2 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



biuret reaction, but they had no other characteristic properties. The 

 anhydrides of aspartic acid were more fully investigated by Schiff in 

 1897-1899, and he assigned formulae to the various products. These 

 formulae show that there existed at this time no concrete idea of the 

 mode of combination of the amino acids in a protein molecule. 1 



Several remarkable compounds were described by Curtius during 

 his investigations of the derivatives of glycine from 1881-1888. They 

 were : 



A, two anhydrides which resulted when glycine ester was kept 

 for some time ; the one anhydride was insoluble and was proved to be 

 derived from two molecules of glycine ; the other anhydride was readily 

 soluble and gave the biuret reaction ; hence it was termed the biuret 

 base. This base and other anhydrides of glycine formed the subject of 

 research by several other observers. The first anhydride of Curtius 

 has been shown to be a compound which we now term glycine anhydride 

 and the biuret base has been shown to be the ester of triglycyl-glycine. 



B, two acids of high molecular weight obtained by the action of 

 benzoyl chloride upon the silver salt of glycine ; the one was found 

 to be hippuryl-glycine or benzoyl-glycyl-glycine, the first definite 

 compound known which contained two amino acid radicles in com- 

 bination ; the other had the formula C 10 H 12 N 3 O 4 . 



The composition of this compound could not then be ascertained 

 but was proved many years later when Curtius returned to the subject 

 in 1890. With the help of his pupils he subsequently prepared a 

 series of compounds in which the amino acids are linked together. 

 These compounds all contain a benzoyl group in their molecule, and 

 have consequently not attracted the attention of other workers ; their 

 mode of synthesis is peculiar and they undergo many interesting re- 

 actions (pp. 19-26). 



Hofmeister's Summary. 



No definite theory of the mode of combination of the amino acids 

 in a protein seems to have been enunciated before the beginning of 

 the twentieth century, although the same idea was already in the 

 minds of several workers at this time. Hofmeister summarised and 

 criticised the exact position of our knowledge of the proteins in 1902 

 as follows : 



1 Textbooks on organic chemistry scarcely mentioned the chemistry of the proteins, and 

 some textbooks on physiology published at this time gave fantastic representations for the 

 formula of a protein. 



