Biological systems dispersion forces

A major advance in force measurement was the development by Tabor, Win-terton and Israelachvili of a surface force apparatus (SFA) involving crossed cylinders coated with molecularly smooth cleaved mica sheets [11, 28]. A current version of an apparatus is shown in Fig. VI-4 from Ref. 29. The separation between surfaces is measured interferometrically to a precision of 0.1 nm the surfaces are driven together with piezoelectric transducers. The combination of a stiff double-cantilever spring with one of a number of measuring leaf springs provides force resolution down to 10 dyn (10 N). Since its development, several groups have used the SFA to measure the retarded and unretarded dispersion forces, electrostatic repulsions in a variety of electrolytes, structural and solvation forces (see below), and numerous studies of polymeric and biological systems. 

Note first that in this older picture, for both the attractive (van der Waals) forces and for the repulsive double-layer forces, the water separating two surfaces is treated as a continuum (theme (i) again). Extensions of the theory within that restricted assumption are these van der Waals forces were presumed to be due solely to electronic correlations in the ultra-violet frequency range (dispersion forces). The later theory of Lifshitz [3-10] includes all frequencies, microwave, infra-red, ultra and far ultra-violet correlations accessible through dielectric data for the interacting materials. All many-body effects are included, as is the contribution of temperature-dependent forces (cooperative permanent dipole-dipole interactions) which are important or dominant in oil-water and biological systems. Further, the inclusion of so-called retardation effects, shows that different frequency responses lock in at different distances, already a clue to the specificity of interactions. The effects of different geometries of the particles, or multiple layered structures can all be taken care of in the complete theory [3-10]. 

When two molecules interact with each other, several types of electrostatic interactions or forces may be involved, some of which have been described in the preceding sections (e.g., charge-charge, charge-dipole, dipole-dipole interactions). Here, we would like to mention two other kinds of electrical interactions which were not described above namely, short-range repulsive interactions and the London dispersion interaction. The latter interaction plays an especially important role in biological systems. 

Lipid bilayers (Section 23.6A) A two-layer noncovalent molecular assembly comprised primarily of phospholipids. The hydrophobic phospholipid tail groups of each layer orient toward each other in the center of the two-layered structure due to attractive dispersion forces. The hydrophilic head groups of the lipids orient toward the aqueous exterior of the bilayer. Lipid bilayers are important in biological systems such as cell membranes. 

Here, V (x) is the van der Waals interaction without screening. For a 0.1 M NaCl solution, the Debye length is k = 0.95 nm. For a distance between the surfaces of 2 nm, the Keesom and Debye interaction will be reduced to less than 8% of the unscreened interaction. The screening of the static contributions by ions is especially relevant for biological systems, where the interactions may be dominated by the Keesom and Debye interactions and physiological solutions typically contain more than 0.1 M salt. For systems where the London dispersion forces are dominating, screening will have a minor effect. 

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