INTRODUCTION 5 



1. Aminobutyric Acid. The presence of aminobutyric acid, which 

 would complete the series of monoaminomonocarboxylic acids, was as- 

 sumed by Schiitzenberger [1879]. A small amount of a substance of 

 the empirical formula C 4 H 9 NO 2 was isolated by Foreman [191 3, 2] from 

 the products of hydrolysis of caseinogen. Foreman considered it to 

 be a-aminoisobutyric acid, but his evidence is not quite sufficient for 

 the identification of the product. Abderhalden [Abderhalden and 

 Weil, 1913, 3] subsequently stated that an amino acid of this composi- 

 tion had been obtained from a great number of proteins but that its 

 constitution was not established. 



2. Norleucine or a-amino-n-caproic Acid. In addition to the two 

 isomers, leucine and isoleucine, of the empirical formula C 6 H 13 NO 2 , 

 another isomer, norleucine, has been shown to be present in the protein 

 of the nervous tissue of the brain [Abderhalden and Weil, 1912, 2 ; 

 1913, 1,2, 3]. It is apparently identical with glycoleucine described 

 by Thudichum [1901]. This unit is most likely present in other pro- 

 teins also. Further evidence of its more general occurrence is given 

 by the results of Skraup and Witt [1907], who obtained ^-valerianic 

 acid, which can only arise from a normal caproic acid, by the action 

 of alkaline hypobromite on caseinogen and of Heckel [1908], who by 

 the same method obtained w-valerianic acid from the leucine fraction 

 of amino acids prepared from caseinogen ; also of Samec [1908], who 

 obtained ^-caproic acid by the reduction of the leucine fraction of amino 

 acids prepared from elastin. 



3. Skraup [1904] added several new units to the list. Two of 

 these, diaminoadipic acid and diaminoglutaric acid, were subsequently 

 [1905, 2] shown to be alanine and a mixture of glycine and alanine. 

 The other compounds were hydroxyaminosuccinic acid, C 4 H 7 NO 5 , di- 

 hydroxydiaminosuberic acid, C 8 H 16 N 2 O 6 , caseanic acid, C 9 H 16 O 7 N 2 , and 

 caseinic acid, C 12 H 16 O 5 N 2 . These substances were not found by Skraup 

 [1905, i] in gelatin, but another compound, C 12 H 25 N 5 O 10 , was obtained. 



Adensamer and Hoernes [1905], who used the same method as 

 Skraup, could only isolate small quantities of caseanic and caseinic 

 acids from egg-albumin. All these substances are most probably 

 mixtures. Caseinic acid resembled in many ways the substance 

 diaminotrioxydodecanic acid. 



4. The two substances, hydroxyaminosuberic acid, C 8 H 15 O 5 N, and 

 hydroxydiaminosebacic acid, C 10 H 20 O 5 N 2 , described by Wohlgemuth 

 [1904, 1905] in the protein of the liver are, like the above, probably 

 mixtures. 



