Physicochemical basis of antibody-antigen interaction

Hydrophobic interactions are a major driving force for the anti- 

Attractive interactions between non-polar residues of the antibody and the antigen are due to dispersion forces (transient mutual perturbation in electron clouds between two residues results in oscillating dipoles). This interactive force decreases with the inverse 7th power of the intermolecular distance until a certain minimum distance (the so-called van der Waals contact distance of about 0.3 to 0.4 nm). Two important characteristics of these forces are that they act, for practical purposes, over small distances only, and their additivity, i.e., the force between two large sites equals the sum of all interactions. A single van der Waals bond accounts for a decrease of only about 4 kJ/mol, which is just slightly more than the average thermal energy of molecules at room temperature (2.5 kJ/mol). 

Several conclusions may be drawn from these considerations. A high affinity of the antibody for the antigen is given by a multitude of weak interactions which require a close fit. The release of energy of 4 kJ/mol of a typical van der Waals bond results in a 5-fold 

As expected, the reaction of the hapten with the antibody has a stabilizing effect on the latter, since the N-terminals of the chains are no longer separated by a cleft, as in the free antibody, but held together by the hapten (Zavodsky et al., 1981). 

Influence of pH, ionic strength, temperature, and organic solvents on the stability of the antigen-antibody complex 

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