Adsorption diffusion-controlled, proteins

Fig. 20. Schematic adsorption isotherms with a constant surface site concentration ([A]s in Fig. 12 is here constant), but with adsorption time as a variable. At very short times, adsorption is diffusion controlled. At short times, the protein has insufficient time to conformationally adjust to the interface, thus adsorption can be reversible and of the Langmuir type. At longer times, conformational adjustments begin leading to the commonly observed semi- orir-reversible behavior of protein adsorption. Other nomenclature same as Fig. 12...

Diffusion controlled adsorption of proteins at an interface can imply either that the molecules diffuse to the interface and adsorb without further spreading, or that spreading or unfolding is so rapid that diffusion becomes the rate-controlling factor. 

As mentioned above, beside the diffusion-controlled models, others exist to describe the adsorption kinetics and exchange of matter. De Feijter et al. (1987) have developed a relation taking into consideration simultaneous adsorption of proteins and surfactants at an interface. As a special case a relation results which describes the equilibrium state of adsorption of polymer molecules at a liquid interface. 

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. 

Some consequences which result from the proposed models of equilibrium surface layers are of special practical importance for rheological and dynamic surface phenomena. For example, the rate of surface tension decrease for the diffusion-controlled adsorption mechanism depends on whether the molecules imdergo reorientation or aggregation processes in the surface layer. This will be explained in detail in Chapter 4. It is shown that the elasticity modulus of surfactant layers is very sensitive to the reorientation of adsorbed molecules. For protein surface layers there are restructuring processes at the surface that determine adsorption/desorption rates and a number of other dynamic and mechanical properties of interfacial layers. 

The protein concentration in the sublayer c(Fi) can be determined via the adsorption isotherm Eqs. (2.117) to (2.119). The Eq. (4.38) is quite complicated for a further analysis and simplifications are necessary. From experimental data, it is known that the adsorption of proteins at the air/water interfaces follows a diffusion-controlled mechanism, at least for small surface pressures n < 2 mN/m [71, 72, 73, 74, 75]. Moreover, the so-called induction time t, the time at which the surface pressure FI starts to increase, can be used for an estimation of the adsorption mechanism. For this time interval the relation cH s const should hold [71, 74, 76]. A diffusion model for the range of small F as approximation was given in [77]... 

Since the slopes of the linear variation of AC = f(t " ) and AC = f(t) varied with protein concentrations at all concentration as according to the corresponding equations giving F (14), the process is diffusion controlled. It also implies immediate adsorption and negligible back reaction. 

Adsorption Kinetics. Figures 2A and 2B show the FN adsorption kinetics on the three surfaces from 0,07 and 0,21 mg/ml FN solutions respectively. Each line is the average of two experiments on a given polymer. At each protein concentration, the initial rate of adsorption is independent of the type of polymer substrate, and adsorption from 0,21 mg/ml is nearly 3 times faster than from 0,07 mg/ml FN, The initial adsorption rates are linear in time until adsorption exceeds 0.06 ug/cm on PEO-PEUU and 0.10 ug/cm on the other polymers (data not shown). This suggests that the adsorption is diffusion controlled up to the above surface concentrations, after which point the adsorption rate decreases and becomes dependent upon the polymer surface chemistry. The amount of FN adsorbed does not reach a plateau within 120 minutes, nor does it reach a plateau when adsorption continues for 18 hours (data not shown). 

Many ingenious methods have been introduced to study protein adsorption. If the kinetics of the adsorption process are important, the ellipsometric method introduced by Rothen (3) is probably the best. In this method protein adsorption can be studied ijn situ from a solution. The method has been used to study the kinetics of both the adsorption of protein in single layers and in double layers that can occur in the immune-reaction. When protein such as bovine serum albumin (BSA) was adsorbed from a dilute solution onto a surface, after a delay of a few seconds, steady-state diffusion controlled the adsorption process and, consequently, the amount bound to the surface increased linearly with time. However, as the surface became covered, adsorption slowed down, because it was now limited by the number of available sites on the surface. The final layer of BSA was roughly 2 nanometer thick. 

The irreversibility of adsorption of some proteins also emphasizes the importance of understanding the kinetics of the adsorption process. Given a situation where transport of the protein to the material surface is diffusion controlled, Eq. (2) can be used during initial stages of adsorption, where the amount of protein on the surface (A) is proportional to the product of the protein concentration in solution (C) and the square roots of protein diffusion coefficient ( )) and time (t) ... 

Since the velocity of the film drainage is small, protein adsorption in films is assiuned to be diffusion controlled. The enrichment was calculated by using the protein concentration in the feed for its pool concentration. The surface concentration of a protein at the foam/liquid interface was calculated by the following equation ... 

RRB and APEE mechanisms represent extreme cases. The RRB mechanism is to be favoured in cases in which strong protein adsorption is not apparent. The APEE mechanism, on the other hand, is to be favoured wherever diffusion-controlled voltammetry is observed despite strong adsorption with saturative electrode surface coverage. For a heterogeneous electrode surface, it is quite likely that both mechanisms can operate simultaneously with RRB and APEE occurring, respectively, at coolspots and hotspots . 

In [31] kinetics of the surface tension decrease was described using the model accounting for diffusion-controlled adsorption of protein molecule and for conformational changes of adsorbed molecule. The model corresponds to one proposed by Serrien [32] and describes diffusion toward a/w surface and subsequent reorientation and other changes in adsorption layer, which usually one gives a sence of conformational changes the adsorbed protein. The model yields the diffusion relaxation time (t) and (kc) - the rate constant of conformational changes. 

The interfacial tension response to transient and harmonic area perturbations yields the dilational rheological parameters of the interfacial layer dilational elasticity and exchange of matter function. The data interpretation with the diffusion-controlled adsorption mechanism based on various adsorption isotherms is demonstrated by a number of experiments, obtained for model surfactants and proteins and also technical surfactants. The application of the Fourier transformation is demonstrated for the analysis of harmonic area changes. The experiments shown are performed at the water/air and water/oil interface and underline the large capacity of the tensiometer. 

For separations comparable with particle dimensions, the most significant transport mechanism becomes diffusion, the rate of which increases with decreased size of particles and increased temperature. The diffusion transport mechanism was the dominating one in various experimental studies on protein and colloid adsorption [2,8-10]. However, the disadvantage of the diffusion-controlled transport is its inherently unsteady character, leading to considerable decrease in adsorption rate with time. 

Diffusion-controlled processes cannot account for most specific DNA-protein interactions, in which the proper site cannot possibly be found by random collisions. A onedimensional random walk along the DNA molecule is the most probable explanation. Here nonspecific binding to the DNA and local adsorption and desorption, with maintenance of many transient states through electrostatic interactions, is facilitated by the existence of grooves in the DNA helix, which considerably reduce the entropic barrier of the kinetics of the recognition process. Still, some processes are so rapid that special kinetic mechanisms have to be envisaged. 

Van Enckevort et al used radiochemical techniques in their studies of adsorption of BSA at the stainless steel/aqueous solution interface for a wide range of solution concentrations. It was shown that the adsorbed protein was essentially nonexchangeable and the adsorption process was not reversed upon dilution. The adsorption isotherm consisted of a plateau region that extended to the most dilute solutions studied, provided adequate time was allowed. They found adsorption from sufficiently dilute solutions to be diffusion-controlled. In ideal cases, they were able to estimate both the kinetics of diffusion-controlled adsorption and saturation coverages from molecular parameters. 

When a protein adsorbs from a solution in which the pH is close to its isoelectric point, the rate of adsorption at mobile interfaces is controlled by the rate of diffusion to the interface and the interfacial pressure barrier. However, when the protein molecule takes on a net electrical charge, an additional barrier to adsorption appears, owing to the electrical potential set up at the interface by the adsorbed protein. 

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