The dependence of the photochemically induced reduction of lumiflavin on pH, light intensity, and concentration of sulphydryl compounds

Abstract

Can. J. Chem. 63, 341 1 (1985). The spectral changes which occur on photolysis of flavin solutions containing P-mercaptoethanol (RSH) and lipoamide (L(SH),) are in accord with the formation of dihydroflavin, except at pH values below 3 for RSH. Here an oxygen-stable product is formed, which has X , , , 360 nm and is probably a C-4a substituted dihydroflavin. ' The fraction of flavin present when FlH2 reaches a photostationary state f(F1H2),, is dependent on pH and on the absorbed light intensity I,. At certain pH values the actual photostationary concentration of FlH2 is linear in the concentration of RSH, but for the lipoamide system shows a more complex dependence on concentration. The initial quantum yields are independent of absorbed light intensity in the pH range 7 to 10 for RSH, and increase strongly with pH in the region 7 to 9 for both sulphydryl compounds. In the pH range 5-6 they are linear in sulphydryl concentration. For RSH the results can be explained by a mechanism in which flavin triplet abstracts a hydrogen atom from RSH to form RS. and .FIH radicals, These radicals undergo back reaction to RSH and F1 or form RSSR and FlH2 by one of two mechanisms: (a) formation of RSSR--. which electron transfers to F1 or .FIH or (b) reaction of RS. with .FIH--RSH complexes. The results with L(SH), can be explained similarly. PARMINDER S. SURDHAR et DAVID A. ARMSTRONG. Can. J. Chem. 63, 34 1 1 (1 985). Les changements spectraux qui se produisent lors de la photolyse de solutions de flavines contenant du P-mercaptoCthano1 (RSH) et de la lipoamide (L(SH),) pourraient Ctre expliques par la formation de la dihydroflavine, i I'exception des pH inftrieurs i 3 dans le cas du RSH. Dans ces cas, il y a formation d'un produit qui est stable vis-i-vis de I'oxygtne, qui prtsente un X, , , , i 360 nm et qui est probablement une dihydroflavine substitute en position C-4a. La fraction de flavine qui est prtsente quand la FIH, atteint un Ctat photostationnaire, f(FIH,),,, qui depend du pH et de l'intensitk de la Lumikre absorbte, I,. A certains pH, la concentration photostationnaire reelle de la FlH, varie d'une f a~o n lintaire avec la concentration en RSH; dans le cas du systtme de la lipoamide, on note toutefois une relation plus complexe avec la concentration. Dans le cas du systtme du RSH et i d e s pH allant de 7 a 10, les rendements quantiques initiaux ne dkpendent pas de I'intensitk de la lumikre absorbCe; toutefois, pour chacun des composes sulfurCs et i des pH allant de 7 2 9, ces rendements augmentent rapidement. Aux pH allant de 5 i 6, ils sont IinCaires avec les concentrations en sulfhydryles. Pour le systkme RSH, les rtsultats peuvent Ctre expliquCs par un mecanisme dans lequel 1'Ctat triplet d'une flavine enlkve une hydrogtne du RSH pour former des radicaux RS et . FIH.Ces radicaux subissent des reactions de retpur vers le RSH et la F1 ou ils forment du RSSR et de la FIH, par I'un des deux mtcanismes suivants: (a) formation de RSSR-qui donne des reactions de transferts d'electron conduisant i la F1 ou au .FIH ...

Electron‐transfer Organic and Bioorganic Photochemistry

“…, , 1987. Kinetic analyses of the photoreduction of lumiflavin by 2-mercaptoethanol or lipoamide at various p H were carried out and the mechanistic details were presented (Surdhar and Armstrong, 1985). Photoreactions of 3-methyllumiflavin with a-sulfide-or cy-disulfidesubstituted carboxylic acids results in decarboxylation of the acids accompanied with dihydroflavin-4a-carbon adduct formation, and other photoproducts including the 6-substituted flavins were isolated and characterized; the suggested mechanism involves electron transfer from the acids to the photoexcited flavin with consecutive radical coupling (Eberlain and Powell, 1984).…”