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-hydrox y-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), a water-solub le model compound for vitamin E, has been studied in aqueous solutions of pH values 6 and 12 and in organic media of different polarity. Exp erimental evidence indicates that the process largely occurs through a Trolox-singlet molecular oxygen O-2((1) Delta(g)) interaction, the ph otooxidation rates increasing with the pH value and the polarity of th e medium. Low polarity solvents favour the physical quenching of O-2(( 1) Delta(g)) by Trolox, as already observed in other tocopherols. High polarity solvents drive the interaction predominantly towards a react ive pathway that yields the same substituted p-benzoquinone that can b e obtained by thermal oxidation of Trolox. Overall rate constants for the quenching of O-2((1) Delta(g)) by Trolox ( determined by time-reso lved phosphorescence spectroscopy) and reactive rate constants (obtain ed by relative methods) are both in the range 10(7)-10(8) M(-1) s(-1). On the basis of these results and their comparison with previous resu lts on the photo-oxidation of tocopherol derivatives, the role of the solvent in the behaviour of vitamin E is discussed.