The effect of medium polarity on the hematoporphyrin-sensitized photooxidation of L-tryptophan.

Abstract

Abstract The 83 μM hematoporphyrin (HP)‐sensitized photooxidation of 0.1 mM tryptophan in aqueous solution buffered at pH 7.4 or in binary mixtures of phosphate buffer and organic solvents of higher (formamide) or lower (N,N‐dimethylformamide, methanol, ethanol, tetrahydrofuran) polarity proceeds by a pure singlet oxygen (1O2) mechanism as suggested by azide quenching experiments, the rate‐enhancing action of deuterated solvents, and the lack of any significant reaction between triplet HP and tryptophan. Both the first‐order rate constant of the photoprocess and the photooxidation quantum yield (φ= 0.011 in phosphate buffer at pH 7.4) increase when the medium polarity is increased (e.g. φ= 0.024 in 90% formamide); this results mainly from the greater quantum yield of 1O2 generation and the longer lifetime of 1O2. The intrinsic reactivity of 1O2 with tryptophan is independent of formamide concentration. A moderate decrease in the medium polarity (e.g. in the range 0‐30% methanol) enhances the efficiency of tryptophan photooxidation (φ= 0.014 in 30% methanol) as a result of the enhanced quantum yields of triplet HP and 1O2 formation. In contrast, the overall photooxidation rate is depressed at high concentrations of low‐polarity organic solvents (e.g. φ= 0.0039 in 90% methanol) due to a 5.5‐fold drop of the rate constant for the 1O2‐tryptophan reaction which counteracts the enhancement of the lifetime and quantum yield of triplet HP and 1O2. The solvent composition also affects the equilibria between monomeric and multimeric forms of HP. However, under our experimental conditions, the aggregation state of HP appears to exert only a minor influence on the efficiency of tryptophan photooxidation.