A Study of the Methylene Blue‐sensitized Oxidation of Amino Acids

Abstract

A study has been made of the effects of eight amino acids and a group of compounds related to tryptophan upon the photofading of methylene blue under anaerobic conditions and upon the excited species generated by flash photolysis of the dye. These effects are discussed in relation to the ease of sensitized photooxidation of the amino acids by methylene blue, in order to test the hydrogen-abstraction mechanism proposed for this reaction.In general, amino acids susceptible to sensitized oxidation also act as reducing agents in the photoreduction of methylene blue, while the insensitive compounds do not reduce the lightexcited dye molecule. Methionine, histidine and tyrosine are exceptions, but this is probably due to our choice of pH for the photofading experiments.The flash-photolysis experiments give no clear-cut evidence for a reaction between amino acid and dye triplet excited state leading to an enhanced yield of the semi-reduced radical, thought to be an intermediate of the photofading reaction. The lifetime of the semi-oxidized radical is reduced by many of the amino acids, and this may be the reactive intermediate in their oxidation.

Sensitizer‐induced Conformational Changes in Lens Crystallin—ii. Photodynamic Action of Riboflavin on Bovine Α‐crystallin

“…Under aerobic condition, two different classes of Trp residues appear to be photooxidized (Fig. 2), one that is rapidly oxidized by the sensitizer-substrate complexation (Knowles and Gurnani, 1972;Foote, 1976;Silva and Gaule, 1977;Spikes, 1977;Amagasa, 1981) and one that is slowly oxidized by the diffusable active species of oxygen, namely, O,, *OH, H202, etc. Under anaerobic condition, these species are not supposed to form, and the oxidation of only the former class of Trp proceeds by faster kinetics via complex formation (type I); the slight bending in the curve after 80 min of irradiation is most probably due to some residual (dissolved) oxygen in the reacting cell.…”

Laser Photolysis Study of Triplet Eosin and Thionine Reactions in Photosensitized Oxidations

“…The table also includes some previously published values. A number of triplet quenching rate constants for methylene blue (closely related to thionine) have also been published recently (Knowles and Gurnani, 1972;Nilsson et al, 1972).…”

A Biochemical Screening Test for the Photosensitizing Potential of Drugs and Disinfectants

“…The amino acids tyrosine and histidine, which are known to be susceptible to photosensitized modification (Knowles and Gurnani, 1972;Spikes and Mac-Knight, 1970;Schothorst et al, 1972;Grossweiner, 1976) were not transformed by irradiation in the presence of chlorpromazine, bithionol, or sulfanilamide.…”

“…Special attention was paid to the photosensitive amino acid tryptophan, which has been studied in some detail (Jori et al, 1969;Knowles and Gurnani, 1972;Grossweiner, 1976). We found that tryptophan was only altered to a certain extent by light in the presence of PP, anthracene and tribromsdlicylanilide.…”