306 THE EQUILIBRIUM OF COLLOID AND 



radicles of the proteins can again be made to unite. In the latter 

 direction the process cannot be carried back quite to that degree 

 of complexity which yields the naturally occurring protein, mainly 

 because at that level the degree of chemical association of the 

 constituents is so delicately balanced that the chemical manipula- 

 tions cause splitting off and decomposition. 



The synthesised products are in fact beginning to possess that 

 delicately balanced power of associating and dissociating which is 

 characteristic in still higher degree of living matter, and it is for 

 this reason that only the living cell has hitherto been able to put 

 the finishing touches upon the delicate unions which finally yield 

 proteins, and beyond these up to living protoplasm, where the 

 complexity and corresponding instability reach their acme. 



The organic radicles, which form the building stones, so to 

 speak, for the structure of the protein molecules, may be divided 

 into three classes viz., those which are purely organic bases, those 

 which are entirely organic acids, and a third and most characteristic 

 class which possess both the properties of organic acids and organic 

 bases in modified degree. The compounds of this third class are 

 known as the amino- or amido-acids, and it is to them that the pro- 

 teins owe their peculiar property of building up into such complex 

 bodies of higher molecular weights. 1 The simplest type of amino- 

 acid contains one organic acid radicle and one basic radicle, the 

 acid character being given by the carboxyl group (COOH) and the 

 basic character by the amidogen group (NH 2 ). As the simplest 

 example, glycocoll, or glycine, which is the amino-acid of acetic 

 acid, may be quoted. Acetic acid is CH 3 .COOH, and is purely 

 acid in its properties, combining with bases ^such, for example, as 

 ammonium to form the acetate (CH 3 .COO.NH 4 ). If the ammonia, 

 instead of neutralising the carboxyl group, becomes attached, 

 with the loss of one atom of hydrogen, as the group amidogen (NH 2 ) 

 in the methyl group (CH 3 ), there is formed instead the body 

 CH 2 (NH 2 ).COOH, which is the amino-acid. The carboxyl group 

 (COOH) being still free, the amino-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 amino- 

 acid now has the peculiar property of being able to function 



1 The terms amido-acid and amino-acid have the same meaning, and are 

 used indiscriminately in describing members of this class of compounds. 



