Competitive protein adsorption

Most practical systems, for example, essentially all biological fluids, are multi protein systems. The various proteins compete with each other (and with other surface-active components) for adsorption at any interface present. As a rule, the interface will at first become covered by the molecules that have the highest rate of arrival (that is, the smaller ones that have the highest diffusion coefficient and the ones that occur most abundantly in the solution). At later stages, the initially adsorbed molecules may be displaced in favor of other molecules that have a higher affinity for the surface. 

Competition between proteins of different types is more complicated. The molecular size may still play an important role but, in view of the discussion in Section 15.5, other variables such as polarity, electrical charge density, and structural stability should be taken into account as well. 

FIGURE 15.21 Adsorption of fibrinogen from diluted blood plasma on glass, displayed (a) as a function of time for varying dilutions, and (b) as a function of dilution at various time intervals. (Data taken from Vroman, L. and Adams, A.L., J. Colloid Interface Sci., Ill, 391, 1986.) The transient adsorption of fibrinogen from blood plasma is known as the Vroman effect . 

FIGURE 15.22 Competitive adsorption between lysozyme (LSZ), ribonuclease (RNase), myoglobin (MGB) and a-lactalbumin (a LA) on surfaces of apolar polystyrene (PS) and polar iron oxide (a-Pe203). The + and - signs indicate the electrical charge of the components. The solutions supplied to the sorbent surfaces contain equal concentrations (0.3 g dm ) of each of the proteins. (Adapted from Arai, T. and Norde, W., Colloids Surfaces, 51,17, 1990.) 

2 Upon attachment at the sorbent surface, adsorbed polymer molecules try to relax, whereby they undergo structural rearranganents. Sketch in one figure the courses of the adsorbed amount F with time t for polymer solutions of three different concentrations c C2, C3, for which c, = 2 2 and = 2C3. 

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Green RJ, Davies MC, Roberts CJ et al (1999) Competitive protein adsorption as observed by surface plasmon resonance. Biomaterials 20(4) 385-391... 

Zhang, Z., Dalgleish, D.G., and Goff, H.D. (2004). Effect of pH and ionic strength on competitive protein adsorption to air/water interfaces in aqueous foams made with mixed milk proteins. Coll. Surf. B. Biointerfaces 34, 113-121. 

Dickinson E. Mixed proteinaceous emulsifiers review of competitive protein adsorption and the relationship to food colloid stabilization. Food Hydrocolloids 1986 1 3-23. 

The in situ adsorption/desorption technique can be used to distinguish between irreversibly and reversibly adsorbed protein. The desorption/adsorption ratio depends on the nature of protein and surface. Further development of this technique would involve the design of a new type of measuring cell for adsorption/desorption measurements in flow conditions and extension of protein adsorption experiments to competitive protein adsorption measurement. Studies of albumin and fibrinogen competitive adsorption versus collagen 12) have recently been initiated. 

Protein diffusivity, however, is not always the main determinant in the composition of adsorbed protein layers. If this were the case, the composition of the adsorbed protein layer would be the same on different materials exposed to the same solution. This condition is not usually observed (Horbett, 1999), indicating that the affinity of each protein is influenced by the surface chemistry of the biomaterial (Horbett and Brash, 1995). Because proteins differ in affinity for various surface chemistries, the competitive protein adsorption process will also differ, leading to unique protein layer compositions upon different materials. Furthermore, the vast majority of protein adsorption studies have been carried out in vitro, assuming that this accurately mimics the in vivo environment. However, differences in implant site (i.e., blood-contacting devices vs solid tissue... 

Choi, S. and Chae, J. 2009. A microfluidic biosensor based on competitive protein adsorption for thyroglobulin detection. Biosens. Bioelectron. 25 118-123. 

Holmberg M, Hou XL (2010) Competitive protein adsorption of albumin and immunoglobulin G from human serum onto polymer surfaces. Langmuir 26 938-942... 

Landstrom, K., Alsins, J., Bergenstahl, B., 2000. Competitive protein adsorption between bovine serum albumin and 3-lactoglobulin during spray-drying. Food Hydrocolloids 14(1) 75-82. 

Peppas et al. [16] presented a new method for calculating protein adsorption on polymeric surfaces. In their model, protein adsorption is regarded as an equilibrium reaction, which takes place on the polymer surface in competition with the adsorption of water. 

Adams, et al. 101 used a curved disc on a flat surface to study the effect of solution volume at constant surface area on competitive plasma protein adsorption. Although the experiment was qualitative, it elegantly demonstrated the importance of exchange, bulk solution concentration, and surface-volume ratio on competitive adsorption 98>. 

Here A//adS is the enthalpy of adsorption, T is the temperature, and AAads is the entropy change associated with the adsorption of the protein onto the surface. Protein adsorption will take place if AGads < 0. Considering a complex system, where proteins are dissolved in an aqueous environment, and are brought into contact with an artificial interface, there are a vast number of parameters that impact AGads due to their small size (i.e., large diffusion coefficient), water molecules are the first to reach the surface when a solid substrate is placed in an aqueous biological environment. Hence, a hydrate layer is formed. With some delay, proteins diffuse to the interface and competition for a suitable spot for adsorption starts. This competition... 

Solutes are one of the major components of foods, and they have significant effects on their adsorption at fluid interfaces. In addition, the study of the effects of ethanol and/or sucrose on protein adsorption at fluid interfaces is of practical importance in the manufacture of food dispersions. The presence of ethanol in the bulk phase apparently introduces an energy barrier for the protein diffusion towards the interface. This could be attributable to competition with previously adsorbed ethanol molecules for the penetration of the protein into the interface. However, if ethanol causes denaturation and/or aggregation of the protein in the bulk phase, the diffusion of the protein towards the interface could be diminished. The causes of the higher rate of protein diffusion from aqueous solutions of sucrose, in comparison with that observed for water, must be different in aqueous ethanol solutions. Since protein molecules are preferentially hydrated in the presence of sucrose, it is possible that sucrose limits protein unfolding in the bulk phase and reduces protein-protein interactions in the bulk phase and at the interface. Both of these phenomena may increase the rate of protein diffusion towards the interface. Clearly, the kinetics of adsorption of proteins at interfaces are highly complex, especially in the presence of typical food solutes such as ethanol and sucrose in the aqueous phase. 

When an oil-soluble emulsifier is present in an oil phase, less proteins (e.g., P-lg and P-casein) are displaced by a water-soluble emulsifier if the emulsion has been aged before the addition of the water-soluble emulsifier (Chen and Dickinson, 1993 Chen et al., 1993). Although the aging time influences the competition of proteins at an interface, protein adsorption is likely to be noncompetitive once one protein has become established at the interface (Dickinson, 1992). The highest surface-active protein would be the major protein at the interface if it is present during emulsification. The protein likely to be predominant in an aged protein film would be the one that was first introduced to the interface, irrespective of whether or not it is the more surface-active of the two proteins. 

This chapter deals with a specific test of blood-surface interaction in vitro platelet retention in a column of beads (due to platelet adhesion and aggregation). Protein adsorption precedes platelet adsorption, and thus the in vitro platelet retention test involves competitive and sequential adsorption of proteins, the outcome of which produces surfaces having widely varying degrees of platelet retention. Except in the case of thrombin (3), plasma protein absorption on these surfaces has not been studied. 

The adsorption of plasma proteins to polymers precedes the interaction of blood cells with the surfaces, and therefore, is likely to be an important initial event in the response of blood to polymers (25, 29). At present, however, little is known about the adsorbed protein layer, even though it has been studied in some detail in recent years (30-36). Because protein adsorption from blood plasma is a competitive process, differences in the adsorbed layer on different polymer substrates could be a primary cause of differences in thrombogenicity. Previous studies of the composition of the adsorbed protein layer have employed 12oI-labeled protein added to plasma (37-39), antibody binding (34) to detect individual proteins, or electrophoretic analysis of detergent-elutable proteins (17, 33, 35). The procedure used in this study does not require the large surface areas used in previous work (35), nor does it rely on incorporation of radiolabels (36) into adsorbed protein. Instead, a staining method at least 100-fold more sensitive than these other techniques has been used. 

Even though zinc is at a low level of only 2% in the alloy, it cannot be overlooked in regard to its effect upon the alloy s interaction with protein. Admittedly, the presence of zinc in the material compounds the issues of protein adsorption that are under investigation. Zinc binds to proteins in various ways, and in some instances, an active competition for zinc exists between certain proteins and amino acids (42). In normal human sera, between 2 and 8% of zinc is ultrafilterable, implying that the remainder is bound to protein. Zinc in human saliva is likewise bound to various molecular... 

A number of important questions revolve around the central one of reversibility of protein adsorption. Are proteins, once adsorbed, able to desorb Why are proteins so difficult to remove from a surface What happens in the situation where there is competitive adsorption between different proteins or between proteins and other species Do the different points on an adsorption isotherm correspond to dynamic equilibria If a protein molecule can desorb, does it revert to its original solution configuration and recover its original biological activity Can the Gibbs Adsorption Equation be applied to protein adsorption Some of these questions may be effectively tackled by studies with the film balance. 

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