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JE Roberts, A Cantrell, DJ McGarvey, T Sarna, TG Truscott; Photochemical Studies of Excited State Intermediates of N-Retinylidene-N-Retinylethanolamine (A2-E) . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4523.
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Purpose: To understand the role of A2-E (a major fluorophore of lipofuscin) in the eye by studying its excited state intermediates. Methods: A2-E was synthesized and purified according to the procedure described by Parish et al. [Proc. Natl. Acad. Sci. USA, (1998) 95:14609]. Laser flash photolysis was used as previously described by Truscott et al.[Biochem Soc Trans (1995)23:252] to determine the properties of excited transient intermediates of A2-E. Results: Direct irradiation of A2-E in organic solvents, in the absence of any sensitizer, leads only to a weak transient that is quenched by oxygen (indicating the formation of a triplet). The lifetime for this A2-E triplet state is ∼ 40 us leading to a quantum yield of the triplet for A2-E of < 0.01 (limit of detection). In contrast to direct photolysis, we found that the triplet of A2-E can be more efficiently generated by energy transfer from a sensitizer (anthracene and 1-nitronaphthalene). The generation of triplets in this fashion is common in carotenoids. Discussion: Singlet oxygen is formed from the triplet state of a compound. If the triplet is not efficiently formed then little or no singlet oxygen will be produced. We have now found that the production A2-E triplet is very inefficient and this explains our previous results [Roberts et al. IVOS (2001) 42:S943, 5049] of inefficient singlet oxygen production by A2-E (quantum yield of singlet oxygen = 0.004). Conclusion: The photochemical properties of A2-E have more in common with endogenous carotonoids than with an endogenous phototoxic agent. Like carotenoids, A2-E does not exhibit significant triplet state or singlet oxygen production and behaves as effective singlet quencher (kq = 10+8 mol dm-3 s-1), as effective as Vitamin E).
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