Roles of hydrophilicities and hydrophobicities of dye and sacrificial electron donor on the photochemical pathway

Relative rates of the photosensitized production of singlet oxygen (1O2) and of superoxide (O2
• —) were determined using different couples of dyes and sacrificial electron donors (SEDs) of either high or low hydrophobicities. Such rates were measured in the absence and presence of single unilamellar vesicles (SUVs) with 9DMPC:1DMPA mol ratio composition. The dyes aluminum phthalocyanine tetrasulfonate (AlPcS4) and pheophorbide-a (PHEO) were used as hydrophilic and hydrophobic photosensitizers, respectively. Xanthine (X) and glutathione (GSH) were used as hydrophobic and hydrophilic SEDs, respectively. The presence of SUVs in the aqueous sample produces the physical separation or encounter of SEDs and photosensitizers according to their membrane binding constants. When both the SED and the photosensitizer are localized within the same phase, a strong decrease in the rate of 1O2 formation, united to a strong increase in the rate of O2
• — formation, is observed, relative to when both of these species are localized in different phases. The lipid phase is always present in the biological milieu. Thus, the use of a hydrophobic couple of both dye and SED (as in the case of X and PHEO), as well as a hydrophilic couple of both dye and SED (as in the case of GSH and AlPcS4), should strongly favor the Type I mechanism over the Type II. Since only a small number of hydroxyl radicals are needed to initiate a chain reaction of phospholipid peroxidation, the latter could be more toxic to the tumor tissue than peroxidation by a much higher concentration of singlet oxygen molecules.

► Role of sacrificial electron donor (SED) in the photochemistry of dyes was studied.
► Study was carried out in the absence or presence of small unilamellar vesicles.
► If SED and dye are at the same phase, a strong decrease in 1O2 formation is observed.
► If SED and dye are at the same phase, a strong increase in O2
• — formation occurs.
► If SED and dye are at different phases, superoxide production is strongly inhibited.