Effect of pressure on antigen-antibody complexes: modulation by temperature and ionic strength

Changes in the equilibrium binding affinity of antigen-antibody complexes subjected to hydrostatic pressures of about 2000 bar provide a potential means for the separation and recovery under mild conditions of biological molecules from immunoadsorbents or immunosensors. We have investigated the ability of temperature and ionic strength to modulate the pressure sensitivity of several antigen-antibody complexes in solution. For two different protein:monoclonal antibody complexes (BSA:9.1 and HEWL:HyHEL-10) exhibiting pressure-induced dissociation (positive association volume), we find little temperature dependence to the association volume. For another complex (digoxigenin:26-10) exhibiting pressure-induced association, the association volume increases with temperature, which, via a Maxwell relation, indicates that enthalpic changes drive the pressure effect. An increase in ionic strength decreases the affinity of binding the HEWL:HyHEL-5 complex, which contains several salt bridges. At low ionic strengths (<0.3 M), no pressure dependence of the free energy of association is observed, but at higher ionic strengths, significant pressure-induced association is observed, suggesting that positive contributions to the association volume provided by the salt bridges are counterbalanced by other (e.g., aromatic stacking) interactions that lead to negative association volumes. These results suggest that ionic strength may be used to modulate the pressure sensitivity of antigen antibody complexes, which may be useful in designing processes that exploit this phenomenon for immunoseparations.