Proteins interface, surface potential

In the above equations, h is the film thickness, n is the munber concentration of z z symmetrical electrolyte and is the surface potential. The surface potential is the potential at the interface of stem and diffuse layers and is usually replaced by the zeta potential of the droplet determined from electrophoretic measurements. When the interface has an adsorbed layer of globular proteins, it may be reasonable to assume that the shear plane is located at the interface of protein layer. When xp > 2L, the disjoining pressure 11 / can be evaluated by replacing with potential and taking as (jCf - 2L,). 

The arrangement of the proteins within the membrane seems to depend to some extent on the electrostatic surface potential and interface permittivity. It is influenced by electrostatic interaction between the proteins, polar head groups of the phospholipid and ions within the aqueous medium of the membrane surface. This can be affected by exogenous molecules such as drugs. Phospholipid-induced conformational change in intestinal calcium-binding protein in the absence and presence of Ca2+ has been described [37]. There is, however, no doubt that hydrophobic interactions between peptides and membrane interfaces play an important role. A general frame-... 

Salts are known to influence several properties of aqueous solutions in a systematic way (122,123). The effect of different aiuons and cations seems to be ordered in a sequence this theory was already proposed by Hofmeister in 1888 (124) from a series of experiments on the salts ability to precipitate hen-egg white protein. Numerous other properties of aqueous salt solutions are also found to be systematically salt dependent, such as the surface tension or the surface potential (122). However, the exact reason for the observed specific cation and anion sequences is still not fully understood (125). Model calculations (126), as well as nuclear magnetic relaxation experiments (127), propose a delicate balance between ion adsorption and exclusion at the solute interface. This balance is tuned by the solvent (water) stmcture modification according to the ion hydration (128, 129) and hence is possibly subject to molecular details. 

Recently a method has been developed for measuring the surface potential of protein monolayers at the oil-water interface (11,12). From... 

Even if conserved residues can be part of interaction surfaces, other residues conserved in subfamilies, the so-called tree-determinants, can also be important for tracing functional interfaces. These tree-determinants point to positively selected changes in a protein family that potentially indicate the presence of functional-specific sites. In this position it is possible to find protein-protein interaction related sites [94,98]. The ability of these methods to predict sites that are specific [99] and the state-of-the-art in current methods for predicting functional sites [100] has been described in recent publications, including experiments demonstrating the capacity of those methods to predict residues that once swapped can produce a exchange of functional specificity between two protein subfamilies [65,101,102]. 

Information about fluidity and viscosity of bilayers of artificial and natural membranes has been obtained from electron spin resonance studies in which the mobility of the spin-labelled species along the surface plane of the membrane is determined (17). However, the monolayer of either lipid, protein, or lipid-protein systems at the air-water interface, makes an ideal model because several parameters can be measured simultaneously. Surface tension, surface pressure, surface potential, surface viscosity, surface fluorescence and microviscosities, surface radioactivity, and spectroscopy may be determined on the same film. Moreover, the films can be picked up on grids from which they may be observed by electron microscopy, studied further for composition, and analyzed for structure by x-ray diffraction and spectroscopy. This approach can provide a clear understanding of the function and morphology of the lipid and lipid-protein surfaces of experimental membranes. However, the first objective is to obtain molecular correlations of surface tension, pressure, potential, and viscosity. 

Bos MA, Shervani Z, Anusiem ACI, Giesbers M, Norde W, Kleijn JM. Influence of the electric potential of the interface on the adsorption of proteins. Colloids Surfaces B 1994 3 91-100. 

The membrane/protein interface with the bulk is dominated by the discontinuity of the physical chemical properties of the reaction space. On one side of the borderline there is a low viscosity, high dielectric constant matrix where rapid proton diffusion can take place. On the other side of the boundary, there is a low dielectric matrix that is covered by a large number of rigidly fixed charged residues. The dielectric boundary amplifies the electrostatic potential of the fixed charges and, due to their organization on the surface of proteins, a complex pattern of electrostatic potentials is formed. These local fields determine the specific reactivity of the domain, either with free proton or with buffer molecules. In this chapter we shall discuss both the general properties of the interface and the manner in which they affect the kinetics of defined domains. 

Studies on polymer monolayers spread at the air-water interface are now in progress in our laboratory. Biocompatible and biodegradable polymers used as nanoparticles carrying biologically active substances are characterized using the surface balance, surface potential and protein adsorption/desorption measurements. The combined data of all these measurements provide information on drug and protein penetration/delivery with these polymers. 

Electrochemical properties are other important physical surface parameters. The existing surface charge density, i.e. the surface potential, has a strong influence on protein adsorption and blood compatibility [81]. In this way the characterization of the interface charge density of a biomaterial by -potential determination delivers an important parameter for understanding blood compatibility of biomaterial surfaces [82-85]. 

The protein monolayer is responsible for the electrostatic properties of the adsorbate-adsorbent (protein-gold) system. However, the potential change is not only a property of the protein layer, but is also affected by the modification of the electronic properties of the surface such as the covalent binding of any sulfur atom in the protein on the gold (Au -S ). As a consequence, the change in surface potential involves two contributions protein layers and protein-gold interface. 

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