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Issue 18, 2011
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Laser flash photolysis study on the retinol radical cation in polar solvents

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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˙+ (λmax = 580 nm) and solvated electrons following LFP of retinol. Subsequently, the solvated electrons are rapidly scavenged by retinol to form retinol˙ (λmax = 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. kBr = 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.

Graphical abstract: Laser flash photolysis study on the retinol radical cation in polar solvents

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Publication details

The article was received on 24 May 2011, accepted on 23 Jun 2011 and first published on 23 Jun 2011


Article type: Paper
DOI: 10.1039/C1OB05814B
Org. Biomol. Chem., 2011,9, 6437-6446

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    Laser flash photolysis study on the retinol radical cation in polar solvents

    A. El-Agamey and S. Fukuzumi, Org. Biomol. Chem., 2011, 9, 6437
    DOI: 10.1039/C1OB05814B

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