Photooxidation of Methionine Derivatives by the 4‐Carboxybenzophenone Triplet State in Aqueous Solution. Intracomplex Proton Transfer Involving the Amino Group

Abstract

Abstract— 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 depro‐tonate 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 photochemistry experiments with the rate constants for the reaction of the CB radical anion with the sulfur‐containing substrates in pulse radiolysis experiments.

Substituent‐Dependent Reactivity in the Photodimerization of N‐Substituted Dibenz[b,f]azepines

“…The fluorescence spectra were identical for concentrations between 10 À2 and 10 À5 mol dm À3 in methanol or between 10 À2 and 10 À6 mol dm À3 in cyclohexane, that is, an emission from potential excited-state aggregates was not observed for these two compounds. At À196 8C, an emission as a result of fluorescence (1±14, 16,19,20,22) and/or phosphorescence (4±7) was observed in ethanol. Examples are shown in Figure 1 b for 4 and 7.…”

Phototransformation of 4-phenoxyphenol sensitised by 4-carboxybenzophenone: Evidence of new photochemical pathways in the bulk aqueous phase and on the surface of aerosol deliquescent particles

“…where p°(λ) is the incident spectral photon flux density of the lamp (Figure 1 (9) are reported in the literature (Hug et al, 1998). Because * 3 CBP Φ was determined from R 1 in equation (7), a first possibility is that 4PP degradation by 3 CBP* is less effective than here hypothesised due to additional side-reactions.…”

Photoinitiating free‐radical polymerization electron‐transfer pairs applying amino acids and sulfur‐containing amino acids as electron donors

“…This additional absorption band appears at a red region of the transient absorption spectra with the maximum at about 500 nm. According to Hug et al 18 this transient can be associated with the dimeric cation radical (S І S) ϩ that is formed with a two-center, three-electron bond between the monomeric sulfur radical cation ϾS ⅐ ϩ and the sulfur of unreacted methionine. The presence of absorption bands at 420 and about 500 nm explicitly suggests not only the electron-transfer reaction between xanthene dye and methionine but also that the electron transfer occurs from methionine sulfur to the excited triplet state of xanthene dye.…”