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Abstract— The photosensitized oxidation of I ‐ , tyrosine, and tryptophan by aqueous eosin has been investigated with a photostationary technique, in which the system is irradiated with sinusoidally modulated light and phase sensitive detection is used to measure the spectra of short‐lived intermediates and their rate constants. It has been shown that the principal oxidizing agent for I ‐ is semi‐oxidized eosin ( X ) produced by the reduction of oxygen by triplet state eosin. The same mechanism obtains when femcyanide ion is substitued for oxygen. A second oxidizing agent is effective in oxygenated solutions at high I ‐ concentrations, attributed to O 2 ‐ . The X mechanism controls when tyrosine or tryptophan are irradiated at low substrate concentrations in the presence of oxygen or femcyanide. However, at high substrate concentrations, or in the absence of the electron accepting agents, the mechanism switches to the ‘dye‐substrate’ process where triplet state eosin oxidizes the aromatic substrate and is reduced to the semiquinone. The conversion quantum yields agree with the predictions based on the measured quenching and reaction rate constants.
Kępka et al. (Mon,) studied this question.