Elementary Photochemical Processes Involving meso-Substituted Anionic Cyanine Dyes in Solutions and in Complexes with Serum Albumins

Primary photochemical processes (photoisomerization, triplet state formation) involving meso-substituted anionic cyanine dyes 3,3'-di(γ-sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine betaine, 3,3'-di(γ-sulfopropyl)-4,5,4',5'-dibenzo-9-methylthiacarbocyanine betaine, 3,3'-di(γ-sulfopropyl)-5,5'-dimethoxy-9-ethylthiacarbocyanine betaine, and 3,3'-di(γ-sulfopropyl)-5,6,5',6'-di(methylenedioxy)-9-ethylthiacarbocyanine betaine in noncovalent complexes with human and bovine serum albumins are studied by flash photolysis. For comparison, the photochemistry of these dyes in solutions (in water and dioxane) is also studied. It is found that upon the flash photolysis of dye solutions in water, photoisomerization and triplet state formation are not observed; however, unstable photoisomers are formed in a nonpolar solvent—dioxane. In this solvent, the formation of the triplet states of the dyes is not observed in the absence of oxygen. At the same time, upon flash photolysis of the dyes in an aqueous solution in the presence of serum albumins, photoisomerization is not observed, but (in the absence of oxygen) the formation (with subsequent decay) of the triplet states of the dyes occur. The quenching rate constants of the triplet states of the dyes by molecular oxygen are determined, which are found to be much lower than the diffusion-controlled quenching rate constant. This is possibly explained by the hindered access of oxygen to the dye molecule in the complex with albumin.