Pulse radiolysis studies of intermolecular charge transfers involving tryptophan and three-electron-bonded intermediates derived from methionine

Abstract

The oxidation processes of the radiation-generated, three-electron-bonded intermediates AcMet2 [S∴S]+ and AcMet [S∴Br] were investigated by pulse radiolysis via their reactions with tryptophan (TrpH). These intermediates were derived from N-acetyl-methionine amide (N-AcMetNH2) and N-acetyl-methionine methyl ester (N-AcMetOMe). The bimolecular rate constant k of the reaction between each intermediate and l-tryptophan (TrpH) was measured. For N-AcMetNH2, k for the reaction of AcMet2 [S∴S]+ with TrpH were 3.4 × 108 and 2.2 × 108 dm3 mol−1 s−1 at pH = 1 and 4.5, respectively. For N-AcMetOMe, k for the reaction of AcMet2 [S∴S]+ with TrpH were 4.0 × 108 and 2.8 × 108 dm3 mol−1 s−1 at pH 1 and 4.5, respectively. The rate constants for the intermolecular transformation of Met [S∴Br] into TrpH+ or Trp were also estimated. For N-AcMetNH2, k for the reaction of AcMet2 [S∴Br] with TrpH were 2.6 × 108 and 3.3 × 108 dm3 mol−1 s−1 at pH 1 and 4.5, respectively. Related mechanisms were discussed.