as the number of double bonds of carotenoids increases, the quenching rate constant of carotenoid increa ses.