GENERAL CHEMICAL CHARACTERS OF PROTEINS 33 



weak acids at the ordinary temperature), they can be readily freed. 

 These conjugated proteins give the same general chemical reactions as 

 the simple proteins. The chief groups are : (i) the nucleo-proteins, or 

 proteins in combination with a nucleic acid complex ; (2) the glyco- 

 proteins, or proteins in combination with a complex which can exert 

 a reducing action on alkaline copper solutions [it has not been 

 definitely proved that the prosthetic group in this class is always a 

 carbohydrate] ; (3) the chromo-proteins, or proteins containing a 

 chromatogenic group. 



It is not proposed to consider under this section the chemical 

 composition of the conjugated proteins; only that of the simple 

 proteins, either those existing as such in nature, or those obtained by 

 the decomposition of conjugated proteins, will be discussed. 



The essential constituents of a protein molecule are carbon, 

 hydrogen, nitrogen, oxygen and, in nearly all cases, sulphur. Some 

 proteins contain in addition phosphorus, and the halogens are also 

 found in a limited number of substances of this class. 



It is extremely difficult to prepare the majority of proteins free 

 from ash ; in some cases the inorganic constituents may be in com- 

 bination with the organic body (the question of salt formation will be 

 discussed later) and in others simply adsorbed. 



The carbon, hydrogen and nitrogen content of a protein varies 

 in substances of very different origin and character within compara- 

 tively very narrow limits ; the numbers obtained by an ordinary 

 elementary analysis of carbon, hydrogen and nitrogen cannot be 

 regarded as very characteristic. The following analyses will serve as 

 an example : 



C H N 



Substance. Origin. Per Cent. Per Cent. Per Cent. Observer. 



Serum-albumin Horse 52*93 7-05 15*89 Abderhalden. 



Albumin Egg 5 2 '75 7' 12 I 5'43 Hopkins. 



Edestin Hemp-seed 51*27 6-85 18-76 Osborne. 



Rye 51-03 6-74 18-19 



Gliadin 52-75 6-84 17-72 ,, 



Caseinogen Cows' milk 52-96 7-05 15*65 Hammarsten. 



Although these bodies yield very different amounts of the typical 

 hydrolysis products, and are undoubtedly differently constituted 

 chemically, their contents of carbon and hydrogen vary within com- 

 paratively narrow limits. The nitrogen shows larger variations, the 

 plant globulins containing generally between 2 and 3 per cent, more 

 than the typical animal proteins. In certain other cases also the 

 nitrogen content is high, viz., in the proteins of more basic character 

 like the histones and protamines, substances which on hydrolysis yield 

 relatively large quantities of diamino-acids. The thymus histone 

 contains, for example, 18*35 P er cent, nitrogen, whilst the protamine 

 salmine, from salmons' testicles, contains no less than 31*69 per cent. 

 Taken as a whole, the nitrogen does not show, however, very large 

 variations ; in fact, the nitrogen content of a mixture is often taken 

 as the index of the amount of protein present. 



The Nitrogen Distribution in Proteins. 



It has been already stated that the proteins are essentially poly- 

 peptides formed by the condensation of varying numbers of groups 

 of diamino- and monoamino-acids, which are obtained from the 

 proteins by hydrolysis. In addition to these acids, another hydrolysis 



