REACTIONS OF EXCITED TRIPLET DUROQUINONE WITH ALPHA-TOCOPHEROL AND ASCORBATE - A NANOSECOND LASER FLASH-PHOTOLYSIS AND TIME-RESOLVED RESONANCE RAMAN INVESTIGATION

Nanosecond laser flash photolysis and time-resolved resonance Raman spectroscopy (TR(3)) have been used to study reactions between the antioxidants a-tocopherol and ascorbate and the triplet excited states of duroquinone (DQ) and ubiquinone (UQ). In nonaqueous solvents (3)DQ is reduced by both antioxidants at near diffusion-limited (> 10(9) dm(3) mol(-1) s(-1)) rates. Rate constants obtained by direct measurement of the rate of decay of (3)DQ as a function of antioxidant concentration were in good agreement with those determined indirectly from reduction in singlet oxygen luminescence yield, showing that the antioxidants are capable of reacting quantitatively to suppress the sensitized formation of singlet oxygen from triplet states. In SDS micellar solution, the rate of reaction of (3)DQ with ascorbate is governed by the rapid exit of (3)DQ from the micelle. Time-resolved resonance Raman (TR(3)) and absorption spectra were used to show that (3)DQ reacts with both 6-palmitoyl-L-ascorbate and alpha-tocopherol in SDS micelles at neutral pH by hydrogen atom transfer. The spectra also allowed observation of deprotonation of the resulting neutral durosemiquinone radical after diffusion from the micelle into the aqueous phase. Implications of these results to the biological role of antioxidants are discussed.