100 THE PROTEIN SUBSTANCES. 



After making alkaline with ammonia or alkalies the color becomes orange- 

 yellow, due to the nitroderivatives of the benzene and indol groups. 



3. Adamkiewicz' s Reaction. If a little proteid is added to a mixture 

 of 1 vol. concentrated sulphuric acid and 2 vols. glacial acetic acid a 

 reddish-violet color is obtained slowly at ordinary temperatures, but 

 more quickly on heating. According to HOPKINS and COLE 1 this reaction 

 takes place only on using glacial acetic acid containing glyoxylic acid. 

 According to them it is better to use a solution of glyoxylic acid, which 

 can be readily prepared by adding sodium amalgam to a concentrated 

 solution of oxalic acid and filtering after the discharge of the gas. A 

 dilute aqueous solution of the acid or some of the solid acid is added to the 

 proteid solution and sulphuric acid allowed to flow down the side of the 

 test-tube, when the reddish-violet color will appear at the point of con- 

 tact of the two liquids or on shaking the mixture. This color reaction, 

 which is generally called the ADAMKIEWICZ-HOPKINS reaction depends 

 upon the tryptophane and therefore gelatin (which does not contain 

 any tryptophane) does not give this reaction. 



As further color reactions we will mention: 4. Biuret Test. If a 

 proteid solution be first treated with caustic potash or soda and if then 

 a dilute copper-sulphate solution be added drop by drop, first a reddish 

 then a reddish-violet, and lastly a violet-blue, color is obtained. 5. Pro- 

 teids are soluble on heating with concentrated hydrochloric acid, produc- 

 ing a violet color, and when they are previously boiled with alcohol and 

 then washed with ether (LIEBERMANN 2 ) they give a beautiful blue 

 solution. This blue color is due, according to CoLE, 3 to a contamination 

 of the ether with glyoxylic acid, which reacts with the tryptophane 

 groups split off by the hydrochloric acid. The violet color obtained 

 with proteins not purified with ether is also considered as a tryptophane 

 reaction with the furfurol (oxymethylfurfurol) formed from the hexose 

 containing protein by the action of the concentrated hydrochloric 

 acid. Reaction 6 with concentrated sulphuric acid and sugar (in small 

 quantities) is explained in the same way. The beautiful red coloration 

 is connected with the formation of furfurol from the sugar. 7. With 

 p-dimethylaminobenzaldehyde and concentrated sulphuric acid thepro- 

 teids give a beautiful reddish- violet or deep- violet coloration (O. NEU- 

 BAUER and E. RoHDE 4 ). Other . aldehydes also give color reactions 

 by virtue of the tryptophane group in proteins. Other reactions are 8; 

 ARNOLD'S reaction 5 is a purple-violet coloration which the proteins give 



1 Proceed. Roy. Soc., 68. 



2 Centralbl. f. d. med. Wissensch., 1887. 

 8 Journ. of Physiol., 30. 



4 Zeitsche. f. physiol. Chem., 44. 



6 Arnold, Zeitschr. f . physiol. Chem., 70. 



