84 THE PROTEIN SUBSTANCES. 



The previously mentioned methods used in studying the structure 

 t>f the protein substances are not of the same value, but they in part 

 substantiate each other. Of these we must mention the hydrolysis 

 by means of boiling dilute mineral acids, or by proteolytic enzymes, 

 as the best methods for obtaining the carbon nuclei in the protein mole- 

 cule. The most important of the carbon nuclei obtained are as follows: 



I. The Nuclei belonging to the Aliphatic Series, 



A. Sulphur free, but containing nitrogen: 1. A guanidine residue (combined 

 with ornithine as arginine). 2. Monobasic monamino-acids: Glycocoll, alanine, 

 valine, leucine, and isoleucine. 3. Bibasic monamino-acids: Aspartic acid 

 and glutamic acid. 4. Oxy monamino-acids: serine oxyaminosuccinic acid and 

 oxyaminosuberic acid. 5. Monobasic diamino-acids: Diaminoacetic acid, ornithine 

 (from arginine) and lysine. 6. Oxy diamino-acids: Oxydiaminosuberic acid, 

 oxydiaminosebacic acid, diaminotrioxydodecanoic acid, caseanic and caseinic acids. 



B. Sulphurized: Cysteine and its sulphide cystine, thiolactic acid, mercaptans, 

 and ethyl sulphide. 



II. The Nuclei belonging to the Carbocyclic Series. 

 Phenylaminopropionic acid and tyrosine. 



III. The Nuclei belonging to the Heterocyclic Series, 

 Proline, oxyproline, tryptophane and histidine. 



In regard to these carbon nuclei it must be remarked that they are 

 not all found in every protein body thus far investigated, and also that 

 one and the same cleavage product, such, for example, as glycocoll, 

 leucine, tyrosine, etc., is obtained in very variable amounts from differ- 

 ent protein substances. 



It is very difficult to say to what extent all the above-mentioned 

 carbon nuclei exist in the protein molecule. It is not inconceivable 

 that in the hydrolysis certain carbon nuclei may be secondarily formed 

 from others. We cannot exclude the possibility, as suggested by LoEW, 1 

 that in the hydrolysis a marked atomic displacement perhaps occurs 

 before cleavage, and for this reason two carbon nuclei, such as leucine 

 and lysine, or tyrosine and phenylalanine, may be produced from the 

 same atomic groupings, each according to the nature of the neighboring 

 groups. Such a possibility cannot be entirely excluded, but it has been 

 made rather improbable by recent investigations. 



Even if we admit the above, still it is undoubtedly true that the chief 

 cleavage products of the protein substances are amino-acids. EMIL 

 FISCHER has shown that the amino-acids have the property of readily 

 grouping together when water is split off and the amide group of one 



1 Loew, Die chem. Energie d. lebenden Zellen, Miinchen, 1898, and Hofmeister's 

 Beitrage, 1. 



