CHEMISTRY OF riiOTEINS 10 



grouped iiiulcr tho h('a(liii|!;.s of i)rotc'iiis, lipins, carbohydrates, salts, 

 and water, and no attempt will i)o made to give here more than the 

 most essential features concerning each. 



Proteins - 



In the last few years we have obtained something approaching a scientific 

 uuderstandinp; of the chemical nature of this great group of the most highly com- 

 ])lcx bodies known to chemistry. Our information has been obtained almost e.\- 

 clusively tiu-ough studies of the products obtained by splitting up the protein 

 molecule, for as yet relatively little has been accomplished through synthesis. 

 Proteins can be decomposed by the action upon them of acids or alkalies in various 

 concentrations, by sui)erhcatcd steam, by digestive ferments, and by bacteria. 

 The products obtained in these different waj's arc not all the same, for some sub- 

 stances may be formed by oxidation, reduction, decomposition, combination, or 

 condensation of the various products of simple cleavage, and it is necessary to 

 distinguish between the primary cleavage products (those which exist as radicals 

 within the molecule) and the secondary products (those not existing preformed in 

 the molecule but formed by transformation of the primary products). This can 

 usually be done, and it is found that so far as the primary products are concerned, 

 it makes little difference which method of cleavage (or hydrolysis, since in the 

 si)litting, water is combined with the organic substances) is used. 



At first the proteins split up into compounds still possessing many of the fea- 

 tures of the typical protein molecule, such as albumoses and peptones, and these 

 bodies are then further resolved into simpler substances, which are not aggregates 

 of several smaller molecules as are the proteins, and which can be obtained in pure 

 crystalline form. No matter which method is used we find the process going 

 through these stages, and, as before mentioned, the primary crj'stalline products 

 obtained are practically the same quantitatively as well as qualitatively. Some 

 methods, e. g., bacterial decomposition, however, lead in the end to more profound 

 or different decomposition of the cleavage products into secondary substances. 

 The similarity of the results obtained in these different ways indicates that there 

 are definite lines of cleavage in the protein molecule along which separation takes 

 place, independent of the nature of the agency at work, and that the substances 

 obtained represent the "building stones" of the entire molecide. 



These substances all have in common one important point: each one is an acid, 

 which has a NH2 group substituted for a hydrogen atom on the carbon nearest the acid 

 radical (the a-position). It makes no difference what the rest of the radicals are, 

 whether they are simple chains (leucine), or members of the cyclic or aromatic 

 series (tyrosine), or sulphur-containing bodies (cystine), without exception this 

 relation of a NH2 group to an acid radical is constant, as in this formula: 



NH2 



/ 

 R— CH— COOH. 



Through this arrangement every one of the constituents of the protein mole- 

 cule is provided with a group with a strong basic character arid a group with a 

 strong acid character, and hence it is possible for them to unite with one another 

 in indefinite numbers, and, because of the great variety of individuals, in practi- 

 cally an infinite number of combinations. It is believed that it is in just this way 

 that the protein molecule is built up. By artificially uniting various cleavage 

 products Emil Fischer succeeded in producing large molecules made up of several 

 amino-acid radicals (called by him "polypeptids")^ which show some of the char- 

 acteristics of the peptones, and this is the nearest that investigators have yet come 

 to synthesizing a protein molecule. The union is accomplished by the splitting 

 off of water, corresponding to the addition of water that occurs when the protein 



- For the complete literature of this subject see "The Chemical Constitution 

 of the Proteins," PHmmer, London, 1917; "The General Character of the Proteins," 

 Schryver, London, 1912; "The Vegetable Proteins," Osborne, London, 1910 (all 

 in the series of "Monographs on Biochemistrv"). Also "The Physical Chemistry 

 of the Proteins," T. B. Robertson, New York, 1918. 



3 Reviewed by Fischer, in Ber. deut. Chem. Gesell., 1906 (39), 530. 



