Solvent-Induced Forces between Macromolecules

The solvent-induced forces operating between any two molecules were discussed in sections 6.4 and 7.14. Here we extend the treatment for two macromolecules, denoted by L and P. These could be two proteins, two DNA molecules, two membranes, or any two walls containing H0I groups on their surfaces. The emphasis here is on the origin of strong and relatively long-range forces induced specifically by water as a solvent. 

Consider the force on L at Xl given P at Xp in water at a given temperature and pressure. The general expression for the indirect force or solvent-induced (SI) force on L is 

FIGURE 8.19. A conformational equilibrium between A and B. In the absence of ligand x% x A In the presence of ligand the equilibrium shifts towards A, so that x b x a.  

In general, Vl / is a six-dimensional vector. Here we shall discuss only one component of the force, the component perpendicular to the surfaces of L and P as shown in Fig. 8.20/4. Since 

Both and F, when considered per unit surface area of L, are expected to be on the same order of magnitude as in the case of a simple small solute L. By unit area of L we mean here an area on an order of magnitude of the cross-section area of, say, a methane molecule. We have already seen in section 7.14 that a strong solvent-specific force can be expected if both L and P contain FGs that can form HBs with the solvent. 

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Double-layer forces are commonly used to induce repulsive interactions in colloidal systems. However, the range of electrostatic forces is strongly reduced by increasing the ionic strength of the continuous phase. Also, electrostatic effects are strong only in polar solvents, which is a severe restriction. An alternative way to create long-range repulsion is to adsorb macromolecules at the interface between the dispersed and the continuous phase. Polymer chains may be densely adsorbed on surfaces where they form loops and tails with a very broad distribution of sizes... 

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