Formation and spectra of flavin radicals in the presence of β-mercaptoethanol at pH 9 and 10

Surdhar, Parminder S.; Ahmad, Rizwan

doi:10.1139/v84-097

Cited by 8 publications

(9 citation statements)

References 15 publications

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“…The pseudo first-order rate constants at pH 7 were plotted against [Fl] to obtain the value of k5. This rate constant was rate has been observed with unsubstituted flavins (6,16). It may be noted that step [6] is too fast to be seen in our buffered solutions (17).…”

Section: Pulse Radiolysissupporting

confidence: 62%

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The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO₂ and dithiothreitol

Surdhar¹,

Massey²

1986

Can. J. Chem.

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. Can. J. Chem. 64,67 (1986). The one-and two-electron reductions of 2-thioriboflavin with .COT and cyclic disulphide anion of dithiothreitol (DS?.) have been studied by the steady state y and pulse radiolysis techniques. The .COT radical reacted with 2-thioriboflavin to give the neutral semiquinone (.FIH) and the radical anion (.F1-) at pH 5 and 10 respectively. The pK of the .FIH radical was determined to be 7.4. In the case of the anion, the 2-thioriboflavin spectrum is similar in shape to that of FAD radical anion, but red shifted by 40-50 nm. Red shifts are also seen in the neutral .FIH form for the 370-nm peak and 580-nm shoulder. However, in addition, there is strong enhancement of the absorbance at 500 nm. The spectrum of 2-thioriboflavin semiquinone produced in the presence of 2-5 mM dithiothreitol was perturbed, as was observed previously for unsubstituted flavin semiquinones in the presence of sulphydryls. The rate constants for the initial one-electron reduction step viz: [5] F1 + .COT -, .F1-+ C 0 2 were 4.0 ? 0.5 X lo9 M-' s-' and 1.3 & 0.2 X 109 M-' s-' at pH 7 and 10 respectively. The corresponding rate for the reaction of DS;. with 2-thioriboflavin at pH 7 was determined to be 2.4 * 0.2 X lo9 M-Is-'. The continuous production of DS;. radicals by y radiolysis reduced 2-thioriboflavin to the dihydro form, and the flavin was regenerated on the addition of air. The .COT radical also effected a two-electron reduction. However, in this case, if the process was taken beyond the equivalence point, the dihydroflavin spectrum was bleached and the oxidized flavin could not be recovered.

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Section: Pulse Radiolysissupporting

confidence: 62%

“…radical as the reducing species. As in the case of lumiflavin semiquinone, formed in sulphydryl-containing solutions (16,18), the spectrum of the 2-thioriboflavin radical was somewhat perturbed, exhibiting increased absorption in the 400-500 nm region and lesser absorbance above 540 nm.…”

Section: Pulse Radiolysismentioning

confidence: 97%

The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO₂ and dithiothreitol

Surdhar¹,

Massey²

1986

Can. J. Chem.

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“…Electron transfer from LipSS .À to FAD or lumiflavin (one of the RF photodegradation products) has been reported. [50,51] Furthermore, oxidizing LipSS . can also be converted into reducing LipSS .À in the presence of thiolate.…”

Section: Electron Transfer From Lipss Radical Anions To Riboflavin Anmentioning

confidence: 99%

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B₂)

Bucher

Sander

2004

ChemPhysChem

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As a powerful natural antioxidant, lipoic acid (LipSS) and its reduced form dihydrolipoic acid (DHLA) exert significant antioxidant activities in vivo and in vitro by deactivation of reactive oxygen and nitrogen species (ROS and RNS). In this study the riboflavin (RF, vitamin B2) sensitized UVA and visible-light irradiation of LipSS and DHLA was studied employing continuous irradiation, fluorescence spectroscopy, and laser flash photolysis (LFP). Our results indicate that LipSS and DHLA quench both the singlet state (1RF*) and the triplet state (3RF*) of RF by electron transfer to produce the riboflavin semiquinone radical (RFH.) and the radical cation of LipSS and DHLA, respectively. The radical cation of DHLA is rapidly deprotonated twice to yield a reducing species, the radical anion of LipSS (LipSS.-). When D2O was used as solvent, it was confirmed that the reaction of LipSS with 3RF* consists of a simple electron-transfer step, while loss of hydrogen occurs in the case of DHLA oxidation. Due to the strong absorption of RFH. and the riboflavin ground state, the absorption of the radical cation and the radical anion of LipSS can not be observed directly by LFP. N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and N,N,N',N'-tetramethyl benzidine (TMB) were added as probes to the system. In the case of LipSS, the addition resulted in the formation of the radical cation of TMPD or TMB by quenching of the LipSS radical cation. If DHLA is the reducing substrate, no formation of probe radical cation is observed. This confirms that LipSS.- is produced by riboflavin photosensitization, and that there is no oxidizing species produced after DHLA oxidization.

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“…The evidence for this stems from recent studies of flavin semiquinone spectra in the presence of RSH or RSmolecules (26,27). In the equations which follow we write…”

Section: The Mechanismmentioning

confidence: 98%

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

Surdhar¹

1985

Can. J. Chem.

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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 ...

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Formation and spectra of flavin radicals in the presence of β-mercaptoethanol at pH 9 and 10

Cited by 8 publications

References 15 publications

The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO₂ and dithiothreitol

The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO₂ and dithiothreitol

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B₂)

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

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Formation and spectra of flavin radicals in the presence of β-mercaptoethanol at pH 9 and 10

Cited by 8 publications

References 15 publications

The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO2 and dithiothreitol

The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO2 and dithiothreitol

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B2)

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

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The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO₂ and dithiothreitol

The one- and two-electron reduction of 2-thioriboflavin by radical anions of CO₂ and dithiothreitol

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B₂)