88 A. I. RINGER 



2. Cystein or o-amino j3-thio-propionic acid. 

 P-CH 2 SH 



a-CHNH 2 



COOH 



3. Cystin or dicystein. 



CH 2 S S CH 2 



I I 



CHNH 2 CHNH 2 



COOH COOH 



These three substances are neutral in reaction, and have properties sim- 

 ilar to those in group "A". The two latter are the only amino acids which 

 contain sulphur, and there is every indication to prove that only the latter 

 exists in protein and that the former is only a product of its hydrolysis. 



4. p-Hydroxyglutamic acid, Dakin (1918, 1919). 



COOH 



CH 2 



P-CHOH 

 <x-CHNH 2 



COOH 



This acid is similar to the dibasic acid glutamic acid, except that it 

 has an hydroxyl radical attached to the P-carbon. This is the youngest 

 member of the amino acid family, naving been discovered by H. D. Dakin 

 in 1918. 



D. Diamino acids. 



1. Lysin or a-E-diamino caproic acid. 

 e-CH 2 NH 2 



8-CH 2 

 Y-CH 2 



P-CH 2 



I 

 a-CHNHj, 



COOH 



