Tryptophan reaction with singlet oxygen

Pyrene can sensitize the photooxidation of 1,3-diphenylisobenzofuran (72) in DTAC micellar solutions 61>. The reaction involves sensitization of singlet oxygen by pyrene which diffuses into another micelle and reacts with (72). Indole and tryptophan, which also react with singlet oxygen, quench the above reaction in ethanol solutions. However, in micellar systems they enhance the rate of reaction. Because of the high local concentrations of the quencher, the pyrene excited state is quenched by indole and tryptophan which leads to the photooxidation of (72) by a Type I process. 

Histidine is susceptible to oxidation in the presence of metals, primarily by reaction with singlet oxygen, and this constitutes a major cause of enzyme degradation. Both histidine and tryptophan are highly susceptible to photooxidation. 

The dye-sensitized photo-oxidation of 1,3-diphenylisobenzofuran has been developed as a teaching experiment.286 1 The production of singlet oxygen by thiazine dye photosensitization as measured by the rate of photo-oxidation of tryptophan has been found to be very pH sensitive. The effect was shown to be related to the pJ5Ta s of the triplet dyes.2866 The occurrence of 2 in the photodynamic oxidations of guanosine,287 the reaction rates of bilirubin with 2 in the 874 877 878... 

The main products of tryptophan oxidation with singlet oxygen are a dioxethane derivative, formed via 1,4-cycloaddition and a hydroperoxide at C-3. Subsequent decomposition of these intermediates yields N-formylkynurenine, whereas their ring closure leads to cis- and trans-isomers of 3a-hydroxypyrroloindoles, that is, 3a-hydroxy-l,2,3,3,8,8a-hexahydropyrrolo[2,3-fo]indole-2-carboxylic acids and 3a,8a-dihydroxypyrroloindoles. The simplified reaction sequences are given in Figure 2.38. 

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