Premium
THE EFFECT OF 8α‐SUBSTITUTION ON FLAVIN TRIPLET STATE AND SEMIQUINONE PROPERTIES AS INVESTIGATED BY FLASH PHOTOLYSIS *
Author(s) -
Edmondson Dale E.,
Rizzuto Frank,
Tollin Gordon
Publication year - 1977
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.1751-1097.1977.tb09169.x
Subject(s) - flavin group , chemistry , flash photolysis , semiquinone , photochemistry , riboflavin , yield (engineering) , oxygen , triplet state , kinetics , reaction rate constant , stereochemistry , quinone , organic chemistry , molecule , biochemistry , physics , materials science , quantum mechanics , metallurgy , enzyme
— Flash photolysis techniques have been used to study the effect of 8α‐substitution on flavin triplet state formation and decay and on the properties of neutral and anionic serniquinones. Compared with riboflavin, the N(1) and N(3) isomers of 8α‐histidylriboflavin show a lower triplet yield (˜10%) and a faster rate of decay (˜ 4‐Cfold). Acetylation of the histidyl a‐amino groups and of the flavin ribityl side chain results in a 2‐fold increase in triplet yield and a 2‐fold slower rate of decay. The yield of neutral 8α‐substituted flavin semiquinones upon flash photolysis in the presence of EDTA was approximately 50% that given by riboflavin. These substituted flavin neutral semiquinones dismutated at a rate 2–3 times slower than the corresponding unsubstituted form, although the anionic semiquinones dismutated at approximately the same rate. In the presence of oxygen, the kinetics of semiquinone decay changed from second order to pseudo‐first order upon raising the pH, thus showing anionic semiquinone oxidation as seen previously with unsnbstituted flavins. The pK values for the ionization of the neutral 8α‐substituted Aavin semiquinones are 1–1.5 units lower than the unsubstituted form. The anionic 8α‐substituted flavin semiquinones react with oxygen at a rate 2–10 times more slowly than does the riboflavin form. Such alterations in properties probably reflect the electron‐withdrawing effect of the 8α‐substituents on the flavin ring system.