32 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The occurrence of alanine in proteins was first shown by Schutzen- 

 berger, 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 trypto- 

 phane, 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. Schutzenberger, in 1879, also obtained 

 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 and other proteins. The preparation 

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

 thetical a-aminoisovalerianic acid, 



CH 3 v 



yCH . CH(NH 2 ) . COOH, 



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

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

 aldehyde ; 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 com- 

 pound in 1906 by E. Fischer. 



