PROTEINS OR ALBUMINOUS SUBSTANCES 435 



" The nineteen amino-acids in the table are the chief hydro- 

 lytic cleavage products of the proteins and those which are 

 generally met with. . . . The proportions in which the various 

 amino-acids are obtained from the different proteins vary very 

 considerably. In some cases they are altogether lacking, as may 

 be proved by application of the definite tests for tyrosine, 

 tryptophane, or glycine ; but it is worthy of note that, as a rule, 

 the amino-acids referred to as isolated from the mixtures pro- 

 duced by subjecting albuminous substances to hydrolysis all 

 occur almost without exception ; especially is this true of the 

 important proteins which play the chief part in animal or 

 vegetable metabolism ; so that the conclusion must be drawn 

 that none of them can be dispensed with in organic life. With 

 the exception of diaminotrihydroxydodecanoic acid they have 

 all been so thoroughly investigated that their structure is well 

 established. The majority also have been synthesised, proof of 

 their structure having, in fact, been given in this way. Only 

 oxyproline, histidine, and diaminotrihydroxydodecanoic acid 

 remain still to be synthesised. 



" With the exception of glycine all the amino-acids derived 

 from natural sources are optically active ; but when prepared 

 by ordinary synthetic methods, as is well known, they are 

 obtained in the first instance in the racemic form. The resolution 

 of the racemoids into their optically active components has been 

 effected quite recently in most cases. Asparagine, however, 

 which is closely related to aspartic acid, had been resolved into 

 the two active forms by recrystallising the inactive synthetic 

 product from water, and separating the two constituents 

 mechanically. Moreover, in the case of some other amino- 

 acids, for example leucine, the antipode of the natural form 

 had been obtained by partially fermenting the synthetic product 

 with moulds. The complete synthesis of the active amino-acids 

 which are obtained from natural sources was first accomplished 

 by the method I introduced based upon the use of the acyl 

 derivatives. The method has been applied with success to the 

 majority of the synthetic products ; its extension to the re- 

 maining cases, proline, lysine, tryptophane, and cystine, is not 

 likely to be attended with any difficulties. The synthetical 



amino groups than in the conjunction in the natural substances of groups of 

 several different kinds. The further study of the proteins tends to show that 

 the intricacy of the subject increases with progress of knowledge. 



