148 LECTURE VIII. 



Glycocoll was one of the first-known cleavage-products of proteins. As 

 early as 1820 Braconnot l obtained it, in conjunction with leucine, on boiling 

 glue with dilute acid or alkali. It is also obtained, as such, from the 

 muscles of the scallop, Pecten irradians. 



Alanine is the next homologue of glycocoll. It is an a-amino-propionic 



acid: CH 3 . CH(NH 2 ). COOH. It contains an asymmetric carbon atom, 

 * 



*, and is consequently optically active, as are most of the other acid 

 cleavage-products of albumin, which itself rotates the plane of polarized 

 light. Alanine, as it occurs in nature, is dextro-rotary. 



An ammo-butyric acid has been described as a cleavage-product from 

 proteins. Later investigations, however, have not shown its presence in 

 the protein molecule. On the other hand, its next homologue, amino- 

 valeric acid, has been obtained very often. The amino-valeric acid so far 

 isolated from the proteins does not have a normal chain, but a branch- 



PTT 



ing one. It is an a-amino-isovaleric acid: nTT 3 >CH . CH(NH 2 )COOH. 



^*l3 -X- 



It is dextro-rotary. 



Leucine also has a branching chain, and is an a-amino-isobutyl-acetic acid: 



>CH . GH 2 . CH(NH 2 )COOH. The leucine, usually obtained by the 

 & 



cleavage of proteins, is Z-leucine. In this form it occurs in many plants 

 and in invertebrates. Penicillium glaucum produces d-leucine from the 

 inactive leucine. The constitution of leucine has been proved by E. 

 Schulze and A. Lickiernik. 2 



Felix Ehrlich 3 has recently separated an isomer of leucine from molasses 

 sludge, and, soon after, also showed its presence in many plant, and 

 animal proteins. It is an a-amino-methyl-ethyl-propionic acid: 



r? w 3 >CH . CHNH 2 . COOH. 

 ^2*15 # 



Its constitution has been proved by its synthesis, and also that it is 

 decomposed by pure-culture yeast into d-amyl- alcohol. The relations 

 of iso-leucine to d-amyl-alcohol are shown as follows. We will com- 



3 



1 H. Braconnot: Ann. Chim. Phys. 13, 113 (1820). 



2 Ber. 24, 669 (1891), and Z. physiol. Chem. 17, 513 (1893). 



3 Felix Ehrlich : Ber. 37, 1809 (1904) ; Z. Ver. Zuckerind, 1904, 975 ; 55, 

 592 (1905). We shall dwell upon the syntheses and decomposition of the amino 

 acids, only as much as is necessary in order to understand biological processes. 

 We may also add, that this form of synthesis has often been utilized in the reac- 

 tions between ammonia and the halogen fatty acids. Cf. E. Fischer: Ber. 39, 530 

 (1906). 



