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Early Events of Photosensitized Oxidation of Sulfur-Containing Amino Acids Studied by Laser Flash Photolysis and Mass Spectrometry.

ABSTRACT: The mechanism of photooxidation of methionine (N-Ac-Met-NH-CH3, 1) and methyl-cysteine (N-Ac-MeCys-NH-CH3, 2) analogues by 3-carboxybenzophenone triplet (3CB*) in neutral aqueous solution was studied using techniques of nanosecond laser flash photolysis and steady-state photolysis. The short-lived transients derived from 3CB and sulfur-containing amino acids were identified, and their quantum yields and kinetics of formation and decay were determined. The stable photoproducts were analyzed using liquid chromatography coupled with high-resolution mass spectrometry. Substantial differences in the mechanisms were found for methionine and S-methyl-cysteine analogues for both primary and secondary photoreactions. A new secondary reaction channel (back hydrogen atom transfer from the ketyl radical to the carbon-centered ?-thioalkyl radical yielding reactants in the ground states) was suggested. The detailed mechanisms of 3CB* sensitized photooxidation of 1 and 2 are proposed and discussed.

SUBMITTER: Pedzinski T

PROVIDER: S-EPMC7498160 | biostudies-literature | 2020 Sep

REPOSITORIES: biostudies-literature

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Early Events of Photosensitized Oxidation of Sulfur-Containing Amino Acids Studied by Laser Flash Photolysis and Mass Spectrometry.

Pedzinski Tomasz T Grzyb Katarzyna K Kaźmierczak Franciszek F Frański Rafał R Filipiak Piotr P Marciniak Bronislaw B

The journal of physical chemistry. B 20200821 35

The mechanism of photooxidation of methionine (N-Ac-Met-NH-CH3, 1) and methyl-cysteine (N-Ac-MeCys-NH-CH3, 2) analogues by 3-carboxybenzophenone triplet (3CB*) in neutral aqueous solution was studied using techniques of nanosecond laser flash photolysis and steady-state photolysis. The short-lived transients derived from 3CB and sulfur-containing amino acids were identified, and their quantum yields and kinetics of formation and decay were determined. The stab ...[more]

PMID: 32790392

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Project description:p-Cresol methylhydroxylase, a heterodimer consisting of one flavoprotein subunit and one cytochrome c subunit, may be resolved into its subunits, and the holoenzyme may then be fully reconstituted from the pure subunits. In the present study we have characterized the reduction kinetics of the intact enzyme and its subunits, by using exogenous 5-deazariboflavin semiquinone radical generated in the presence of EDTA by the laser-flash-photolysis technique. Under anaerobic conditions the 5-deazariboflavin semiquinone radical reacts rapidly with the native enzyme with a rate constant approaching that of a diffusion-controlled reaction (k = 2.8 X 10(9) M-1 X s-1). Time-resolved difference spectra at pH 7.6 indicate that both flavin and haem are reduced initially by the deazariboflavin semiquinone radical, followed by an additional slower intramolecular electron transfer (k = 220 s-1) from the endogenous neutral flavin semiquinone radical to the oxidized haem moiety of the native enzyme. During the steady-state photochemical titration of the native enzyme at pH 7.6 with deazariboflavin semiquinone radical generated by light-irradiation the haem appeared to be reduced before the protein-bound flavin and was followed by the formation of the protein-bound anionic flavin radical. This result suggests that the redox potential of the haem is higher than that of the flavin, and that deprotonation of the flavin neutral radical occurred during the photochemical titration. Reduction kinetics of the flavoprotein and cytochrome subunits were also investigated by laser-flash photolysis. The protein-bound flavin of the isolated flavin subunit was reduced rapidly by the deazariboflavin semiquinone radical (k = 2.2 X 10(9) M-1 X s-1), as was the haem of the pure cytochrome c subunit (k = 3.7 X 10(9) M-1 X s-1). Flash-induced difference spectra obtained for the flavoprotein and cytochrome subunits at pH 7.6 were consistent with the formation of neutral flavin semiquinone radical and reduced haem, respectively. Investigation of the kinetic properties of the neutral flavin semiquinone radical of the flavoprotein subunit at pH 7.6 and at longer times (up to 5s) were consistent with a slow first-order deprotonation reaction (k = 1 s-1) of the neutral radical to its anionic form.

Dataset Information

Early Events of Photosensitized Oxidation of Sulfur-Containing Amino Acids Studied by Laser Flash Photolysis and Mass Spectrometry.

Publications

Early Events of Photosensitized Oxidation of Sulfur-Containing Amino Acids Studied by Laser Flash Photolysis and Mass Spectrometry.

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