Transient phenomena in the pulse radiolysis of retinyl polyenes. 1. Radical anions

Abstract

The spectra and kinetics of formation and decay of radical anions of a number of retinyl polenes have been studied in methanol and 2-propanol at room temperature, using pulse radiolysis and kinetic spectrophotometry. The bimolecular rate constants for the attachment of solvated electrons, e/sup -//sub MeOH/, to the retinyl polyenes are in the diffusion-controlled limit (8.6 x 10/sup 9/-1.6 x 10/sup 10/ M/sup -1/ s/sup -1/). The radical anions of retinol and retinol acetate have their spectral maxima at 370 to 390 nm, and undergo decay very slowly with second-order kinetics. On the other hand, the radical anions of retinal, retinal n-butylamine Schiff base, and retinoic acid/ester have spectral maxima at 430 to 510 nm, and decay by first-order kinetics in methanol with rate constants in the range 1 x 10/sup 4/ - 1 x 10/sup 6/ s/sup -1/. The decay rates of radical anions of retinal and retinoic acid/ester become considerably longer on going from methanol to less acidic alcohol, 2-propanol, suggesting that protonation by sovent is the major mode of their decay in protic media. In the case of retinal Schiff base, an additional slow process with bimolecular rate constant 9.0 x 10/sup 7/ M/sup -1/ s/sup -1/more » in methanol is observed for the formation of radical anion and is ascribed to the electron-transfer reaction from the methanol radical, CH/sub 2/OH. 6 figures.« less