132 THE PROTEINS AND THE AMINO-ACIDS 



Incomplete hydrolysis, however, results in the production of a 

 number of intermediate substances, variously designated, in the order 

 of decreasing complexity, Proteoses (Albumoses) , Peptones, Polypep- 

 tides and Dipeptides. The hydrolysis of the proteins, therefore, occurs 

 in stages, just as, in the hydrolysis of starch, intermediary stages (the 

 dextrins and maltose) are passed through before the attainment of the 

 last stage of hydrolysis and the quantitative conversion of the starch 

 into glucose. 



It is not certain, however, whether the various intermediate pro- 

 ducts of protein hydrolysis represent successive stages of hydrolysis 

 or whether in some instances comparatively simple products may not 

 be split off from the proteins or proteoses, leaving complex residues, 

 so that complex and simple intermediate substances are produced 

 simultaneously. Probably both types of cleavage occur at different 

 points in the protein molecule. Those linkages which are most acces- 

 sible to the action of the particular catalyzer employed will be dis- 

 rupted firstj and if some of them chance to lie near the extremities of 

 the molecule, simple products and a complex residue will result, while 

 disruption of more internal linkages will break the molecule into parts 

 of more equal weight and complexity. 



It was early recognized that amino-acids form the chief part of the 

 decomposition-products which result from the hydrolysis of protein. 

 The separation of the individual amino-acids from one another, and 

 their quantitative estimation, was a much more difficult matter. The 

 first attempts to isolate individual amino-acids from the mixture which 

 the complete hydrolysis of a protein yields, depended upon the frac- 

 tional crystallization, either of the free amino-acids or of their salts. 

 Except in the case of the very slightly soluble amino-acids, such as 

 tyrosine, these methods were not even approximately quantitative, 

 and even the isolation and identification of a given amino-acid could 

 only be effected with certainty when that acid was present in relatively 

 large amounts. The attainment of our present relatively extensive 

 knowledge of the nature and quantities of the amino-acids which 

 result from protein hydrolysis, is an achievement of the past twenty 

 years, and we owe it in the first place to the investigations of Kossel 

 and of Emil Fischer and their pupils. 



The various amino-acids which are yielded by the proteins are limited 

 in number, and probably do not exceed eighteen or nineteen. These 

 however, fall into several very distinct classes, namely: 



Monoamino-monocarboxylic acids, general formula: 



H 2 N.R.COOH 



Monoamino-dicarboxylic acids, general formula: 



/coon 



H 2 N.R 



