Single protein system, adsorption

Though most studies on protein adsorption at interfaces have been conducted in solutions having a single well characterized protein, evidence has emerged in recent years that film properties in mixed protein systems are much more complex than in single protein systems. 

Adsorption of Single Protein System. One minute of rinsing (at a flow rate of 10 ml/sec) was sufficient to reach a constant surface con-... 

The above discussion provides some indication of how kinetic and affinity factors may interact in competitive situations. The more complex problem of how to synthesize all of the possible factors into a quantitative predictive model of competitive adsorption remains to be solved and is certainly worthy of considerable effort. Data for such models are currently unavailable. They could be obtained in two ways. First it may be assumed that affinity constants in single protein systems could be measured and with a knowledge of diffusion coefficients could be used to predict adsorption layer compositions in mixtures. This approach may have some validity at low surface concentrations but ignores the effect of interlayer interactions at higher coverages. 

Protein affinity constants both in single protein and multi-component systems, including theories for adsorption isotherms. 

Most real protein-containing fluids of interest, such as blood, are multicomponent systems so that the influence of one protein on another may become important. With respect to adsorption, the main consideration is whether adsorbed amounts are in proportion to solution concentration or whether surfaces "select one protein in preference to another. Presumably if proteins act independently of each other one should be able to predict adsorbed amounts in mixtures from relative affinities derived from single protein studies. Although there have been no systematic attempts to make such predictions it seems likely that they would fail. In general it has been found that preferential or selective adsorption occurs so that certain proteins may be enriched in the surface relative to the solution and vice versa. There have as yet been no attempts to determine the properties of protein-surface systems that govern the relative surface affinity of different proteins. More will be said on this topic when adsorption from plasma is discussed. 

An example of an expanded bed process for recovery of a Pseudomonas exotoxin from an E. coli system has been recently reported in pilot plant scale.28 A single-step recovery of a secreted recombinant protein has been carried out in expanded-bed mode directly from the fermentation broth without prior cell removal. The fusion protein was designed to have relatively low isoelectric point to enable anionic exchange adsorption at pH 5.5 where most of the E. coli host proteins are not adsorbed. The gene product was secreted to the culture medium of E. coli in high yield and the recovery of the protein was 90% in one step.29... 

In the mixed systems, the behavior was similar to that observed for surface pressure. In the presence of surface-active PGA (Figure 25.3a and b) at low concentrations in the bulk phase (0.1 wt%), competition between the biopolymers at the interface results in a lower Ed than that expected from the observation of the single components. However, at higher concentrations of PS and long adsorption times, a cooperative adsorption can be deduced. This result could be explained by a concentration of (3-lg at the interface caused by the incompatibility with different biopolymers (that is more evident at higher concentrations). These phenomena would lead to an increase in the protein association in the film with the resultant increase in viscoelasticity. 

For the non-ideal behaviour of a system which contains a single component able to exist in different states, the rigorous thermodynamic expressions are far more complicated than those given above. The relevant mathematical formalism becomes yet more involved if the contributions of ionisation to the surface pressure of the adsorption layer and the chemical potentials of surface active ions located in the diffuse region of the double electric layer are taken into account. Therefore, to describe the adsorption behaviour of a protein/surfactant mixture, some assumptions have to be introduced which simplify the problem significantly. 

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