Osmotic stress method

Figure 9 shows force laws for phosphatidylcholine bilayers (Lis et al, 1982), determined by the osmotic stress method. Similar data were obtained for DNA samples (Rau et al., 1984). The characteristic length governing decay of the force is about 3 A for both systems. Interactions of this kind can also be important for protein aggregates. Prouty et al. (1985) used the osmotic stress method to determine the phase diagram of sickle cell hemoglobin (Fig. 10). At a critical osmotic pressure, which is temperature dependent, a solution of deoxyhemoglobin S collapses to a gel, with a large change in volume. One of the strengths of the osmotic stress method is that it provides additional information that can be used for thermodynamic analysis of the system.

The influence of various cosolvents on protein stability has been discussed by Timasheff [50]. There has been a considerable debate in the literature on the number of water molecules that are taking part in protein-protein or protein-DNA interactions as estimated by various methods. A recent theoretical analysis suggests that the osmotic stress method may overestimate the number of waters involved [51]. These models assume that the cavities that are formed at the interface between macromolecules do not contribute to the measured volume changes as suggested by Silva and Weber [31]. 

Experimental Results. The DLVO theory, which is based on a continuum description of matter, explains the nature of the forces acting between membrane surfaces that are separated by distances beyond 10 molecular solvent diameters. When the interface distance is below 10 solvent diameters the continuum picture breaks down and the molecular nature of the matter should be taken into account. Indeed the experiment shows that for these distances the forces acting between the molecularly smooth surfaces (e.g., mica) have an oscillatory character (8). The oscillations of the force are correlated to the size of the solvent, and obviously reflect the molecular nature of the solvent. In the case of the rough surfaces, or more specifically biomembrane surfaces, the solvation force displays a mono tonic behavior. It is the nature of this solvation force (if the solvent is water, then the force is called hydration force) that still remains a puzzle. The hydration (solvation) forces have been measured by using the surface force apparatus (9) and by the osmotic stress method (10, II). Forces between phosphatidylcholine (PC) bilayers have been measured using both methods and good agreement was found. 

Figure 2. Illustration of how the osmotic stress method is used to measure the volume of water associated with the opening and closing of a membrane...

The osmotic stress method was first applied to the large voltage dependent anion channel (VDAC), also known as mitochondrial porin , from the outer membrane of mitochondria. The protein was placed in an artificial... 

Figure 4. The osmotic stress method used to measure net repulsive pressure between membranes as it varies with membrane separation. Solid symbols represent stearoyloleoylphosphatidylcholine open symbols represent palmitoyl-oleoylphosphatidylethanolamine.

Figure 5. The osmotic stress method used to determine the relation between structural dimensions of the hexagonal phase and the applied stress.

In parallel, another important (although less direct) technique for measuring forces between macromolecules or lipid bilayers was developed, namely, the osmotic stress method [39-41]. A dispersion of vesicles or macromolecules is equilibrated with a reservoir solution containing water and other small solutes, which can freely exchange with the dispersion phase. The reservoir also contains a polymer that cannot diffuse into the dispersion. The polymer concentration determines the osmotic stress acting on the dispersion. The spacing between the macromolecules or vesicles is measured by X-ray diffraction (XRD). In this way, one obtains pressure-versus-distance curves. The osmotic stress method is used to measure interactions between lipid bilayers, DNA, polysaccharides, proteins, and other macromolecules [36]. It was particularly successful in studying the hydration... 

Characteristic decay lengths Xo determined with the SFA, the osmotic stress method, or the AFM range from 0.2 to 1.4 nm. Typical amphtudes are A = 10 to 10 J m . In contrast to the electrostatic double-layer force, hydration forces tend to become stronger and longer ranged with increasing salt concentration, especially for divalent cations. 

Several devices for measuring surface forces have been developed, including the surface force apparatus [13,14], the force balance [15], the osmotic stress method, and the total internal reflectance microscope [16]. But in all these methods there are limitations. 

The interaction of water soluble polymers with lamellar dispersions has implications in many practical applications such as consumer products, drug delivery, bioseparations, and industrial processes. The osmotic stress method for studying membrane biophysics involves the addition of water soluble polymers such as poly (ethylene glycols) (PEGs) to phospholipid vesicles and measuring the changes in... 

While in the beginning the biological application of the osmotic stress method was mainly used to investigate forces between lipid bilayers (reviewed in Ref [1239]), it has also become a valuable tool to study the hydration forces between biomolecules [1243], for example, DNA [1155], collagen fibers [1244], or polysaccharides [1245]. 

The osmotic stress method has also been applied to colloidal dispersions [1238,1246], emulsions [1247], colloidal crystals [1248], clays [1249, 1250], block copolymers [1251], a mixed nanoparticle/polymer system [1252], and colloids with polyelectrolyte multilayers [1253]. 

Direct quantitative measurements of steric repulsion were made with the surface forces apparatus [1353-1360] and the atomic force microscope [1361-1364]. Although we focused on the interaction between solid surfaces, steric forces also act between fluid interfaces. The first force versus distance curves of steric repulsion were recorded across a liquid foam lamellae with a thin film balance by Lyklema and van Vhet [1365]. Another example is the force measurement between vesicles using the osmotic stress method by Kenworthy et cd. [1366]. Experimentally, the Milner, Witten, and Cates and the de Gennes model both fit force curves measured between polymer bmshes in good solvents reasonably well. 

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