Protein adsorption electrostatic forces

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

At present, a wide range of solid substrates are available for protein immobilization. According to the protein attachment strategies, namely, adsorption, affinity binding, and covalent binding, all these substrates can be separated into three main parts. Surfaces like ploy(vinylidene fluoride) (PVDF), poly(dimethylsiloxane) (PDMS), nitrocellulose, polystyrene, and poly-1-lysine coated glass can adsorb proteins by electrostatic or hydrophobic forces. A potential drawback of such substrates is the difficulty... 

Whatever the mechanism is, particles adhere spontaneously if, at constant temperature and pressure, the Gibbs energy G of the system decreases. The main contributions to the Gibbs energy of particle adhesion A Gad are from electrostatic, hydrophobic and dispersion forces,1 5 and, furthermore, in case of protein adsorption, from rearrangements in the structure of the protein molecule.6 9 When the sorbent surface is not smooth but hairy , additional, mainly steric, interactions come into play.4,10 12 Hairy surfaces are often encountered in nature as a result of adsorbed or grafted natural polymers, such as polysaccharides, that reach out in the surrounding medium with some flexibility. Interaction of particles with such hairy surfaces will be dealt with in section 3. 

Several different forces may be involved in protein adsorption at the solid-liquid interface hydrogen bonding, electrostatic forces, and hydrophobic interactions. Entropic factors such as loss of water, structural deformation of the protein onto hydrophobic patches and dehydration of the protein may drive the adsorption process when there are non favourable electrostatic interactions. 

Apart from polymer adsorption for uncharged macromolecules, charged macromolecules (polyelectrolytes), such as proteins can also adsorb at surfaces [20, 21]. Adsorption of a charged macromolecule is different from adsorption of an uncharged polymer in that there is a high dependency on the salt concentration. At a low salt concentration, repulsive electrostatic forces between charged polymer chains will inhibit formation of loops and tails (Fig. 4). This has been predicted and confirmed, for instance for adsorption of humic acids on iron-oxide particles [22]. 

In some cases the protein adsorption was found to be slower than expected from diffusion-controlled adsorption kinetics, which was attributed by van Dulm Norde (1983) to some barrier the adsorbing molecules have to break through before they adsorb at the interface. This barrier has been considered to be caused by electrostatic repulsion between protein molecules and the surface. This phenomenon can be associated with the objective of the present section and the observed barrier could be caused by long range forces. 

In addition to electrostatic forces, hydrophobic interactions are also implied in the interaction of proteins with soil constituents and this results in an interplay between different driving forces in adsorption. For example, hydrophobic interactions with clays can result from an electrostatic exchange of the hydrophilic counter-ions on the clay surface, leaving a hydrophobic siloxane surface (Staunton and Quiquam-poix, 1994). The rearrangement of the enzyme structure on the surface can be facilitated when hydrophobic amino acids come into contact with the clay hydrophobic siloxane layer and remain shielded from the water molecules of the solution. If this structural modification is accompanied by a decrease in ordered secondary structures, it will result in an additional increase in conformational entropy. This will lower the Gibbs energy of the system. The combination of all these different sub-processes gives rise to an irreversibility of the modification of conformation of enzymes on clay surfaces. 

The adsorption of proteins from aqueous solution to solid surfaces is the result of a combination of hydrophobic, steric, and electrostatic interactions between the protein, solid surface, and solution [ 1-3]. Numerous studies have been conducted to identify the driving forces governing protein adsorption and dynamics at liquid-solid interfaces and have been reviewed elsewhere [4—8], In the adsorbed state, protein stmcture is likely to be perturbed (Figure 15.1). The unfolded or partially unfolded protein can then adopt various flexible conformations depending on the natures of the solid surface the protein [1, 4, 9-13]. While this has been exploited for various applications [12], uncontrolled adsorption can cause protein degradation, compromised function, and even life-threatening immunogenic responses. The molecular mechanisms of protein adsorption have not been fully elucidated and remain a current area of research [ 10]. 

The adsorption of HPF (initial Chpf=0.0795 wt% in the solution) is maximal at pH 5-6 which is close to the pH value of the isoelectric point (lEP) of HPF. The adsorption slightly decreases (by 20% at pH 8) at pH >6 (Figure 6.14b) because the adsorption of proteins is maximal at their lEP. Consequently, the state of HPF is stable at pH 5-8 that is of importance because HPF can easily denature in strongly acid or base solutions, as weU as on the adsorption. A similar type of the pH dependence of the adsorption is characteristic for the systems, in which interaction of proteins with a silica surface occurs predominantly due to electrostatic forces and the hydrogen bonds (the main portion of which is due to electrostatic forces). 

Heme proteins have also been adsorbed to TCO surfaces as modifiers of their electrochemical properties. These adsorption processes are undoubtedly a combination of electrostatic forces, H-bonding, and metal ion coordination in the TCO surface, and produce reasonably robust and active modifying layers. Cytochrome C (cyt C) has been the most extensively studied adsorbed heme protein. Hawkridge and coworkers have examined the electrochemical behavior at gold, platinum, and metal oxide electrodes [81]. Electron transfer rates as high as 10 cm were obtainable at ITO electrodes. Bowden and coworkers have determined that the... 

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