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 α-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.
Highlights
The mechanisms of photosensitized and radiation-induced oxidation of amino acids and peptides have been investigated mainly due to the biological significance of such processes.[1−8] One of the sites primarily attacked by oxidative agents such as short-lived excited states, free radicals, or reactive oxygen species is the thioether moiety of methionine (Met) residues
Met oxidation can cause serious consequences during oxidative stress;[1] despite the numerous studies focused on the one-electron oxidation processes of the methionine residue, some aspects of the process still remain unclear or controversial
Rational to use relatively simple model structures, such as the compounds investigated in this paper, to carry out these complementary time-resolved laser flash photolysis and stationary photochemical irradiations experiments
Summary
The mechanisms of photosensitized and radiation-induced oxidation of amino acids and peptides have been investigated mainly due to the biological significance of such processes.[1−8] One of the sites primarily attacked by oxidative agents such as short-lived excited states, free radicals, or reactive oxygen species is the thioether moiety of methionine (Met) residues. One-electron oxidation of Met-containing peptides and proteins in solution occurs e.g., by using strongly oxidizing hydroxyl radicals (OH) from water radiolysis or through photosensitization using carboxybenzophenone (CB) excited triplets as electron acceptors.[3,6,11] The transients formed in the oxidation of Met-containing peptides by various one-electron oxidants have been well-characterized.[3,6,11−15] The initially formed sulfur radical cation can interact with electron-rich atoms (O, N, or S), yielding two-centered three-electron bonds. There are only a few reports that combine complementary timeresolved and steady-state techniques in the photoinduced and radiation-induced oxidation of Met-containing peptides.[9,10,16] It is, rational to use relatively simple model structures, such as the compounds investigated in this paper (see Figure 1), to carry out these complementary time-resolved laser flash photolysis and stationary photochemical irradiations experiments. All LFP and stationary irradiation experiments were performed in oxygen-free aqueous solutions at neutral pH
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