Explicit solvent effects on macromolecular interactions from a solvent-augmented contact value theorem

The Derjaguin-Landau-Verywey-Overbeek (DLVO) theory has been a remarkably accurate framework for thecharacterization of macromolecular stability in water solvent. In view of its solvent-implicit nature neglectingthe electrostatics of water molecules with non-negligible charge structure and concentration, the precision of theDLVO formalism is somewhat puzzling. In order to shed light on this issue, we derive from our earlier explicitsolvent formalism [S. Buyukdagliet al.,Phys. Rev. E87, 063201 (2013)] a solvent-augmented contact valuetheorem and assess the contribution of solvent molecules to the interaction of charged membranes. We findthat in the case of hydrophobic membranes with fixed charges embedded in the membrane surface, the nearlyexact cancellation of various explicit solvent effects of substantially large magnitude but opposite sign keepsthe intermembrane pressure significantly close to the double-layer force of the DLVO theory. Then, in the caseof hydrophilic surface charge groups within the aqueous region, due to the spatial separation of the membranesubstrate from the location of the fixed charges where the nonlocal dielectric response of the structured solventis sharply localized, the interfacial field energy and the contact charge densities remain unaffected by the explicitsolvent. As a result, the hydration of the lipid head groups suppresses the signature of the solvent molecules fromthe membrane interaction force.

Automated summary

The DLVO theory is highly accurate for characterizing macromolecular stability in water solvent, but the reason behind its accuracy is puzzling. Through research, it has been found that for hydrophobic membranes with fixed charges embedded in the surface, the various explicit solvent effects are nearly canceled out, keeping the intermembrane pressure close to the double-layer force of the DLVO theory. For hydrophilic surface charge groups within the aqueous region, the solvent molecules' influence on the membrane interaction force is suppressed by the hydration of the lipid head groups.