HISTIDIXE. 157 



Histidine, C 6 H 9 N 3 O2, according to the investigations of H. PAULY, 

 .OOP and WINDAUS, and F. Kxoop 1 , is an a-amino-/?-imidazolpropionic 

 CH-NHv 



C X^ C 



acid, = CH 2 



CH(NH 2 ) 



COOH 



Histidine was first discovered by KOSSEL in the cleavage products of 

 sturine. It was then found by HEDIN in the cleavage products of pro- 

 teins by acid hydrolysis, and by KUTSCHER among the products of 

 tryptic digestion, and finally also as a cleavage product of different pro- 

 tein substances. It does not occur in the protamines, with the excep- 

 tion of sturine. Of the protein bodies globin (from horse-hsemoglobin) 

 seems to be richest in histidine, as ABDERHALDEN found 10.96 per cent. 

 It also occurs in germinating plants (E. SCHULZE 2 ) . 



Histidine crystallizes in colorless needles and plates and is readily 

 soluble in water, but less soluble in alcohol, and has an alkaline reaction. 

 It is precipitated by phosphotungstic acid, but this precipitate is soluble 

 in an excess of the precipitant (FRANKEL). With silver nitrate alone 

 the aqueous solution is not precipitated; on the careful addition of 

 ammonia or baryta-water an amorphous precipitate, which is readily 

 soluble in an excess of ammonia, is obtained. Histidine can be precip- 

 itated by mercuric chloride, or, still better, by the sulphate acidified 

 with sulphuric acid, and can in this way be separated from the other 

 diamino-acids (KOSSEL and PATTEN). The hydrochloride crystallizes in 

 beautiful plates (BAUER), dissolves rather readily in water, but is insolu- 

 ble in alcohol and ether. With hydrochloric acid and methyl alcohol 

 it gives the dihydrochloride of histidine methyl ester, which melts at 

 196. Histidine is levorotatoiy, (a) D = 39.74, while its solution in 

 hydrochloric acid is dextrorotatory. On warming it gives the biuret test 

 (HERZOG), and it also gives WEIDEL'S reaction if performed as sug- 

 gested by FISCHER (see Xanthine, Chapter V) (FRANKEL 3 ). On adding 

 sufficient bromine water and warming, a reddish coloration ensues 



1 Pauly, Zeitschr. f. physiol. Chem., 42; Knoop and Windaus, Hofineister's Bei- 

 trage, 7 and 8; Knoop, ibid., 10. 



2 Kossel, Zeitschr. f. physiol. Chem., 22; Hedin, ibid., Kutscher, ibid., 25; Wetzel, 

 ibid., 26; Lawrow, ibid., 28, and Ber. d. d. chem. Gesellsch., 34; Kossel and Kutscher, 

 Zeitschr. f. physiol. Chem., 31; Hart, ibid., 33; Abderhalden, ibid., 37; Schulze, ibid., 

 24 and 28. 



3 Kossel and Patten, Zeitschr. f. physiol. Chem., 38; Bauer, ibid., 22; Herzog, 

 ibid., 37; Frankel, Sitz.-Ber. d. Wien. Akad., 112, II. B., 1903, and Hofineister's Bei- 

 trage, 8. 



