THE PROTEINS 101 



proportion of the total number of ami no- acids which have as yet been 

 isolated from acid digests of proteins. The differences in various 

 proteins cannot therefore be determined by qualitative differences 

 in their constituent molecules, but must depend on the relative amounts 

 of the amino-acids which are present and on their arrangement in 

 the whole molecule. As regards relative amounts of amino-acids we 

 find very striking differences. Thus, glutamic acid, which forms 8 per 

 cent, of egg albumin and only 1-7 per cent, of globjn (derived from 

 haemoglobin), amounts to 31-5 per cent, in gliadin, the protein extracted 

 from wheat flour. Striking differences are also noticeable in the 

 relative proportions of the diamino- acids and bases, the so-called hexone 

 bases. Whereas in casein they form about 12 per cent, of the total 

 molecule, in globin they form about 20 per cent., and in the prota- 

 mines, salmine and sturine, about 85 per cent, of the total molecule 

 consists of these bodies. On this account the two last-named bodies 

 have a strongly basic character. From these figures it is evident also 

 that certain of the amino-acids must occur many times over in the pro- 

 tein molecule. Thus in globin, if we assume the presence of one 

 tyrosine molecule, there must be at least thirty-two leucine and ten 

 histidine molecules. On these data the molecular weight of haemo- 

 globin would come out at about 14,000, a figure which agrees with that 

 derived from a study of the amounts of sulphur and iron respectively 

 in its molecule. 



THE DISTRIBUTION OF NITROGEN IN THE PROTEIN 



MOLECULE 



Attempts have been made to differentiate among the proteins 

 by a method which, while less laborious than the isolation and recogni- 

 tion of the individual amino-acids. may yet throw some b'ght on the 

 manner in which the nitrogen is combined within the molecule, and on 

 the relative amounts of the different classes of nitrogen groups which 

 may be present. One method, which was devised by Hausmann, is 

 carried out as follows. One gramme of the protein is dissociated 

 by boiling with strong hydrochloric acid. The nitrogen, which has 

 been split off as ammonia and is present in the solution as ammonium 

 chloride, is then distilled off with magnesia and received into deci- 

 normal acid, where its amount can be determined by titration. This 

 nitrogen is variously designated as amide nitrogen, ammonia nitrogen, 

 or easily displaceable nitrogen. The remaining fluid, freed from 

 ammonia, is precipitated with phosphotungstic acid. By this means 

 all the diamino- acids and bases are thrown down. The nitrogen in 

 the precipitate is determined by Kjeldahl's method and is called 

 diamino- or basic nitrogen. In the remaining fluid, which contains 

 mono -amino-acids, the total nitrogen, the mono-amino-nitrogen, is 



