Protein behavior at interfaces

It is likely that experimentally found values of molecular cross-sectional areas do not correspond to the equilibrium states of the protein layers but reflect only the transition state configuration, as assumed by MacRitchie [16] and depicted in Fig. 1. The problem of adsorption reversibility is basic for understanding the protein behavior at interfaces. The belief in the protein adsorption irreversibility is mainly based on drastic conformational changes in interfacial film and the great difficulty of desorbing a protein from this film [15], However, these criteria are not always a proof of irreversibility. It was shown in many cases [3,24,39-41] that proteins can be desorbed... 

Since the question of whether protein adsorption can be considered to be a reversible process is basic to an understanding of protein behavior at interfaces, it is proposed to discuss the problem in some detail. Apart from the loss of solubility on adsorption, other observations that have been interpreted as evidence for irreversibility are ... 

As we expand our observations from the behavior of a few proteins to that of plasma, we may begin to feel justified in asking why the system performs rather than how it does. The question "Why do plasma proteins interact at interfaces " can then be interpreted to mean "what aspects of the behavior and interactions among purified plasma proteins can be seen as well in their natural habitat - the plasma -, and can these aspects be explained as being beneficial to our survival " What thus far had appeared as senselessly complex behavior of purified proteins at interfaces may become more reasonable in the context of many plasma proteins interacting at the mottled surfaces of cells in a way that will allow the survival of their host. Meanwhile, we will discover that purified proteins behave unlike their sibblings in vivo and that in the eyes of our plasma a purified protein adsorbed out of an artificial solution will not look like a film of the same protein deposited by the plasma itself. 

Proteins generally adsorb onto solid surfaces from solution. This process is of importance in a number of applications. For instance, the composition, conformation, and orientation of adsorbed proteins are believed to influence cell/substrate interactions (1-3). Also, adsorption of serum proteins onto biomaterials is generally recognized as the initial event in the sequence that culminates in thrombus formation (4,5). Consequently, protein behavior at solid-liquid interfaces has been extensively studied (6-11). Many fundamental questions about the protein adsorption phenomenon, however, remain unanswered (12). 

In our laboratory, two techniques have been extensively used for studying protein behavior at various interfaces. The first technique censists of in situ measurement of protein adsorption with labeled proteins the second technique based on multiple-beam interferometry measures surface forces between two mica sheets with adsorbed proteins (Tabor-Israelachvili technique). While the in situ measurements enable quantitation of protein adsorption, force-distance measurements provide direct experimental data on the extension of adsorbed protein layers towards the solution and on their conformation. 

Sah H. Protein behavior at the water/methylene chloride interface. Journal of Pharmaceutical Sciences. December 1999 88(12) 1320-1325. PubMed PMID 10585229. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

If we consider that cell adhesion under biological circumstances is mainly brought about with the aid of preadsorbed protein on the material s surface, we may explain the unique behavior of amino-containing materials against the cell-adhesion process in terms of the reduced residence-time of protein molecules at the interface. Actually, a recent study [129] revealed that the surface of polyamine-gra/t-polystyrene copolymer (SA) containing 6 wt.% polyamine portion exhibited a minimal adsorptive property against bovine plasma fibronectin (FN) and vitronectin (VN), both of which are known to mediate cell-adhesion processes. 

An important issue that has to be emphasized is that the experimentally determined dependence ofthe stability ratio on electrolyte concentration, at low ionic strengths, exhibits (at least for NaSCN) a strongly non-DLVO behavior, in a range in which the DLVO theory is considered fairly accurate. Therefore, we are inclined to believe that the electrolyte (even at low ionic strength) induces indeed structural modifications of the adsorbed protein layer at least near the interface. 

Polybrene and Heparin on the Behavior of Plasma, Plasma Proteins, Platelets and Factor XII Activity at Interfaces, Thrombosis Res. (1972) 1, 507. 

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