Reaction of histidine with the biuret reagent in the presence of oxygen

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A simplified system is described which involves the reaction of the amino acid histidine with oxygen and the biuret reagent, yielding a biuret-positive color on standing or being heated. The reaction product appears to be a three-membered complex of histidine or a derivative of histidine, oxygen or possibly a derivative of oxygen (H 2O 2), and copper. The participation of cuprous copper in the complex is indicated by the facility with which a variety of reducing agents effect immediate biuret positivity at room temperature, and by the speed of the reaction in the presence of cuprous copper without the addition of any other reducing agent. In the absence of a prolonged incubation period or the application of heat, a reducing agent such as cysteine must be added to enable the reaction to proceed rapidly to biuret positivity at room temperature. It is concluded that, in the absence of added reducing agents, histidine itself may act as the reducing agent. The reaction differs in hue and color intensity from the immediate blue color-producing reaction obtained by mixing relatively large quantities of histidine with biuret reagent; it also differs in respect to other variables such as the necessity for O 2. A brief discussion of the present significance of this reaction, its use as a quantitative method for estimating histidine, and its potential for introducing gross error in the biuret determination of protein and peptide material is given. The reaction may hold some promise as a sensitive chemical dosimeter for ionizing radiation.