Zinc and cobalt picrates as reagents for specific precipitation of glycine from protein hydrolysates.

The defects inherent in the older methods (l-5) for isolating glycine from protein hydrolysates may be overcome by the direct precipitation of glycine from such mixtures as a complex salt with copper picrate (6). This reagent has been subsequently employed with success for the removal of glycine from the hydrolytic products of proteins prior to the isolation of other amino acids (7). Difficulties, however, have been encountered in the recrystallization and drying of the glycine-copper picrate complex. These have been shown to be due to the tendency of the complex to undergo decomposition by treatment with ethanol or when its concentrated aqueous solutions are heated above 90” (6). The present work has accordingly been undertaken with the object of developing new glycine-metal picrate complexes which are free from these effects. It has been previously reported (6-8) that glycine forms complex salts with calcium, strontium, and barium picrates which are distinguished by their remarkably low solubilities and rapid crystallization. Glycine, however, cannot be directly isolated from amino acid mixtures by precipitation with these alkaline earth metal picrates, which also form sparingly soluble derivatives with a number of amino acids. A search has, therefore, been made for other picrates capable of specifically precipitating glycine from protein hydrolysates. Of the various metal picrates tested, only zinc and cobalt picrates have been found to satisfy this condition. The reaction of these picrates with all of the amino acids which are usually present in protein hydrolysates has been investigated. The results have indicated that glycine is the only amino acid which is capable of furnishing sparingly soluble complexes with these reagents. It should be pointed out, however, that arginine is very slowly and incompletely precipitated as a complex derivative by treatment of its pure aqueous solutions with zinc and cobalt picrates. These compounds, on the other hand, are far more soluble than the corresponding glycine complexes, and they do not separate from protein hydrolysates under the experimental conditions employed. Study of the properties of the zinc and cobalt picrate complexes of glycine has demonstrated that each of them is composed of 2 moles of