60 THE CHEMISTRY OF CATTLE FEEDING 



prepared from brain tissue, is very poisonous, whereas 

 choline (hydroxy ethyl - trimethyl - ammonium hydroxide), 



//-TT \ 



C ^"^^' * s non "Pi sonolls ' The last is a normal 



constituent of bile. 



Peptides. It has been shown (p. 50) that the amino-acids 

 can form salts, (a) with acids and (b) with bases. As they are 

 themselves at once acidic and basic they can form salts with 

 each other, i.e. by reaction of the carboxyl group of one 

 molecule with the amino group of another. The compound 

 formed by the reaction of two molecules of glycocine (amino- 

 acetic acid) is called glycyl-glycine. 



H H 



H-N CH 2 CO. |OH + Hi N CH 2 CO.OH 

 H 



_> H N CH 2 CO NH CH 2 CO.OH + H 2 O 



(Glycyl-glycine.) 



If the two reacting molecules are not of the same kind two 

 different, isomeric compounds can be formed. Thus alanine 

 and leucine combine in two different ways as shown below. 



NH 2 CH(CH 3 ) CO NH 



(Alanyl-leucine. 1 ) 



_ CR<^^ 2 CH(CH 3 ) 2 



< CONHCH(CH 3 ) CO.OH 



(Leucyl-alanine.) 



Such compounds are called peptides. They retain the 

 essential characteristics of the amino-acid type, i.e. they are 

 neutral substances, but are potentially both acidic and basic. 

 It is probable therefore that, like the simple amino-acids, they 

 have a cyclic constitution which may be represented as 

 follows : 



