Protein-surface interactions solution properties

At present, the protein/surface interactions that determine adsorption kinetics are unclear. To clarify these interactions, the effects of polymer surface properties on protein adsorption and desorption rates have been investigated. BSA adsorption from a 1 mg% solution (1 mg% = 1 mg/100 mL) was studied using several polymers chosen for their wide range of surface properties and functionalities (22, Cheng, Y.L. et al.. J. Coll. Int. Sci., in press). The polymers and their surface properties (under the conditions of the BSA adsorption experiments) are listed in Table I. 

Figure 8.12 illustrates the effect of complex formation between protein and polysaccharide on the time-dependent surface shear viscosity at the oil-water interface for the system BSA + dextran sulfate (DS) at pH = 7 and ionic strength = 50 mM. The film adsorbed from the 10 wt % solution of pure protein has a surface viscosity of t]s > 200 mPa s after 24 h. As the polysaccharide is not itself surface-active, it exhibited no measurable surface viscosity (t]s < 1 niPa s). But, when 10 wt% DS was introduced into the aqueous phase below the 24-hour-old BSA film, the surface viscosity showed an increase (after a further 24 h) to a value around twice that for the original protein film. Hence, in this case, the new protein-polysaccharide interactions induced at the oil-water interface were sufficiently strong to influence considerably the viscoelastic properties of the adsorbed biopolymer layer. 

To understand the structures and functions of proteins, you must be familiar with some of the distinctive properties of the amino acids, which are determined by their side chains. The side chains of different amino acids vary in size, shape, charge, hydrophoblclty, and reactivity. Amino acids can be classified into several broad categories based primarily on their solubility in water, which is influenced by the polarity of their side chains (Figure 2-13). Amino acids with polar side chains are hydrophilic and tend to be on the surfaces of proteins by interacting with water, they make proteins soluble in aqueous solutions and can form noncovalent interactions with other water-soluble molecules. In contrast, amino acids with nonpolar side chains are hydrophobic they avoid water and often aggregate to help form the water-insoluble cores of many proteins. The polarity of amino acid side chains thus Is responsible for shaping the final three-dimensional structure of proteins. 

In a dilute protein solution, the nano length scale or the molecular structure of protein molecules determines the thermodynamic equilibrium between protein-protein and protein-water interactions. The consequent surface and hydrodynamic properties of proteins are resulted from the proportion of hydrophobic, hydrophilic, and charged amino acid residues. For example, caseins could adopt a random coil structure due to their flexible structure as a result of phosphorylated serine residues caseins indeed lack the ordered structures of a-helix, 3-sheet, and 3-turn found in globular proteins. This gives rise to better multifunctionality of caseins over globular proteins. 

Adsorption at solid/liquid interfaces has some peculiarities as compared with fluid/fluid interfaces. The chemical nature of the solid surface and its properties (charge, hydrophobicity, etc.) determine the mode and strength of binding, as well as, in many cases, the conformational changes in adsorbed protein molecules. The solid surfaces can be easily modified and tuned up for specific types of interactions. Usually, in contrast to fluid surfaces, solid surfaces are not chemically or energetically uniform, and their heterogeneity may result in nonuniform adsorption of protein layers. Finally, adsorption from solutions is always a competitive process, and in the simplest case competition between a protein and a solvent takes place. 

The first point 1) is very important since complex formation that is strong enough to cause the protein to interact intimately with the electrode surface may alter intrinsic properties and change the reduction potential. This is particularly relevant since the promoters are free in solution. One example in which two promoters yield different reduction potentials is in the voltammetry [105] of Azotobacter chroococcum 7Fe ferredoxin, which is discussed later. 

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