On the mechanism of the photo-oxidation of amino acids sensitized by methylene blue

Abstract Photo-oxidation of histidine, methionine, tryptophan, and tyrosine, and their derivatives, as the function of pH were investigated. Photo-oxidation of histidine and methionine as a function of temperature has shown that oxidation proceeds through an initial fast reaction, which is followed by an additional light-sensitive slow reaction, the latter being more pronounced with increasing temperature. Photo-oxidation of tyrosine and tryptophan have also shown a marked temperature dependence. It was proposed that the first step in the photo-oxidation of histidine, methionine, tryptophan, and tyrosine consists of oxidation of these amino acids with a simultaneous reduction of the dye. Photo-oxidation of the reactive amino acids obey the Michaelis-Menten reaction scheme. A possible cyclic free radical mechanism for the photo-oxidation of amino acids mediated by methylene blue is proposed.