Solubilities and Compositions of the Phospho-12- Tungstates of the Diamino Acids and of Proline, Glycine, and Tryptophane

Phosphotungstic acid (which will be designated as PTA) has long been used in separations of the amino acids. It was employed by Drechsel (1) in the discovery of lysine in casein, and by Hausmann (2), by Osborne, Leavenworth, and Brautlecht (3), and by Van Slyke (4-8) for quantitative precipitation of the diamino acids, arginine, lysine, and histidine, in protein hydrolysates. Cystine was found to be about half changed under the conditions of acid hydrolysis to a non-precipitable form (4) although the natural I form is precipitable with about the same completeness as histidine. The change was shown by Hoffman and Gortner (9) to be racemization, dl-cystine forming a more soluble phosphotungstate than that of I-cystine. The monoamino acids also form crystalline phosphotungstates. Although many times more soluble t,han the PTA salts of the diamino acids, some of the monoamino phosphotungstates have been found useful for separations of individuals within the monoamino group (10). A nearly quantitative separation of alanine from valine has thus been obtained (11). The phosphotungstates of the diamino acids, when precipitated under the conditions customarily employed with protein hydrolysates (2-S), viz. about 1 N HCl or HtS04 and room temperature, have solubilities sufficiently great to necessitate corrections for them in accurate quantitative work (4-6). The effect.s of varying temperature, acidity, and of coprecipitation of the different diamino acids together have, however, not heretofore been subjected to a systematic study. It appeared that such a study might increase the accuracy of solubility corrections, and perhaps also lead to discovery of conditions of precipitation which would make the phosphotungstate separation of the diamino acids from the monoamino group more sharp and certain.