Premium
TRANSIENT PHENOMENA IN THE PULSE RADIOLYSIS OF RETINYL POLYENES—7. RADICAL ANIONS OF VITAMIN A AND ITS DERIVATIVES
Author(s) -
Bhattacharyya K.,
Bobrowski K.,
Rajadurai S.,
Das P. K.
Publication year - 1988
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1988.tb02697.x
Subject(s) - chemistry , radiolysis , radical ion , photochemistry , tetrahydrofuran , protonation , solvated electron , deprotonation , medicinal chemistry , radical , electron transfer , ion , solvent , organic chemistry
— Upon e ‐ ‐pulse irradiation in nonprotic solvents, all‐ trans retinol (ROH) and retinylmethyl ether (ROMe) form transient species (τ= 0.5–7μs, λ max =575–590 nm) identifiable as radical anions. Similar species are also formed upon laser pulse photoexcitation of these retinyl derivatives in the presence of N,N‐dimethylaniline in acetonitrile. In contrast, electron transfer or attachment to all‐ trans retinyl acetate (ROAc) and palmitate (ROPa) results in ‘instantaneous’ loss of carboxylate anions from electron adducts giving the retinylmethyl radical (R‐, λ max = 395 nm, τ k > 100 μ,s); the radical anions in these cases are too short‐lived to be detected by nanosecond pulse radiolysis. The lifetimes of radical anions of ROH and ROMe are very sensitive to water and alcohols (e.g. k q = 10 7 M ‐1 s ‐1 with methanol as quencher for ROH ‐ in tetrahydrofuran). Based on these findings, the spectral dissimilarity of the one‐electron reduction products from ROH and ROAc in alcohols and aqueous micelles becomes explainable in terms of fast formation of protonated radical anions (RH(OH), τ 1/2 , > 100 μs, λ max =370–375 nm) in the case of ROH and of retinylmethyl radical via loss of AcO ‐ from radical anion in the case of ROAc. In tetrahydrofuran, the complexation of ROH ‐ with cations such as Na + and Bu 4 N + affects the relative importance of its major decay modes, namely, protonation and dehydroxylation, the latter process being significantly enhanced by the presence of Na + .