GENERAL PROPERTIES AND REACTIONS OF PROTEIDS. 47 



Third group The indol group, of which indol, skatol, and skatol- 

 carbonic acid are the most important members. 



We can now proceed to the consideration of the proteid colour 

 reactions. 



1. The xanthoprotcic reaction. This is characterised by the yellow 

 colour given by boiling with nitric acid, turned orange by ammonia. 

 0. Loew x considered that the yellow material was a mixture of oxynitro-, 

 trinitro-, and hexanitro-albumin ; but these substances are very doubtful 

 as chemical individuals. Salkowski found the reaction to be given by all 

 the members of his first and third groups of aromatic substances. 

 Pickering 2 found that salicylic acid, and salicylsulphonic acid, cholesterin, 

 cholalic acid, and taurocholic acid also give the test. A large number 

 of other organic substances which were tested did not give the same 

 result. It was noticed that bodies with a benzene nucleus with one 

 hydrogen replaced by hydroxyl, give the xanthoproteic reaction, whereas 

 substances which contain a benzene nucleus without the hydroxyl, as 

 phenylacetic and benzoic acids, do not. 



Millon's reaction. A brick-red coloration occurs when proteid matter 

 is boiled with Millon's reagent (a mixture of the nitrates of mercury 

 with excess of nitric acid) ; the reaction was thought by Kiihne 3 to 

 be due to tyrosine. Salkowski also took this view, as the reaction is 

 given by the substances in his first group, the most prominent member 

 of which is tyrosine. Those in the second and third groups do not give 

 the test. Nasse, 4 however, demonstrated that Millon's reaction is due 

 to benzene derivatives, in which one hydrogen atom has been replaced 

 by hydroxyl (hydroxybenzene nucleus) and not to tyrosine. That 

 Nasse's view is correct is shown by the following considerations : 

 Kiihne and Chittenden 5 have found that certain anti-products of diges- 

 tion, which yield neither leucine nor tyrosine on further digestion, or on 

 decomposition with sulphuric acid, do not give the reaction. Schiitzen- 

 berger 6 found that tyrosine is absent from the putrefaction products of 

 gelatin. Now, Salkowski stated that gelatin does not react with Millon's 

 reagent. But Chittenden and Solley 7 have found that the products of 

 gelatin digestion give a characteristic reaction, and Pickering that pure 

 gelatin and gelatinoses give it in a marked manner ; thus confirming the 

 statement made by Millon 8 in his original memoir. Gelatin, therefore, 

 owes this property to something which is not tyrosine, but which, like 

 tyrosine, contains a hydroxybenzene nucleus. 



Adamkiewicz reaction? If glacial acetic acid in excess and then 

 concentrated sulphuric acid are added to proteid, a violet colour with 

 feeble fluorescence is produced. The test is by no means a oertain one, 

 and is given by proteoses and peptones in concentrated solutions only. 

 It is not given by gelatin (Hammarsten). 



This test is only given by the aromatic substances of Salkowski's 

 third (indol) group. The strong reagents added are likely to produce 



1 Journ. f. pralct. Chem., Leipzig, N.F., Bd. iii. S. 180. 



2 Journ. Physiol., Cambridge and London, 1893, vol. xiv. p. 372. 



3 Ztschr. f. d. ges. Naturw., Halle, Bd. xxix. S. 506 ; Virchow's Archiv, Bd. xxxix. S. 130. 



4 Chem. Ccntr.-BL, Leipzig, 1879, Bd. x. 



5 Ztschr. f. Biol., Miinchen, Bd. xxii. S. 423. 



6 Article in Wurtz' "Diet, de chim.," 1886, Suppl. 1 A, p. 58. 



7 Journ. Physiol., Cambridge and London, vol. xii. p. 23. 



8 Qompt. rend. Acad. d. sc., Paris, tome xxviii. p. 40. 



9 Ber. d. deutsch. chem. Gesellsch., Berlin, Bd. viii. S. 761. See also Wurster, Chem. 

 Ztg., Cothen, Bd. xi. S. 187. 



