Resolution of the Optical Isomers of DL-Tryptophan, 5-Hydroxy-DL-tryptophan and 6-Hydroxy-DL-tryptophan by Paper and Thin-layer Chromatography

Abstract

IT is known that the optical isomers of many amino-acids can be metabolized by different pathways although it is often difficult to obtain suitable preparations for investigation. Several methods, both biological and chemical, are available for obtaining individual isomers in a pure state^1,2, but many of these tend to be practicable only if a large amount of the racemic form is available; yields obtained by classical chemical separation methods are low, while biological procedures usually lead to the destruction of one of the isomers. Separation has been achieved, however, by paper chromatography^3–6. This approach seemed to be best suited to our requirement, the isolation of pure optical isomers of ¹⁴C-5-hydroxy-DL-tryptophan, a comparatively expensive compound. During our investigation, we noted that 5-hydroxy-DL-tryptophan (DL-5HTP), 6-hydroxy-DL-tryptophan (DL-6HTP) and DL-tryptophan (DL-TP) separated into two diffuse but distinct spots when chromatographed on Whatman No. 1 paper with butanol–pyridine–water (1 : 1 : 1, by vol.) as solvent system. The possibility of improving this separation by thin-layer chromatography was therefore explored.