THE PROTEINS 39 



sulphuric acid is added (Hopkins). Serum albumin (from come 

 animals) has also been similarly crystallised (Giirber). 



Action on Polarised Light. All the proteins are levo-rotatory, 

 but the amount of rotation they produce varies with the kind of 

 protein. See Appendix. Several of the compound proteins (for 

 instance, haemoglobin and nucleo-proteins) are dextro-rotatory, 

 though their protein components are levo-rotatory (Gam gee). 



Colour Reactions. The principal colour reactions have been 

 already described in the heading of this lesson. 



(1) The xantho-proteic reaction depends on the conversion of the 

 aromatic group of the protein molecule into nitro-derivatives. 



(2) Millon's reaction is due to the presence of the tyrosine group, 

 and is given by all benzene derivatives which contain a hydroxyl 

 group (OH) replacing hydrogen. 



(3) The formaldehyde reaction (and the Adamkiewicz reaction, 

 which is probably the same thing) is due to the presence of the trypto- 

 phan radical (indole-amino-propionic acid). 



The presence, absence, or intensity of these colour tests in various 

 proteins depends respectively on the presence, absence, or amount of 

 the groups to which they are due. 



(4) In the copper sulphate test the proteoses and peptones 

 behave differently from the native proteins ; the latter give a violet 

 and the former a rose-red colour, which is called the biuret reaction, 

 because the same tint is also given by the substance called biuret. 1 

 The name does not imply that biuret is present in protein, but both 

 biuret and protein give the reaction because they possess the same 

 group or groups which are probably two CONH 2 groups linked either 

 to a carbon atom, or to a nitrogen a t tom, or directly to one another 

 (Schiff). 



Precipitants of Proteids. Proteids are precipitated by a large 

 number of reagents ; the peptones and proteoses are exceptions in 

 many cases, and will be considered separately afterwards (see 

 Lesson VII.). 



Solutions of the proteins are precipitated by 



1. Strong acids, like nitric acid. 



2. Picric acid. 



3. Acetic acid and potassium ferrocyanide. 



4. Acetic acid and excess of neutral salts like sodium sulphate. 



1 Biuret is obtained by heating solid urea ; ammonia is given off and leaves 

 biuret thus : 



2CON.H 4 = CAN 3 H 5 +NH 3 



[urea] [biuret] [ammonia] 



