Rhodanilic acid

BlO. Bergmann, M., Complex salts of amino acids and peptides. II. Determination of f-proline with the aid of rhodanilic acid. The structure of gelatin. J. Biol. Chem. 110, 471-479 (1935). 

Ammonium salt of rhodanilic acid ammonium rhodanilate. 

Precipitate (Fraction E) dissolved in 500 cc. HiO added 200 cc. of 5% solution of rhodanilic acid in CHtOH in cold room 48 hours filtered... 

In a later report (106), SubbaRow and Jacobson described briefly the properties of another fraction— fraction F, obtained from fraction B— which appeared to be a polypeptide. It yielded crystalline salts with rhodanilic acid and Reinecke salt. The rhodanilate, showing a positive biuret, diazo and glyoxylic but negative Milton s reactions, ras precipitated by phosphotungstic, picric, rufianic and tannic acids as well as by ammonium sulfate the Reineckate, showing a positive ninhydrin and a weak diazo reaction, was precipitated by phosphotungstic and rufianic acids. Fraction F was inactive in the guinea pig as well as the pernicious anemia patient. 

Many relatively specific reagents, particularly a variety of metal complexes, have been developed as an aid to the isolation of individual amino acids. An example is provided by the isolation of L-proline from a gelatine hydrolysate using the chromium complex ammonium rhodanilate ammonium dianiline-... 

Suspend the purified salt in 850 ml of water in a stoppered bottle, add 25 ml of pure pyridine and shake the mixture for 4-5 hours. Remove the insoluble pyridine rhodanilate by filtration and wash it with 100 ml of cold water (2). Combine the pale pink filtrate and washings, and add glacial acetic acid dropwise until the formation of a small pink precipitate is complete. Filter, evaporate the almost colourless filtrate to dryness (rotary evaporator) and suspend the residue in absolute ethanol and re-evaporate twice. Dry the resulting faintly pink crude proline in a vacuum desiccator over silica gel the yield is about 18 g. Recrystallise from the minimum volume of absolute ethanol to obtain 11 g (7.3% based on gelatin) of L-proline, m.p. 218-219 °C (decomp.), [oc]d8 —85.6° (c3.0 in H20). Check the purity of the product by t.l.c. on silica gel using the solvent system butan-l-ol-acetic acid-water, 4 1 1 Rf 0.26 (yellow spot with ninhydrin). 

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