102 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Serine. 



Serine is, as yet, the only member of the oxyamino acids of the 

 aliphatic series which has been isolated with certainty from the mixture 

 of decomposition products of the proteins. It was first obtained in 

 1865 by Cramer from silk-gelatin, and was not again obtained until E. 

 Fischer isolated it from the various proteins which he and his pupils 

 have examined. Its occurrence in the free state, in human sweat, has 

 been noticed by Embden and Tachau. 



Cramer, its discoverer, showed that, when serine was treated with 

 nitrous acid, it was converted into glyceric acid, and he recognised it as 

 an aminolactic acid. It was regarded as a-amino-/3-oxypropionic acid, 

 but this was only definitely proved when it was synthesised by Fischer 

 and Leuchs in 1902 from glycollic aldehyde, hydrogen cyanide and 

 ammonia, which is the first instance of the employment of Strecker's 

 method of building up oxyamino acids from oxyaldehydes : 

 CH 2 OH NH 3 CH 2 OH CH 2 OH 



NH 



H 2 O 



CH. 



CHO HCN C NH 2 | 



\ COOH 



N CN 



Serine is another of the amino acids which Erlenmeyer jun. syn- 

 thesised in 1902 from hippuric acid, and which he described in detail 

 with Stoop in 1904; by condensing formic ester and hippuric ester 

 with sodium ethylate they obtained oxymethylene hippuric ester or 

 formyl hippuric ester : 



H . COOC 2 H 5 + NaOC 2 H 5 = NaO . HC(OC 2 H 5 ) 2 



/NH . COC 6 H 5 /NHCOC 6 H 5 



NaO . CH(OC 2 H 5 ) 2 + H 2 C/ = NaO . CH=C\ + 2 C 2 H 5 OH 



^COOC 2 H 



The free ester, obtained from the sodium salt as a thick oil, on 

 reduction with aluminium amalgam gave N-benzoyl serine ester : 



/NHCOC 6 H 5 /NHCOC 6 H 5 



HO . CH=C/ + H 2 = HO . CH 2 . CH( 



X COOC 2 H 5 X COOC 2 H 5 



which, when hydrolysed with dilute sulphuric acid, was converted into 

 benzoic acid and serine : 



/NH . COC 6 H 5 



H' + 2 H 2 = HO.CH 2 .CH(NH 2 )COOH 



X COOC 2 H 3 



