34 BACTERIOLOGICAL CHEMISTRY 



lyopliilic because they contain a high proportion of polar 

 hydroxyl groups, — OH, and oxygen links, — — . 



Adsorption. — It is the attractive forces between these 

 polar groups in neighbouring molecules which are largely 

 responsible for holding groups of molecules together, as, 

 for example, in protein or cellulose fibres. They also 

 account in very large measure for adsorption phenomena, 

 and particularly for specific adsorption. Large molecules, 

 like those of proteins, will have definite distribution 

 " patterns " of polar groups according to the amino - 

 acids of which they are built up, and depending on the 

 order in which the amino -acids are arranged in the 

 molecule. Other molecules which may have a distribution 

 of polar groups giving a " pattern " corresponding to that 

 on the protein molecule will be adsorbed ; if the polar 

 patterns do not correspond there will be less and less 

 adsorption or weaker and weaker adsorption as the pat- 

 terns differ more and more from one another. This offers 

 a physical explanation for the well-known " lock and 

 key " simile which Emil Fischer suggested to explain 

 enzyme specificity and which Ehrlich used of antibody 

 specificity. 



Now if the polar groups on a protein are in close 

 proximity to those on the second adsorbed molecule, 

 which may be represented diagrammatically thus : — 

 + ^ + + ^) the charges due to the polar groups will 

 neutralise one another, to a certain extent at least, if 

 not completely, and the complex of the two molecules 

 will no longer be polar in effect, and as a result will no 

 longer have such an attraction for water molecules. If 

 enough of the polar groups are masked in this way the 

 system will more and more lose its lyophilic character 

 and more and more come to resemble a lyophobic colloid. 

 In this state it will be readily aggregated by metallic 

 ions. If the protein is an antibody and the adsorbed 

 molecule an antigen we get a picture of the sort of thing 



