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 N₂O, 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 × 10⁹ and 1.51 × 10¹⁰ 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.