Singlet-oxygen Generation from A2E¶

Abstract

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,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimeth-yl-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-d 6 , acetone-d 6 , 80% cyclohexane-d 12-20% acetone-d 6 (vol/vol) and hexafluoroben-zene. Using 420 nm irradiation and assuming a singlet-oxygen quantum yield of 0.60 6 0.12 for TPP, the singlet-oxygen quantum yields were 0.8 6 0.3 3 10 23 , 1.2 6 0.4 3 10 23 , 2 6 1 3 10 23 and 4 6 1 3 10 23 , respectively. In acetone-d 6 , the quantum yields were smaller at longer wavelengths, with values of 0.3 6 0.1 3 10 23 and 0.4 6 0.2 3 10 23 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.