SPECIAL REACTIONS OF THE ALBUMINS. 39 



outcome of quantitative variations and of differences of chemical 

 union, rather than of qualitative composition. These radicles may 

 be classified as follows : 



I. Radicles of the aliphatic series. 



1. Amino acids : 



(a) Mono-amino acids. 



(ft) Radicles of mono-amino-monocarbonic acids ; the leucin 

 group. The most notable radicle of this order is leucin (-amino- 

 isobutyl-acetic acid) ; it predominates quantitatively and is present 

 in all typical albumins. Next in frequency follow glycocoll (amino- 

 acetic acid) and alanin (Vamino-propionic acid) ; more rarely amino- 

 valerianic acid and possibly also isoleucin (a-amino-methyl-ethyl 

 propionic acid). 



(ft) Radicles of mono-amino-dicarbonic acids ; the glutaminic acid 

 group. Of these, aspartic acid (ft-amino-succinic acid) and gluta- 

 minic acid (-amino-glutaric acid) are probably never absent in 

 typical albumins, the latter predominating in quantity. 



(j) Radicles of mono-amino-oxymonocarbonic acids ; only one 

 member of this group has been found, but is probably present in all 

 albumins, viz., serin (a-amino-/?-oxypropionic acid). 



(6) Diamino acids. 



Two radicles belonging to the diamino-monocarbonic acids have 

 been found, viz., ornithin (--diamino-valerianic acid), which is 

 obtained on hydrolysis with acids in combination with the guanidin 

 remnant in the form of arginin ; and lysin (a-e-diamino-capronic 

 acid). 



In addition, a representative of the diamino-oxymonocarbonic 

 acids has been encountered (in casein), viz., diamino-trioxy-dodecanic 

 acid. 



(c) Sulphur-containing amino acids. 



The sulphur complex of the albumins is usually obtained on 

 hydrolysis as cystin, viz., as the disulphide of cy stein (a-amino-/?- 

 thiolactic acid ; hence a-diamino-/9-dithio-dilactic acid). From 

 serum-albumin and the keratins Friedmann further obtained the 

 non-nitrogenous a-thiolactic acid, and suggests that this also may 

 occur in the albuminous molecule preformed. Some albumins 

 further contain sulphur radicles, which are obtained as mercaptans 

 or as ethyl sulphide. 



(d) Histidin (-amino-/9-imidazol-propionic acid). 



2. Amino alcohols (hexosamins, the carbohydrate group of the 

 albumins). 



In many but not in all albumins there is a radicle which on hy- 

 drolysis appears as a hexosamin and apparently always as chitosamin 

 (^lucosamin). In its place or in addition there may be other nitrog- 

 enous or non-nitrogenous carbohydrate radicles. 



II. Radicles of the aromatic series. 



1. To judge from the constancy with which phenyl-alanin 

 is encountered among the hydrolytic decomposition-products there 



