THE CHEMICAL CONSTITUTION OF THE UNITS 87 



The occurrence of alanine in proteins was first demonstrated by 

 Schiitzenberger, who did not actually identify his product with the 

 synthetical one; Weyl in 1881 obtained it as a decomposition product 

 of silk and showed that his preparation was similar in properties to 

 Strecker's synthetical alanine. He thus established it as a constituent 

 of a protein molecule. The researches of Emil Fischer have shown that 

 alanine is a constant constituent of all proteins. It is worthy of note 

 that of the eighteen definitely determined units of a protein molecule, 

 six of them, namely, isoleucine, phenylalanine, tyrosine, serine, histidine 

 and tryptophane, are derivatives of a-aminopropionic acid. 



Valine. 



A body of the composition C 5 H n NO 2 was obtained in 1856 by 

 v. Gorup-Besanez from an aqueous extract of pancreas, and on account 

 of its similarity in properties to leucine he regarded it as a homologue of 

 leucine and termed it butalanine. Schiitzenberger, in 1879, also ob- 

 tained a substance which had this empirical formula and properties like 

 that of leucine. 



An aminovalerianic acid was described in 1883 by Schulze and 

 Barbieri as occurring in the seedlings of yellow lupines, and subsequently 

 Schulze again isolated it from the extracts of other seedlings. It 

 appeared to correspond to n-aminovalerianic acid, which had been 

 synthesised by Lipp. 



In 1899 Kossel isolated a similar substance from the protamine 

 clupeine of herring milt, and since then E. Fischer and his pupils have 

 obtained it from caseinogen, horn a'nd other proteins. The preparation 

 from horn, when racemised, corresponded in properties with the syn- 

 thetical a-aminoisovalerianic acid, 



CH 3 \ 



)CH . CH(NH 2 ) . COOH, 

 CH 3 / 



which had been first prepared by Clark and Fittig in 1866 from the 

 corresponding bromo-compound and later by Lipp in 1880 from iso- 

 butyraldehyde ; its derivatives were identical with those of this acid 

 which were prepared by Slimmer in 1902. The exact identity of the 

 natural and synthetical substances was only established in 1906, when 

 Fischer prepared d-aminoisovalerianic acid from the synthetical product, 

 and showed that its specific rotation was the same as that of Schulze 

 and Barbieri's natural substance. The name valine was given to this 

 compound in 1906 by E. Fischer. 



