4-Carboxybenzophenone-sensitized photooxidation of sulfur-containing amino acids. Nanosecond laser flash photolysis and pulse radiolysis studies. [Pulsed radiolysis]

Journal Article · Wed Dec 16 04:00:00 EST 1992 · Journal of the American Chemical Society; (United States)

DOI:https://doi.org/10.1021/ja00052a025· OSTI ID:6566394

Bobrowski, K; Marciniak, B; Hug, G L ^[1]

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1. Univ. of Notre Dame, IN (United States)

Sulfur-containing amino acids were oxidized via photosensitization by 4-carboxybenzophenone (CB) in neutral aqueous solutions. The mechanism of this reaction was investigated with flash photolysis and pulse radiolysis techniques. The rate constants were determined for the quenching of the CB triplet state by 12 amino acids (with variable relative location and number of terminal functions COOH and NH[sub 2] with respect to the sulfur atom) and were found to be 1.8 [times] 10[sup 8]-2.9 [times] 10[sup 9] M[sup [minus]1] s[sup [minus]1]. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet state, the ketyl radical of CB, the radical anion of CB, and the (S[therefore]S)[sup +] radical cations of some of the amino acids. The presence of the radical ions showed the nature of the quenching process to be electron-transfer in character. Two temporally distinct processes were observed for ketyl radical formation. A fast component occurred on a nanosecond time scale. It is ascribed to electron transfer from the sulfur atom to the triplet state of CB followed by (1) the diffusion apart of the charge-transfer (CT) complex and (2) the intramolecular proton transfer within CT complex. The first process was the more efficient one and led to the formation of sulfur-centered radical cations and ketyl radical anions which undergo fast protonation. A slower formation of ketyl radicals occurred on a microsecond time scale and is characterized by pseudo-first-order rate constants. This dark reaction is assigned to the one-electron reduction of CB by the [alpha]-aminoalkyl radicals produced from the free-radical cation of the amino acids as a result of intramolecular electron transfer from the carboxyl group to the sulfur-centered radical cation followed by decarboxylation. A detailed mechanism for the CB-sensitized photooxidation of sulfur-containing amino acids is proposed and discussed. 40 refs., 8 figs., 2 tabs.

• OSTI ID: 6566394
• Journal Information: Journal of the American Chemical Society; (United States), Journal Name: Journal of the American Chemical Society; (United States) Vol. 114:26; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
400500* -- Photochemistry
400600 -- Radiation Chemistry
AMINO ACIDS
AQUEOUS SOLUTIONS
CARBOXYLIC ACIDS
CHARGED PARTICLES
CHEMICAL RADIATION EFFECTS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DECARBOXYLATION
DECOMPOSITION
DIFFUSION
DISPERSIONS
ELECTRON TRANSFER
IONS
KINETICS
MIXTURES
ORGANIC ACIDS
ORGANIC COMPOUNDS
OXIDATION
PHOTOCHEMICAL REACTIONS
PHOTOLYSIS
QUENCHING
RADIATION EFFECTS
RADICALS
RADIOLYSIS
REACTION KINETICS
REDUCTION
RESOLUTION
SOLUTIONS
SULFUR COMPOUNDS
TIME RESOLUTION
TIMING PROPERTIES
YIELDS