Dark and photoinduced interactions between Trolox, a polar-solvent-solublemodel for vitamin E, and riboflavin

The aerobic riboflavin (Rf)-sensitized visible-light irradiation of Trolox (TX), a polar-solvent-soluble model for vitamin E, has been studied employi ng stationary photolysis, polarographic detection of oxygen uptake, station ary and time-resolved fluorescence spectroscopy, and laser flash photolysis . Results indicate that in methanolic solution, no dark complexation exists between Rf and TX. The latter quenches singlet and triplet states of Rf wi th rate constants of 6.2x10(9) M-1 s(-1) and 4.7x10(9) M-1 s(-1), respectiv ely. The photodecomposition of Rf, a known process taking place from triple t Rf, has been found to depend on the concentration of dissolved TX: at gre ater than or equal to 30 mM very slight Rf photodecomposition occurs due to the massive quenching of excited singlet Rf, while at TX concentrations le ss than or equal to1 mM triplet Rf is photogenerated and subsequently quenc hed either by oxygen, giving rise to O-2((1)Delta (g)), or by TX, yielding semireduced Rf through an electron transfer process. Complementary experime nts performed in pure water employing superoxide dismutase and sodium azide inhibition of the oxygen uptake, in coincidence with flash photolysis data , indicate that superoxide anion and singlet molecular oxygen are generated , likely by the reaction of the anion radical from Rf with dissolved oxygen , also yielding neutral, ground state Rf or by energy transfer from triplet Rf to ground-state oxygen, respectively. The final result is that both TX and Rf are photodegraded, likely through oxidation with activated oxygen sp ecies. In the absence of oxygen no degradation of TX can be detected, but R f photodegradation is favoured because Rf regeneration is avoided. (C) 2001 Elsevier Science BY. All rights reserved.