title: Laser flash photolysis study on the retinol radical cation in polar solvents.
abstract: Laser flash photolysis (LFP) of retinol in argon-saturated methanol gives rise to a transient at 580 nm (transient A). Formation of transient A is accompanied by a transient growth at 370 nm. The rate of this growth is retinol concentration-dependent. The transient growth at 370 nm was removed in the presence of N(2)O, which is known to scavenge solvated electrons. These results can be interpreted by formation of retinol˙(+) (λ(max) = 580 nm) and solvated electrons following LFP of retinol. Subsequently, the solvated electrons are rapidly scavenged by retinol to form retinol˙(-) (λ(max) = 370 nm in methanol). On the other hand, transient A is not ascribed to the retinyl cation, as was previously proposed, because the retinyl cation, generated from LFP of retinyl acetate, and transient A show different reactivities towards halide ions (e.g. k(Br) = 1.7 × 10(9) and 1.51 × 10(10) M(-1) s(-1) respectively, in acetonitrile). After demonstrating the identity of transient A as retinol˙(+), its reactions with carotenoids were examined in air-saturated polar solvents. In the presence of carotenoids, an enhancement in the decay of retinol˙(+) was observed and was accompanied by formation of the corresponding carotenoid radical cations via electron transfer from carotenoids to retinol˙(+). Furthermore, the reactivity of retinol˙(+) towards pyridine derivatives was investigated in air-saturated polar solvents. It was found that the decay of retinol˙(+) was accelerated with concomitant formation, with the same rate, of a transient at 370 nm. Similar observations were obtained with increasing pH of air-saturated aqueous 2% Triton X-100 of retinol˙(+). The 370 nm (or 380 nm in the case of Triton X-100) transient is attributed to the base adducts or deprotonated neutral radicals. On the basis of these results, the reactivities of the retinyl cation and retinol˙(+) are compared and the consequences of retinol˙(+) formation within biological environments are discussed.
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka, Japan. a_el_agamey@yahoo.co.uk
authors.forename: Ali
authors.initials: A
authors.lastname: El-Agamey
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D002412:Cations / Q000737:chemistry; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry*; D000432:Methanol; D009609:Nitrous Oxide / Q000737:chemistry; D010782:Photolysis; D014801:Vitamin A / Q000737:chemistry*
chemical_list: D002412:Cations; D005609:Free Radicals; D014801:Vitamin A; D009609:Nitrous Oxide; D000432:Methanol