Quenching of singlet oxygen by Trolox C, ascorbate, and amino acids: Effects of pH and temperature

The pH dependence of singlet oxygen quenching by histidine, N-acetyltyrosin e ethyl ester (ATEE), ascorbic acid, Trolox C, and tryptophan has been obse rved using time-resolved infrared luminescence measurements in a D2O/aceton itrile (50:50 v/v) solvent. Deprotonation of ascorbic acid, the protonated imidazole ring of histidine and the phenolic group of ATEE leads to an incr ease in the quenching rate constants by between 2 and 3 orders of magnitude . Such changes appear to be the basis for wide variations in quoted literat ure values of singlet oxygen quenching constants for these and related comp ounds. It is estimated that these pH-dependent quenching rate constants pre dict a modest (approximately 2- to 3-fold) change in singlet oxygen lifetim e between the extremes of cellular pH. Activation data for singlet oxygen q uenching show that the enthalpies of activation are low in all cases (betwe en 0 and 11 kJ mol(-1)) and that substantially negative entropies of activa tion (between -49 and -116 J K-1 mol(-1)) result in rate constants being mu ch lower than the diffusion-controlled limit. In all cases the data are con sistent with quenching via reversible formation of an exciplex, all reactio ns being at the preequilibrium limit over the available temperature range.