;6 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



If 1-serine has the configuration represented, d-alanine and 1-cystine 

 will have the groups attached to the asymmetric atom arranged in the 

 same order as in 1-serine : 



CH 3 CH 2 OH CH 2 S S CH 2 



NH 2 C H <= NH 2 C H > NH 2 C H NH 2 C H 



COOH COOH COOH COOH 



d-alanine 1-serine 1-cystine 



The foundation for the configuration of these amino acids will be 

 obtained if a-amino-/3-chloropropionic acid can be converted into 

 aspartic acid, the configuration of which is known from its relationship 

 to malic acid, (d-aspartic acid is converted into d-malic acid by nitrous 

 acid.) The configuration of malic acid can be referred to that of tartaric 

 acid and thence to d-glucose. 



Besides the above-mentioned products, several other amino acids 

 have been described as occurring in the protein molecule. Of these, 

 the presence of aminobutyric acid, which would complete the series 

 of monoaminomonocarboxylic acids, was assumed by Schutzenberger, 

 but has not been demonstrated by any of the subsequent investigators. 

 A large number of new products were added to the list by Skraup in 

 1904, but he has since shown that two of them were mixtures of 

 glycine and alanine. Another amino-oxy acid was described, as also 

 caseanic and casein ic acids ; the latter is apparently identical with 

 Fischer and Abderhalden's diaminotrioxydodecanic acid. Another 

 product, diamino-oxysebacic acid, was stated by Wohlgemuth to be a 

 constituent, but its presence as well as that of those described by 

 Skraup has not been definitely proved ; they cannot therefore be re- 

 garded as units of the protein molecule. 



Numerous amino acids including diamino- and oxyamino-acids 

 have also been synthesised of recent years by Neuberg and his co- 

 workers and by Sorensen. Our knowledge of these acids should render 

 the task of identifying a new unit in the protein molecule less laborious 

 than it has hitherto been. Their preparation was no doubt due to the 

 possibility of the presence of other units than those above described, 

 which possibility will not be excluded until the quantity of products 

 isolated reaches 100 per cent. 



The presence of glucosamine in the protein molecule is also a dis- 

 puted question ; there is no doubt that a carbohydrate containing nitrogen 

 is contained in the glucoproteins in their prosthetic group, but it is 

 doubtful if it is present in the protein part of the molecule, although a 



