Electron Transfer Quenching of the Rose Bengal Triplet State

Abstract

The potential for electron transfer quenching of rose bengal triplet (jRB2--) to compete with energy transfer quenching by oxygen was evaluated. Rate constants for oxidative and reductive quenching were measured in buffered aqueous solution, acetonitrile and in small unilamellar liposomes using laser flash photolysis. Biologically relevant quenchers were used that varied widely in structure, reduction potential and charge. Radical ion yields (+i) were measured by monitoring the absorption of the rose bengal semireduced (RBo3-) and semioxidized (RB'-) radicals. The results in solution were analyzed as a function of the free energy for electron transfer (AG) calculated using the Weller equation including electrostatic terms. Exothermic oxidative quenching was about 10-fold faster than exothermic reductive quenching in aqueous solution. The quenching rate constants decreased as AG approached zero in both aqueous and acetonitrile solution. Exceptions to these generalizations were observed that could be rationalized by specific steric or electrostatic effects or by a change in mechanism. The results suggest that electron transfer reactions with some potential quenchers in cells could compete with formation of singlet oxygen [O,(lA,)]. Values of +i were generally greater for reductive quenching and, for oxidative quenching, greater in acetonitrile than in buffer. Electran transfer quenching of 3FU3-in liposomes, below the phase transition temperature was slower than in solution for both lipid-soluble and water-soluble quenchers indicating that these reactions may not compete with formation of O,(lAg) during cell photosensitization.

Effect of temperature on the photobehavior of Rose Bengal associated with dipalmitoylphosphatidyl choline liposomes

“…These changes indicate adsorption of the dye by the liposomes [8,19]. This type of data allows an evaluation of the dye distribution (from the change in absorbance at a given wavelength with DPPC concentration).…”

“…1, implies that RB, in spite of its hydrophilicity, interacts with the liposome and that the microenvironment of the bound dye is different than that sensed in bulk water. These changes can be employed to estimate the microenvironment sensed by the dye molecules bound to the liposomes [8,19]. The red shift observed in presence of the liposomes is indicative of a less polar environment.…”

A Reducing and Oxidizing System Minimizes Photobleaching and Blinking of Fluorescent Dyes

“…[13] The solvent-dependent Coulombic attraction energy C can be neglected due to the high polarity of water. If T 1 of Cy5 is reduced by AA, we obtain with E red (Cy5, vs. SCE) = À0.84 V, and E ox (AA, vs. SCE) = 0.06 V, [22] DG cs = À0.70 eV. Similarly, the oxidation of the Cy5 triplet state by MV is also exergonic (E ox (Cy5) = 0.97 V, E red (MV) = À0.45 V) [23] by DG = À0.18 eV.…”

Singlet molecular oxygen photosensitized by Rhodamine dyes: correlation with photophysical properties of the sensitizers

“…The T state is deactivated by intermolecular interaction either with 3 O 2 ( Fig. 5(i, j)) (predominant energy transfer, under certain conditions electron transfer [57]), with an added triplet quencher Q (Fig. 5(h)) or with other triplet quenchers (Fig.…”