PRIMARY INTERMEDIATES IN THE PULSED IRRADIATION OF RETINOIDS

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Abstract

Abstract Laser flash photolysis and pulse radiolysis have led to the characterisation of several shortlived intermediates formed after irradiation of retinoic acid and retinyl acetate in hexane or methanol.

For retinoic acid, the triplet state, wavelength maximum 440 nm, extinction coefficient 7.3 × 104 dm3 mol−1 cm−1, decay constant 6.2 × 105 s−1, is formed with a quantum yield of 0.012 for 347 nm excitation. The radical cation, absorption maximum 590 nm, extinction coefficient ∼7 × 104 dm3mol−1 cm−1, is formed in a biphotonic process. The radical anion, absorption maximum 510nm in hexane, 480 nm in methanol where its extinction coefficient is 1.2 × 105 dm3mol−1 cm−1, appears to decay partially in methanol into another longer-lived neutral radical, wavelength maximum 420 nm, by loss of OH.

For retinyl acetate, the triplet state, absorption maximum 395 nm, extinction coefficient 7.9 × 104dm3mol−1 cm−1, decay constant 1.2 × 106s−1 is formed with a quantum yield of 0.025 for 347 nm excitation. Monophotonic photoelimination of OCOCH3− in methanol produces the retinylic carbenium ion, wavelength maximum 590 nm, whose decay is enhanced by ammonia, k ∼ 2 × 106 dm3 mol−1 s−1 and retarded by water. The radical cation also has a wavelength maximum at 590 nm, its extinction coefficient being ∼ 1.0 × 105 dm3mol1 cm−1. The long-lived transient absorption with maximum at 385 nm, extinction coefficient 1.0 × 105 dm3mol−1 cm−1, obtained from the reaction of the solvated electron with retinyl acetate in methanol may be due to either the radical anion itself or more likely the radical resulting from elimination of OCOCH3− from this anion.

These results suggest that skin photosensitivity caused by retinyl acetate might be greater than that due to retinoic acid.

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