Singlet-oxygen generation from A2E

Singlet-oxygen generation was measured in solutions containing equilibrium mixtures of the retinal lipofuscins, 2-[2, 6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)- 1E,3E,5E,7Eoctatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E-hexatrienyl]-pyridinium (A2E) and double bond isomer of A2E (iso-A2E), using steady-state irradiation and using cholesterol as a singlet-oxygen trap. The amount of singlet oxygen generated by equilibrium mixtures of A2E and iso-A2E was compared with that generated by tetraphenylporphine (TPP) under the same conditions. Studies were carried out in ethanol-d6, acetone-d6, 80% cyclohexane-d(12)-20% acetone-d6 (vol/vol) and hexafluorobenzene. Using 420 nm irradiation and assuming a singlet-oxygen quantum yield of 0.60 +/- 0.12 for TPP, the singlet-oxygen quantum yields were 0.8 +/- 0.3 x 10(-3), 1.2 +/- 0.4 x 10(-3), 2 +/- 1 x 10(-3) and 4 + 1 x 10(-3), respectively. In acetone-d6, the quantum yields were smaller at longer wavelengths, with values of 0.3 +/- 0.1 x 10(-3) and 0.4 +/- 0.2 x 10(-3) at 461 and 493 nm, respectively. Singlet-oxygen generation was greatest in solvents with the lowest dielectric constants. In view of the relatively small quantum yields, the contribution of singlet-oxygen generation to the phototoxic properties of A2E and of iso-A2E will require further study.