Reactions of Xanthydrol

The method to be described for the determination of tryptophan is based on the reaction between xanthydrol and tryptophan in acid solution to form a highly colored, acid-stable product (1). In the presence of excess xanthydrol the formation of xanthydryltryptophan is directly related to the quantity of tryptophan in the solution. Although other indole derivatives likewise react with xanthydrol to form colored products, the fact that tryptophan is the only substance of this type known to occur in proteins, or in normal blood plasma in appreciable quantities, suggested that the reaction might be applied to the quantitative determination of this amino acid in solutions of biological interest. The procedure is quite rapid and sensitive in the range 10 to 80 y of tryptophan with a standard error of 2 per cent. The analysis of solutions which contain only tryptophan is readily accomplished. More complex solutions, such as deproteinized human blood plasma, contain constituents in addition to tryptophan which form a slight cblor with xanthydrol, and a modified procedure has been devised to avoid this interference. The determinateion of the tryptophan content of proteins requires a longer reaction period than that which is necessary for the analysis of free tryptophan. Under these conditions tyrosine also reacts with xanthydrol. The extent of the tyrosine interference can be evaluated and a correction applied by measurement of the absorbancy of the solution at two wave-lengths.