Tryptophan singlet oxygen oxidation

Oxidation is one of the major causes of protein degradation and has been widely studied. The side-chains of histidine (His), methionine (Met), cysteine (Cys), tryptophan (Trp) and tyrosine (Tyr) residues in proteins are potential oxidation sites. The reactive oxygen species include singlet oxygen O2, superoxide radical O2, alkyl peroxide ROOH, hydrogen peroxide H2O2, hydroxy radicals (HO or HOO ), and halide complexes (CLO )." The reactivity of these oxidants is ... 

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 dependence on pH of an extrinsic band in the c.d. spectrum of a lysozyme with tryptophan-62 selectively (2-nitrophenylsulphenyl)ated has been studied. Analysis of the peptides in tryptic hydrolysates of two forms of lysozyme oxidized with singlet oxygen established that tryptophan-62 is destroyed. This was confirmed by the effect exerted by chitin, which protected tryptophan-62 from oxidation. The possible oxidation of other tryptophanyl residues by singlet oxygen was also discussed. 

Tryptophan (free and bound in proteins) is a very oxylabile compound, especially in acid solutions. Free tryptophan and tryptophan residues in proteins are oxidised by reactive oxygen species, by hydrogen peroxide, fatty acid hydroperoxides, sulfoxides, perox-yacids and other oxidation agents and can also react via physical quenching, which results in energy transfer and de-excitation of singlet oxygen. 

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. 

Gracanin M., Hawkins C.L., Pattison D.J., Davies M.J. Singlet-oxygen-mediated amino acid and protein oxidation Formation of tryptophan peroxides and decomposition products. Free Radical Biology Medicine, 47 92-102 (2009). 

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