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Evidence for Heterolytic Cleavage of C-S Bonds in the Photolysis of 1,3,5-Trithianes

Hug G. L., Janeba-Bartoszewicz E., Filipiak P., Pedzinski T., Kozubek H., Marciniak B.

Polish Journal of Chemistry

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2008

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Vol. 82, nr 4

883-892

EN

The homolytic/heterolytic nature of photolytic C-S bond cleavage was studied in 1,3,5-trithianes. The mechanism of photolysis was refined from previous studies. First, evidence was presented for the existence of a precursor of the biradical-like transient (I) which it self was identified in previous studies. Second, the nature of I was further clarified through methanol-scavenging experiments where the results could be interpreted as lending credibility to the notion that I has significant bipolar character. Kinetic and spectral analyses of transient absorptions, following laser excitation of the trithianes, showed that I was reacting with methanol. Complementary steady-state photolytic quantum yields supported this finding, and additional, but unidentified, stable products from irradiations in methanol were seen in the HPLC. The formation of these products was interpreted as likely arising from a nucleophilic attack of methanol at the carbocationic end of the bipolar structure of I.

2

pH effects on the photooxidation of methionine derivatives by the 4-carboxybenzophenone triplet state

Hug G., Bobrowski K., Kozubek H., Marciniak B.

Nukleonika

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2000

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Vol. 45, nr 1

63-71

EN

Reduction of the triplet state of 4-carboxybenzophenone (CB) by a series of five substituted methionines was monitored under laser flash photolysis conditions in aqueous solution. Spectral resolution techniques were employed to determine initial quantum yields of the photoproducts. Branching ratios were evaluated for the decay of the charge-transfer complex by the competing processes of back electron transfer, proton transfer, and escape of radical ions. The relative prominence of these processes was discussed in terms of the proton-transfer tendencies of the nominal sulfur-radical-cationic species. A scheme was proposed to account for the pH effects observed. The kinetic equations were solved with a minimum of assumptions, and simulations were performed that qualitatively support the proposed scheme.

3

Electron Spin Resonance study of radiation-induced radicals in polycrystalline oligopeptides containing methionine

Strzelczak G., Bobrowski K., Kozubek H., Michalik J.

Nukleonika

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2000

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Vol. 45, nr 1

93-100

EN

The radicals formed on g-radiolysis of oligopeptides containing methionine and glycine residues in the polycrystalline state at 77 K and on subsequent warming over the temperature range of 77-293 K were identified using the electron spin resonance (ESR) technique. In all oligopeptides studied, anion peptide radicals and monomeric sulfur radical cations over the temperature range of 77-95 K and decarboxylation and deamination radicals over the temperature range of 180-250 K were detected. In addition, in oligopeptides containing adjacent methionyl residues, three-electron-bonded dimeric sulfur radical cations (S\S)+ were detected at 110 K. In oligopeptides containing N-terminal methionine residue, H-abstraction radicals in the side chain of methionine residue were observed over the temperature range of 180-250 K. H-abstraction radicals at the a-carbon of the C-terminal amino acid residue were detected at 293 K in most of oligopeptides studied, in contrast to Met-Gly-Met and Met-Gly-Met-Met peptides were H-abstraction radicals at the alfa-carbon of the internal glycine residue were obserwed.