THE PROTEINS 99 



of digestive ferments on the proteins. He distinguishes di-, tri-, 

 tetra-, &c., peptides according to the number of individual amino-acids 

 taking part in the formation of the compound. The polypeptides 

 resemble in all respects the peptones. Most of them, even if derived 

 from relatively insoluble amino-acids, are soluble in water, insoluble 

 in absolute alcohol. They dissolve in mineral acids and in alkalies 

 with the formation of salts, thus resembling in their behaviour the 

 amino-acids. They have a bitter taste, although the amino-acids 

 from which they are formed have a slightly sweet taste, in this way 

 again resembling the natural peptones. The higher members of the 

 series give certain reactions, such as the biuret reaction, which are 

 regarded as characteristic of peptones, and like the latter are pre- 

 cipitated by phosphotungstic acid. Their behaviour with trypsin 

 depends on the optical behaviour of the amino-acids from which they 

 are formed. If synthetised from the amino-acids identical with those 

 occurring in the disintegration of natural proteins, they resemble the 

 peptones in undergoing hydrolysis and disintegration into their 

 constituent amino-acids. Trypsin, however, has no influence on poly- 

 peptides compounded of the inactive amino-acids, or of the amino- 

 acids which are the optical opposites of those which form the 

 constituents of normal proteins. Though most of the amino-acids 

 which occur naturally are Isevo-rotatory, the polypeptides formed from 

 them are generally strongly dextro-rotatory. 



Thus in the building up of the protein molecule there is an almost 

 indefinite coupling up of numerous amino-acid groups, the connecting 

 element in each case being the nitrogen. Of the two or more optical 

 isomers possible of each amino-acid containing more than two carbon 

 atoms, only one is made use of for this purpose. A still further 

 flexibility in its reactions to its environment is conferred on the protein 

 molecule by changes occurring with great readiness in the intra- 

 molecular grouping of its constituent atoms. Thus, if we take the 

 simplest member of the class of polypeptides, glycyl glycine, four 

 structural formulse are possible, namely : 



(1) NH 2 CH 2 CO - NH . OH 2 . COOH 



(2) NH.CH 2 .CO X 



(3) NH 2 . CH 2 . C(OH) = N . CH 2 . COOH 



(4) N.CH 2 .CO 





C(OH)CH 2 .NH 

 (2) and (4) being the intramolecular form of the 



