- Photooxidation of Methionine Derivatives by the 4-Carboxybenzophenone Triplet State in Aqueous Solution. Intracomplex Proton Transfer Involving the Amino Group
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Oxidation of the triplet state of 4-carboxybenzophenone (CB) by a series of five substituted methionines and three methionine-containing dipeptides was monitored under laser flash photolysis conditions in aqueous solution. Spectral resolution techniques were employed to follow the concentration profiles of the intermediates formed from the quenching events. From these concentration profiles, quantum yields for the intermediates were determined. 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 systematic decrease was observed in the quantum yields for the escape of radical ions along with a correlated increase in the proton-transfer yields. The enhanced propensity of the sulfur radical cations to deprotonate is due to deprotonation at the carbons adjacent to the sulfur-cationic site and at the unsubstituted amino groups when present. This scheme was supported by an observed decrease in the yields of dimeric sulfur radical cations with an increase in the electron-withdrawing abilities of the substituents, making the radical-cationic species stronger acids. The involvement of protons on the amino groups was implicated by the correlation of the quantum yields of ketyl radical formation in the photo-chemistry experiments with the rate constants for the reaction of the CB radical anion with the sulfur-containing substrates in pulse radiolysis experiments.
- Hug, Gordon L.,Bobrowski, Krzysztof,Kozubek, Halina,Marciniak, Bronislaw
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p. 785 - 796
(2007/10/03)
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- Side chain fragmentation of N-terminal threonine or serine residue induced through intramolecular proton transfer to hydroxy sulfuranyl radical formed at neighboring methionine in dipeptides
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The reaction of hydroxyl radicals with Ser-Met and Thr-Met at slightly acidic to neutral pH results in the side chain fragmentation of the Ser and the Thr moiety into formaldehyde and acetaldehyde, respectively. The efficiency of this process depends on the concentration of the peptide and protons with maximum yields at low peptide concentrations at naar neutral pH. Significantly less aldehyde formation is observed for the reaction of hydroxyl radicals with AlaMet, Val-Met, Gly-Ser-Met, Met-Ser, Gly-Met-Ser, Ser-Leu, Gly-Thr-Met, and Gly-Met-Thr. These results indicate that the formation of aldehyde requires (i) an N-terminal Ser or Thr residue and (ii) the presence of Met in the sequence. The underlying mechanism involves an intramolecular proton transfer from the protonated N-terminal amino group to an initially hydroxy sulfuranyl radical at the Met residue. This process leads to the elimination of water and the simultaneous formation of a three-electron-bonded [>S∴NH2]+-peptide intermediate which absorbs at λmax = 185 and has been identified by pulse radiolysis. This intermediate decays with t1/2 = 310 ns into aldehyde and an α-amino radical of the structure H2N-C·H-C(=O)NH-peptide, which has been identified by ESR spectroscopy. Mechanistically, the latter process involves the formation of an intermediate nitrogen-centered radical cation which undergose subsequent heterolytic scission of the Cα-Cβ bond of the Ser or Thr side chain, respectively. One-electron oxidation of Thr-Met by SO4·- at slightly acidic pH (≈5.5-6) results in significantly lower yields of acetaldehyde as compared to the hydroxyl radical initiated process, indicating the importance of the intermediary formed hydroxy sulfuranyl radical. It is proposed, however, that generally sulfur-centered radical cations, derived through one-electron oxidation of the methionine sulfur, might convert into hydroxy sulfuranyl radicals which subsequently undergo the proton-transfer process with adjacent N-terminal Ser or Thr residues.
- Sch?neich, Christian,Zhao, Fang,Madden, Keith P.,Bobrowski, Krzysztof
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p. 4641 - 4652
(2007/10/02)
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- FORMATION CONSTANTS OF SILVER(I) COMPLEXES OF SOME SULPHUR-CONTAINING DIPEPTIDES AND VALYLVALINE
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Formation constants at 25 deg C and l = 0.10 mol dm-3 (KNO3) have been determined for the complexes of AgI with a range of nine dipeptides which incorporate side-chains containing one (glycylmethionine and methionylglycine) or two sulphur donor atoms.In the latter case dipeptides formed from amino acids of the same and of different chiralities were studied (e.g.L-methionyl-L-methionine and L-methionyl-D-methionine).The results are compared with those for valylvaline.Values for the formation constants are interpreted in terms of the preferred conformations of the dipeptides, and the tendency for AgI to bond to S-donor atoms or to adopt linear co-ordination through the formation of dimeric complexes.
- Lyons, Anthony Q.,Pettit, Leslie D.
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p. 2305 - 2308
(2007/10/02)
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