Quenching of Singlet Oxygen by Trolox C, Ascorbate, and Amino Acids:  Effects of pH and Temperature

Abstract

The pH dependence of singlet oxygen quenching by histidine, N-acetyltyrosine ethyl ester (ATEE), ascorbic acid, Trolox C, and tryptophan has been observed using time-resolved infrared luminescence measurements in a D2O/acetonitrile (50:50 v/v) solvent. Deprotonation of ascorbic acid, the protonated imidazole ring of histidine and the phenolic group of ATEE leads to an increase 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 literature values of singlet oxygen quenching constants for these and related compounds. It is estimated that these pH-dependent quenching rate constants predict a modest (approximately 2- to 3-fold) change in singlet oxygen lifetime between the extremes of cellular pH. Activation data for singlet oxygen quenching show that the enthalpies of activation are low in all cases (between 0 and 11 kJ mol-1) and that substantially negative entropies of activation (between −49 and −116 J K-1 mol-1) result in rate constants being much lower than the diffusion-controlled limit. In all cases the data are consistent with quenching via reversible formation of an exciplex, all reactions being at the preequilibrium limit over the available temperature range.

Gum Arabic: More Than an Edible Emulsifier

“…In the present case, k r GA = 1.110 7 M -1 s -1 , a typical value for the reaction of aminoacid moieties with 1 O 2 (Michaeli & Feitelson, 1994;Bisby et al, 1999). By comparison with the total quenching rate constant, k t GA = 2.710 7 M -1 s -1 , it can be concluded that almost 60% of the interaction with 1 O 2 is through physical quenching and about 40% of the reactive moieties of GA are oxidized by 1 O 2 .…”

“…In order to separate the contribution of physical and chemical quenching, we monitored the consumption of dissolved 3 O 2 by GA using a FOXY-R oxygen-sensitive luminescent sensor coupled with to CCD detector from OceanOptics, Figure 3a. The dye methylene blue was used as sensitizer (   0.52) and histidine as actinometer, since this aminoacid reacts completely with 1 O 2 with k r His = 4 10 7 M -1 s -1 , (Bisby et al, 1999).…”

Irradiation-induced protein inactivation reveals Golgi enzyme cycling to cell periphery

“…Nevertheless, increased levels of protein expression (up to 50% higher expression by fluorescence intensity) were non-toxic and yielded similar results. The antioxidant Trolox was added as a quenching agent to further reduce the likelihood of off-target effects (Bisby et al, 1999). Trolox concentration and irradiation time and power were optimized using the Golgi-localized control construct to achieve bleaching without affecting Golgi HeLa cells stably expressing GalNacT2-GFP as a Golgi marker were transiently transfected with Killer Red constructs and, after 2 days, were placed in medium containing 375 mM Trolox.…”

Ascorbate Reacts with Singlet Oxygen to Produce Hydrogen Peroxide

“…Singlet oxygen is a highly reactive, electrophilic species that reacts rapidly with electron-rich moieties, such as the double bonds of lipids [1,2], thiols [3,4], and ascorbate [5,6,7,8,9]. We have observed previously that in neutral solutions singlet oxygen reacts with ascorbate (AscH-) to produce H 2 O 2 [9].…”