100 THE PROTEIN SUBSTANCES. 



dehyde and concentrated sulphuric acid the proteids give a beautiful 

 reddish-violet or deep-violet coloration (O. NEUBAUER and E. RoHDE 1 ). 



Many of these color reactions are obtained as shown by SALKOwsi, 2 by the 

 aromatic or heterocyclic cleavage products of the proteids. MILLON'S reaction 

 is given only by the substances of the phenol group ; the XANTHOPROTEIC reac- 

 tion by the phenol group and skatol or the indol group. LIEBERMANN'S reaction 

 depends, according to COLE, upon the indol group, and the reactions with sul- 

 phuric acid and sugar (COLE) and with diamethylaminobenzaldehyde (ROHDE) 

 are also caused by this group. ADAMKIEWICZ'S reaction is given only by the 

 bodies of the indol group. The biuret reaction is not only given by protein 

 substances, but also by many other bodies. According to H. SCHIFF 3 this reaction 

 occurs with those bodies containing amino groups, CONH 2 , CSNH 2 , C(NH)NH 2 

 or also CH 2 NH 2 , united either directly by their carbon atoms or by means 

 of a third carbon or nitrogen atom. As examples of such bodies we can men- 

 tion several diamines or aminoamides, such as oximide, biuret, glycinamide, 

 a- and /3-aminobutyramide, aspartic-acid amide, etc., although we are not clear 

 as to the conditions necessary for the bringing about of this reaction. The biuret 

 reaction alone is therefore no proof as to the protein nature of a substance for 

 example, urobilin gives a very similar color reaction and a protein substance can 

 still retain its protein nature, as by the action of nitrous acid or by a splitting 

 off of ammonia, although it does not give the biuret reaction. 



The delicacy of the various reagents differs for the different proteids, 

 and on this account it is impossible to give the degree of delicacy for 

 each reaction for all proteids. Of the precipitation reactions, HELLER'S 

 test (if we eliminate the peptones and certain proteoses) is recommended 

 in the first place for its delicacy, though it is not the most delicate reac- 

 tion, and because it can be performed so easily. Among the precipita- 

 tion reactions, that with basic lead acetate (when carefully and exactly 

 executed) and with alcohol and the reactions given under 6, are the most 

 delicate. The color reactions 1 to 4 show great delicacy in the order in 

 which they are given. 4 



No proteid reaction is in itself characteristic, and, therefore, in testing 

 for proteids one reaction is not sufficient, but a number of precipitation 

 and color reactions must be employed. 



For the quantitative estimation of coagulable proteids the determina- 

 tion by boiling with acetic acid can be performed with advantage, for 

 by operating carefully, it gives exact results. Treat the proteid solution 

 with a 1-2 per cent common-salt solution, or if the solution contains 

 large amounts of proteid dilute with the proper quantity of the above 

 salt solution, and then carefully neutralize with acetic acid. Now deter- 

 mine the quantity of acetic acid necessary to completely precipitate 

 the proteids in small measured portions of the neutralized liquid which 



1 Zeitschr. f . physiol. Chem., 44. 



2 Ibid., 12. 



3 Ber. d. d. chem. Gesellsch., 29 and 30. 



4 In regard to the precipitation and coloration reactions of proteids with aniline 

 .dyes see Heidenhain, Pfliiger's Arch., 90, 96. 



