PRIMARY DISSOCIATION-PRODUCTS 



57 



. CH 2 CH . NH 2 CO j OH 



S . CH 2 CH . NH 2 CO . OH 



A-cystin or Protein-cystin 



(a-amino-/3-thioglyceric acid 



'disulphide.') 



CH 2 . NH 2 CH . S CO . OH 

 CH 2 . NH 2 CH . S CO . OH 



B-cystin or Stone-cystin 



(a-thio-/3-amino-glyceric acid 



' disulphide.') 



Both cystins may be regarded as sulphuretted serins (see p. 33) ; 

 being isomers they resemble one another in certain respects, but differ 

 markedly in the following points, according to Neuberg and Mayer l 



5. 



B-cystin or Stone-cystin. 



1. Pure, optically active compound 



crystallises in needles. 



2. Racemic cystin is amorphous, and 



cannot be made to crystallise by 

 means of protein-cystin. 



3. Specific rotary power is - 206. 



4. Melts at 190-192, giving rise to a 



foam. 



5. The mercury salt prepared in the 



same way has no constant composi- 

 tion ; it becomes reddish brown on 

 drying, and then black. 



It is very stable and pure white. 



In addition to these characteristics Neuberg and Mayer give six 

 other points of difference, so that there cannot be any doubt as to real 

 existence of two isomeric cystins. 



Protein-cystin when dissociated with HC1 under pressure gives rise 

 in addition to H 2 S and NH 3 to a-alanin, while stone-cystin gives rise 

 to a-thiolactic acid, according to K. A. H. Morner, 2 who was the first 

 to point out the existence of two kinds of sulphur atoms in albumin. 



A-cystin or Protein-cystiu. 



1. Pure, optically active compound 



crystallises in six-sided plates. 



2. Racemic compound crystallises in 



needles arranged in bunches resem- 

 bling tyrosin, and also in globules. 



3. Specific rotatory power is - 224. 



4. No melting point ; decomposes be- 



tween 258-261. 



The mercury salt, made from 1 

 molecule of cystin and 2 molecules 

 of NaOH and HgCl 2 corresponds 

 exactly to the formula 

 S . CH 2 . CH . NH 2 . COO. 



Hg 



CH 2 . SH CH . NH 2 COOH 



Protein-cystin. 



CH 3 CHNH 2 COOH 



a-alanin. 



CH . NH 9 CH. SH COOH -> CH Q CH. SH COOH + NH C 



+ H 2 S 



+ sulphuretted 

 hydrogen. 



Stone-cystin. 



->l 



a-thiolactic acid. 



+ ammonia. 



An inactive modification of what Neuberg and Mayer call 'stone- 

 cystein ' has been prepared by Gabriel, 3 who calls his compound iso- 



1 Carl Neuberg and Paul Mayer, Zeitschr. f. physiol. Ghem. 44. 472 (1905). 



2 K. A. H. Morner, ibid. 34. 295 (1902). 

 3 S. Gabriel Ber. d. deutsch. chem. Gesell. 38. 637 (1905). 



