Interfacial behavior of proteins

Electrochemical Investigations of the Interfacial Behavior of Proteins Electrochemistry and Electrochemical Catalysis in Microemulsions Interfacial Infrared Vibrational Spectroscopy Some Aspects of the Thermodynamic Structure of Electrochemistry... 

The interfacial behavior of protein-surfactant complexes is important in several areas such as the stability of emulsions and foams and the adsorption of proteins and surfactants from their binary solutions onto solid surfaces. Of particular interest is the adsorption of the milk proteins /3-lactoglobulin and /3-casein at the oil-water interface in the presence of nonionic surfactants in relation to food emulsions [56-58] and foam stability [59]. The adsorption of gelatin at the air-water [52,53,60], oil-water [6], and solid-water [62] interfaces in the presence of surfactants has also been studied. Other studies reported include adsorption from aqueous solutions of lysozyme plus ionic surfactants at solid surfaces [63,64], /3-lactoglobulin plus SDS onto... 

Previously we have postulated several stages in the interfacial behavior of proteins (3). 

Since previous reviews provide excellent coverage of the generally well understood or frequently studied aspects of the interfacial behavior of proteins, this chapter will focus on several facets of protein adsorption that have so far not been examined in much detail. While this approach is atypical for an overview chapter, it is in keeping with the intent of this book to provide information to the reader that reflects more recent developments in this field. Furthermore, as will be seen, the topics to be discussed necessitate reexamination of previous studies and provide some unifying views of this rather diverse science. 

Instead, the large effect of single amino acid substitutions on hemoglobin surface activity points to a very important role for structural stability in the interfacial behavior of proteins. Since structural transitions in proteins occur in a cooperative fashion, e.g., protein "melting occurs over a narrow range of temperature, structural stability could be strongly influenced by single amino acid substitutions. 

Surveying the literature, it appears that the interfacial behavior of proteins is a controversial subject. The main reason is that many studies have been performed under insufficiently defined conditions and/or that conclusions have been drawn on the basis of too scanty experimental evidence. Furthermore, the theoretical description of adsorbed layers of simple, flexible polymers is still in its infancy (5,6). As the structure of proteins is much more complex than that of those simple polymers, theories of polymer adsorption need to be greatly extended to become applicable to proteins. Clearly, our current knowledge of protein adsorption mechanisms and of the structure of the adsorbed layer is far from complete. 

It is evident that elucidation of the interfacial behavior of proteins is not a simple matter and requires contributions from several disciplines. In recent years considerable progress has been made in applying spectroscopic techniques to proteins in the adsorbed state (e.g., 7,8,9). In such studies a (small) part of the molecule is analyzed in detail. In our laboratory we study protein adsorption from a more classical, colloid-chemical point of view. Arguments are derived from experimental data referring to whole protein molecules or to layers of them. Information is obtained from adsorption isotherms, proton titrations and both electrokinetic and thermochemical measurements. Recently, topical questions such as reversibility of the adsorption process and changes in the protein structure have been considered. This more holistic approach has produced some insights that could not easily be obtained otherwise. 

In order to control protein adsorption, to enhance it in some cases and prevent it in others, it is necessary to understand the various stages involved in the process. The interaction of protein molecules with polystyrene (PS) latex particles having a well-defined surface has proved to be a very useful model system with which to study the interfacial behavior of proteins. Other colloidal systems, including silica and metal particles, have also been used in these investigations, and although this review concentrates mainly on interactions between proteins and latex particles, other systems are also mentioned where appropriate. Before looking at the interactions of proteins with PS latex particles in detail, it is worthwhile to take a brief overview of the two major components in the system. 

Proteins adsorbed onto polystyrene latex particles have proved to be good models for the interfacial behavior of proteins in processed foods. The behavior of caseins on latex particles appears to be veiy similar to that at the oil/water interfaces of emulsions, at the air/water interface of foams, and at the surface of casein micelles. 

Proteins are themselves surface-active compounds with an amphiphilic nature. The interfacial behavior of proteins is different from that of low-molecular-weight amphiphiles with a simple structure, namely, detergents, because proteins are highly complex polymers made up of a combination of 20 different amino acids (this point is described in detail in Chapter 3 of this book). Normally, proteins take on the folded compact structure, in which nonpolar amino acid residues are located in the interior and hydrophilic residues are exposed to molecular surfaces. Since hydrophobic interactions play dominant roles in the adsorption of surfactants to the air-water and oil-water interfaces, such a native structure of proteins should be modified to make fiiU use of the surface activity of proteins [1]. 

The interfacial behavior of proteins has been particularly important in the design and practical medical introduction of an artificial pancreas and other insulin-releasing systems, because it is extremely important to... 

II. TECHNIQUES USED FOR THE STUDY OF THE INTERFACIAL BEHAVIOR OF PROTEINS... 

The emphasis in this section will be on the interfacial behavior of proteins at electrode surfaces. Thus, studies of direct electron transfer from redox proteins will be discussed because of their relevance to surface adsorption properties. Details of mediated electron transfer will not be described here. Thus, this section will review electrochemical measurements made on the interfacial behavior of a variety of proteins at a number of different solid electrode surfaces. 

There have been a number of papers and reviews on the subject of the conformational behavior of proteins, both in bulk solution and at interfaces. In order to understand and inteipret results obtained from electrochemical studies of the interfacial behavior of proteins, it is important to be able to relate these results to the conformational behavior of proteins. This section will give a general overview of globular protein behavior and the theories associated with it. 

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