5-Methyltryptophan: An internal standard for tryptophan determination by ion-exchange chromatography

The use of 5-methyltryptophan as an internal standard to facilitate tryptophan determination is described. Protein is hydrolyzed in the presence of 5-methyltryptophan for 18 hr at 120° in 5.0n NaOH in the absence of oxygen and in the presence of starch or thiodiglycol as an antioxidant. Ion-exchange chromatography of the hydrolysate on Durrum DC-2 resin using pH 5.43 citrate (0.175n Na⁺) completely resolved tryptophan and 5-methyltryptophan from one another, other amino acids, and artifacts of the alkaline hydrolysate. The chromatographic conditions and stability of tryptophan and 5-methyltryptophan were established initially by demonstrating quantitative recovery of both amino acids that had been added prior to hydrolysis of ribonuclease A, a protein devoid of tryptophan. The tryptophan content of several well-characterized proteins was determined, and the results, after correction to 100% recovery of 5-methyltryptophan, agreed well with values obtained by established procedures.