Solvent influence on the kinetics of the photodynamic degradation of trolox, a water-soluble model compound for vitamin E

The kinetics of the aerobic dye-sensitized photo-oxidation of 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), a water-soluble model compound for vitamin E, has been studied in aqueous solutions of pH values 6 and 12 and in organic media of different polarity. Experimental evidence indicates that the process largely occurs through a Trolox-singlet molecular oxygen O₂(¹Δ_g) interaction, the photo-oxidation rates increasing with the pH value and the polarity of the medium. Low polarity solvents favour the physical quenching of O₂(¹Δ_g) by Trolox, as already observed in other tocopherols. High polarity solvents drive the interaction predominantly towards a reactive pathway that yields the same substituted p-benzoquinone that can be obtained by thermal oxidation of Trolox. Overall rate constants for the quenching of O₂(¹Δ_g) by Trolox (determined by time-resolved phosphorescence spectroscopy) and reactive rate constants (obtained by relative methods) are both in the range 10⁷-10⁸ M^-1s^-1. On the basis of these and their results and their comparison with previous results on the photo-oxidation of tocopherol derivatives, the role of the solvent in the behaviour of vitamin E is discussed.