456 



bacteriological chemistry 

 Table 34 



Pauling has suggested that the forces which hold the 

 complex of antigen and antibody together are a com- 

 bination of the weak van der Waals forces, which need 

 very close juxtaposition of the atoms concerned if they 

 are to be effective ; the weak forces exerted by the 

 polarisation of one atom by the dipolar character of 

 another ; and the stronger electrostatic attraction 

 between positively charged amino groups and negatively 

 charged carboxyl groups, for example, and the even more 

 powerful hydrogen bonds. The energy of electrostatic 

 attractions may be quite considerable, of the order of 

 5 Calories per gram molecule, if the appropriately charged 

 groups can come into close apposition. The hydrogen 

 bond, which results from the attraction of a hydrogen 

 nucleus from one electronegative atom by the unshared 

 electron pair of another electronegative atom, depends 

 for its strength on the degree of electronegativity of the 

 two atoms, the most electronegative, oxygen and nitrogen, 

 giving the strongest hydrogen bonds, with an energy of 

 about 5 Calories per bond. Specificity is due to the 

 complementary configuration and arrangement of the 

 groups of atoms in the antigen and antibody molecules 

 which could form hydrogen bonds, or give rise to electro- 

 static attraction, and this in turn depends on the size 

 and dispositions of the areas on antigen and antibody 

 molecules which could come into contact. If the arrange- 

 ments of groups of atoms in tlie molecules are such that 

 the molecules could only come into close contact at a 

 few 2)olnts, combination would bo weals, wJiereas if the 



