88 PHYSIOLOGY 



CLCH 2 .CO - NH.CH(CH 3 )COOH + 2NH 3 = 

 NH 2 .CH 2 .CO - NH.CH(CH 3 )COOH + NH 4 C1. 



Different halogen acyl chlorides are used for introducing the various amino-acid 

 radicals, e.g. chloracetylchloride for glycyl, a-bromopropionylchloride for alanyl, &c. 



By various such methods Fischer has succeeded in combining compounds 

 containing as many as eighteen amino-acids, e.g. alanyl leucine, glycyl 

 tyrosine, dialanyl cystine, dileucyl cystine, leucyl pentaglycyl glycine, and 

 so on. The last named would be built up out of one molecule of leucine and 

 six molecules of glycine. These compounds have been designated by 

 Fischer as polypeptides, to signify their close connection with the peptones 

 produced by the agency of digestive ferments on the proteins. He dis- 

 tinguishes di-, tri-, tetra-, &c., peptides according to the number of individual 

 amino-acids taking part in the formation of the compound. The poly- 

 peptides 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 precipitated 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 pep- 

 tones in undergoing hydrolysis and disintegration into their constituent 

 amino-acids. Trypsin however has no influence on polypeptides com- 

 pounded 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 laevo-rotatory, 

 the polypeptides formed from them are generally strongly dextro-rotatory. 



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

 nite 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 formulae are possible, namely : 



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



(2) NH.CH 2 .CO 



I > 



CO.CH 2 .NH 8 



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



