36 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



These groups he terms peptides or polypeptides : many of these 

 have been made synthetically in his laboratory, and so the synthesis 

 of the protein molecule is foreshadowed. 



We may take as our examples of these peptides some of the 

 simplest, and may write the formulae of a few amino-acids as 



follows : 



NH 2 .CH 2 .COOH glycine 



NH 2 .CH 4 .COOH alanine 



NH 2 .C 5 H 10 .COOH leucine 



or in general terms 



HNH.R.COOH 



Two amino-acids are linked together as shown in the following 

 formula : 



HNH.R.CO'OH.HNH.R.COOH 



What happens is that thehydroxyl (OH) of thecarboxyl (COOH) 

 group of one acid unites with one atom of the hydrogen of the next 

 amino (HNH) group, and water is thus formed, as shown within the 

 dotted lines : this is eliminated and the rest of the chain closes up. 

 In this way we get a dipeptide. The names glycyl, alanyl, leucyl, 

 &c. are given by Fischer to the NH 2 .R.CO groups which replace 

 the hydrogen of the next NH 2 group. Thus glycyl-glycine, 

 glycyl-leucine, leucyl-alanine, alanyl-leucine, and numerous 

 other combinations and permutations are obtained. If the same 

 operation is repeated we obtain tripeptides (leucyl-glycyl-alanine. 

 alanyl-leucyl-tyrosine &c.) ; then come the tetrapeptides and so on. 

 In the end, by coupling the chains sufficiently often and in appro- 

 priate order, Fischer has already obtained substances which give some 

 of the reactions of peptones. 



TESTS FOR PROTEINS 



Solubilities. The proteins as a class are insoluble in alcohol and 

 in ether. Some are soluble in water, others insoluble. Many of the 

 latter are soluble in weak saline solutions. Some are soluble, others 

 insoluble in concentrated saUne solutions. It is very largely on 

 these varying solubilities that proteins are separated into classes, 

 and from each other. 



When, however, one speaks of the solution of a protein, the kind 

 of solution obtained is usually what physical chemists call colloidal 

 solution : the condition here is something intermediate between true 

 solution and a suspension. Many of the properties of proteins, such 



