CARBON NUCLEI. 85 



plex or whether they are united with the protein only as impurities. Sev- 

 eral observations 1 show that in working with crystalline proteins a con- 

 tamination with ot'her protein substances is unfortunately not excluded, 

 and this must not be lost sight of, especially as the quantity of carbohy- 

 drates obtained is often very small. In this connection we must call 

 attention to the findings of OSBORNE and collaborators that on recrystalliz- 

 ing ovalbumin six times they found that the glucosamine content was 

 reduced to 1.23 per cent while other investigators give 7-8-15 per cent. 

 Under these circumstances we are not warranted in considering the 

 carbohydrate groups as belonging to the carbon nucleus produced on 

 the destruction of the real protein complex. 



The previously mentioned methods used in studying the structure 

 of 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 (amino valeric acid), leucine, and isoleucine. 3. Bibasic monamino- 

 acids: Aspartic acid and glutamic acid. 4. Oxy monamino-acids: serine oxy- 

 aminosuccinic 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). 



n. The Nuclei belonging to the Carbocyclic Series. 



Phenylalanine 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, tryosine, 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 



1 Osborne, D. B. Jones and Leavenworth, Amer. Journ. of Physiol., 24. 



