Photo-oxidation of tryptophan sensitized by methylene blue

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Abstract
The photo-oxidation of tryptophan sensitized by methylene blue takes place by two competing mechanisms, one involving singlet oxygen and the other through the direct reaction of the triplet state of methylene bule with tryptophan. The rate constant for the reaction between singlet oxygen and tryptophan has been measured in a number of solvents covering a wide range of polarities. The rate constant was found to increase with increasing polarity. The rate constant for the reaction between the triplet state of methlyene blue and tryptophan was measured in an aerated 1:1 molar methanol and water mixture. The rate of reaction is well below the diffusion controlled rate but greater than that would be predicted on the basis of the ionization potential of tryptophan. It is suggested that the reaction probably involves some electron transfer and some hydrogen atom transfer.

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