8 THE EQUILIBRIUM OF COLLOID AND 



amidogen (NH 2 ) in the methyl group (CH 3 ), there is formed instead 

 the body CH 2 (NH 2 ).COOH which is the amido-acid. The carboxyl 

 group (COOH) being still free, the amido-acid retains acid properties, 

 but in lessened degree, on account of the presence in the molecule 

 of the basic group (NH 2 ). The presence of the basic group also at 

 the same time confers the properties of a base, so that the amido- 

 acid now has the peculiar property of being able to functionate 

 either as acid or base. Thus with copper it forms a deep blue 

 soluble compound called copper glycocoll, and when in union with 

 other organic acids it forms well-known and important substances 

 found in the body, for example, the compound with benzoic acid 

 known as hippuric acid, and the compound with cholalic acid 

 occurring in the bile as glycocholic acid. 



Amido-acids possessing only one amidogen group are termed 

 mon-amino-acids ; others exist possessing two such basic groups 

 in their molecule, and these are called di-amino-acids. A number 

 of both classes occur amongst the products of hydrolytic cleavage 

 of the proteins. Again, there is in the majority of cases only one 

 acidic or carboxyl group, but there are sometimes two or more 

 such acid groups, and then the amido-acids are referred to as 

 mono- and di-basic, &c., as in the case of ordinary organic acids. 



The most striking chemical characteristic of all these amido- 

 acids, and that which from the point of view of protein formation 

 interests us most at present, is that of undergoing conjugation or 

 condensation with one another or with other organic bodies to 

 form long chains in single series, or it may be main chains with 

 side or branch chains arising from them. 



In each union of this kind the elements of a molecule of water 

 are eliminated, a hydrogen atom being yielded by one of the two 

 combining molecules and a hydroxyl radicle by the other, and in 

 the great majority of instances the union occurs between the 

 amidogen group of one and the carboxyl group of the other. 



For the reader who is not acquainted with the technical terms 

 of organic chemistry, the nature of the process of combination to 

 form protein, and further of proteins to form bioplasm, may be 

 illustrated by the use of electrical terms. The amido-acid, on 

 account of its possessing both an acidic and a basic group, may 

 be considered as possessing a sort of polarity (indeed it does possess 

 a kind of chemical polarity) ; as a result of this polarity a chemical 

 attraction exists between acidic pole and basic pole of different 



