Abstract
Picrolonic acid, used by Steudel to precipitate the hexone bases, and later shown by Mayeda to form salts with the aromatic amino acids tryptophane and phenylalanine, also forms crystalline salts of normal composition with the other monoamino acids obtained on hydrolysis of proteins. The salts are made by dissolving molecular proportions of amino acid and picrolonic acid in a minimum amount of boiling water. The picrolonates crystallize from the cooling solutions, usually while they are still warm. In cold water many of them are very insoluble. In alcohol they are all more soluble than in water. Following is a list of the amino acids of which picrolonates were prepared, after each being given the solubility of the picrolonate in grams per 100 c.c. of water at 20°-23°, and the melting point. The picrolonates are arranged in order of solubility. Dl-phenyl-alanine, 0.12, 212° (decomp.); Tyrosine, 0.29, blackens at 260°; l-phenyl-alanine, 0.34, 208°; dl-leucine, 0.53, indefinite above 140°; l-leucine, 0.55, indefinite at about 150°; d-isoleucine, 0.58, 170°; dl-valine, 0.81, indefinite above 150°; dl-serine, 0.98, decomposed 265°; glycocoll, 0.99, 214°; dl-alanine, 1.01, 216°; d-valine, 1.20, 180°; d-alanine, 1.61, 214°; dl-aspartic acid, 1.69, blackens at 130°; dl-glutaminic acid, 2.37, 194°.
Nearly all the picrolonates decompose more or less on melting, and the melting points are not sharp. L-Phenyl-alanine picrolonate is much more soluble in alcohol than the racemic salt, and can be separated from most of the racemic substance by solution in alcohol. The optically active compound has a sp. rotation in absolute alcohol of + 30°. The specific rotation of d-isoleucine picrolonate in alcohol is + 33.3°. Phenyl-alanine can be separated from glutaminic or aspartic acid by dissolving the mixture with enough picrolonic acid to combine with the phenyl-alanine only.
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