4 THE GENERAL CHARACTERS OF THE PROTEINS 



Ri Rii Riii Riv 



CH NH CO CH NH CO CHNH CO CH 

 COOH NH 2 



The presence of both an amino and a carboxyl group would 

 indicate the possibility of the formation of two kinds of salts, viz., 

 salts with acids and salts with bases. The acidity or basicity of a 

 protein is the property which most obviously requires investigation. If 

 all were constituted like the substance of the formula given above, with 

 R 1 , R", . . . R iv representing simple radicals, containing only carbon 

 and hydrogen, each molecule of protein would react chemically with 

 one molecule of acid or one molecule of base. In practice, however, 

 the problem of the acidity or basicity is not so simple as might at first 

 sight appear. In the first place, proteins do not yield on hydrolysis 

 simple monoamino acids ; other products of a more basic character 

 are obtainable, such as arginine, lysine (which is a simple diamino 

 acid) and histidine. Similarly on hydrolysis proteins yield amino 

 acids containing more than one carboxylic acid group, such as aspartic 

 acid, glutamic acid, etc. Simple polypeptides, yielding either 

 strongly acid or basic amino acids on hydrolysis, could be represented 

 by the typical formula, in which R 1 , R", etc., instead of representing 

 simple radicals of carbon and hydrogen, would represent radicals in 

 which one or more hydrogen atoms are substituted by a carboxyl 

 or amino group. 



The existence of such extra amino or carboxyl groups gives rise 

 to a further possible complication, for they can condense with 

 carboxyl and amino groups of other polypeptides, and thus give rise 

 to substances of highly complex molecular structure with branching 

 chains of polypeptide nature. The proteins, considered as polypeptides, 

 which yield on hydrolysis both diamino- and dicarboxylic acids, can 

 act, therefore, either as basic or acidic bodies, according to whether the 

 amino or carboxyl groups predominate. 1 



As a matter of fact, certain proteins, such as the protamines, first 

 described by Miescher, have very strongly marked basic characters ; 

 these substances yield on hydrolysis more than 80 per cent, of basic 

 products, and the molecules contain, therefore, a large predominance 

 of amino groups. Other proteins are of distinctly acid character, 



1 Nothing is known as to the stereochemical configuration of proteins. H. E. and 

 E. F. Armstrong have suggested that the main polypeptide can assume a spiral form, 

 and have constructed models to represent it according to this hypothesis. In this case 

 the more reactive groups might be represented as the freely moving groups external to 

 the spiral. The stereochemical configuration would probably also affect the reactivity 

 of the amino and carboxyl groups ; adopting the Armstrong configuration it is conceiv- 

 able that those which are external to the spiral would be reactive, and those protected 

 by groups of other atoms inert. Furthermore, it is conceivable that certain stereo- 

 chemical configurations would be favourable to the formation of imino groups through 

 the elimination of ammonia from contiguous amino groups, or of acid anhydrides by 

 elimination of water from carboxyl groups, diminishing the basicity in the former and 

 the acidity in the latter case. It is therefore not possible to predict the acidic or basic 

 character of the protein from the mere estimation of the hydrolysis products, ascertaining 

 thus whether diamino or dicarboxylic groups predominate, as stereochemical configura- 

 tion may render either sets of groups inactive. Furthermore, a certain amount of 

 ammonia is obtainable from most proteins on hydrolysis (see p. 33) ; this is assumed to 

 be derived by the hydrolysis of acid amides containing the group - CO - NH 2 . The 

 existence of such groups would diminish the acidity due to the presence of carboxyl 

 groups. 



