72 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The third method of separating optically active substances by com- 

 bining them with optically active bases or acids had not been employed 

 with any success until E. Fischer took up this question, the study of 

 the optically active amino acids being his first work upon the chemical 

 constitution of the proteins. The non-success of this method was in all 

 probability due to the small affinity which the simple amino acids them- 

 selves have for combining with acids and bases ; even the attempts to 

 separate the monoaminodicarboxylic acids, which are fairly strong acids, 

 were not successful. 



Hippuric acid, or benzoylglycine, has been known for a long time, 

 and by preparing the benzoyl derivatives of the other amino acids, 

 Fischer found that their acidic character was greatly increased, and that 

 they then combined with the optically active bases brucine, strychnine, 

 cinchonine, morphine, forming stable salts. These salts were much less 

 soluble and their power of crystallising much greater than the salts of 

 the amino acids themselves, and consequently they were more easily 

 isolated ; further, they were easily reconverted into the amino acids. 



These benzoyl derivatives were prepared by shaking the amino acid 

 with excess of benzoyl chloride in the presence of sodium bicarbonate 

 instead of in the presence of excess of alkali, i.e. y by the Schotten- 

 Baumann method, which gave poor and varying yields of the benzoyl 

 compound. 



Alanine, aspartic acid, glutamic acid, tyrosine, leucine, phenylalanine 

 and also a-amino-n-caproic acid and a-aminobutyric acid have in this 

 way been separated by Fischer into their optically active isomers. To 

 these must be added ornithine which was synthesised by Sorensen in 

 1903, and separated into d- and 1-ornithine in 1905. 



Not only can the benzoyl derivative be employed for this purpose 

 but also the formyl derivative which is prepared by heating the amino 

 acid with anhydrous formic acid at 100 C. These formyl derivatives 

 also give beautifully crystalline salts with the optically active bases, and 

 they possess one great advantage over the benzoyl derivatives, namely, 

 that the formyl group is easily removed by hydrolysis, whereas the ben- 

 zoyl group requires prolonged heating with a large excess of acid for 

 its removal. The formyl derivative is of enormous advantage also for 

 building up optically active polypeptides, as it admits of the preparation 

 of large quantities of the optically active amino acids. 



Fischer and his pupils have thus prepared the optically active forms 

 of leucine, phenylalanine, and valine, and also of phenylaminoacetic acid, 

 and Locquin has prepared, by means of the formyl derivative, d-isoleucine. 



In the case of serine, no separation could be effected by means of 



