Proteins interfacial behavior

From the kinetic analysis described above of the yt-curves of the three food proteins at different ionic strengths and concentrations (4), the evaluated interfacial behavior is given schematically in Figure 5. 

Figure 5. A highly schematic representation of the differing interfacial behavior of the three food proteins at different ionic strengths. (B) in the bulk medium (S) at the interface (D) diffusion rate (AA) is assumed to be inversely related to the rate of rearrangements in the protein film.

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... 

TiTuch of our understanding of the phase behavior of insoluble - monolayers of lipids at the air-water interface is derived from Adam s studies of fatty acid monolayers (I). It is now clear that the phase behavior of phospholipid monolayers (2) parallels that of the fatty acids we make use of these structure variations in our study of the interactions of phosphatidylcholine (lecithin) monolayers with proteins. Because of the biological significance of the interfacial behavior of lipids and proteins, there is a long history of studies on such systems. When Adam was studying lipid monolayers, other noted contemporary surface chemists were studying protein monolayers (3) and the interactions of proteins with lipid monolayers (4). The latter interaction has been studied by many so-called 4 penetration experiments where the protein is injected into the substrate below insoluble lipid monolayers that are spread on the... 

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). 

An understanding of protein adsorption behavior is applicable in numerous fields including blood-synthetic materials interfaces, macromolec-ular-rnembrane interactions, receptor interactions, enzyme engineering, adhesion, and protein separation on chromatographic supports. Many methods have evolved to study interfacial adsorption, but no single independent method seems adequate. The ideal technique should produce quantitative, real-time, in situ data concerning the amount, activity, and conformation of proteins adsorbed on well-characterized surfaces. All adsorption techniques are approximations to this optimum. 

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. 

Interfacial Behavior of Food Proteins Studied by the Drop Volume Method... 

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. 

Chapters deal with carbon-mineral hybrids and their mosaic surface structures, and interfacial phenomena at the surface of natural and synthetics polymers. They also explore a variety of biosystems that are much more complex, including biomacromolecules (proteins, DNA, and lipids), cells and tissues, and seeds and herbs. The authors cover trends in interfacial phenomena investigations, and the final chapter describes NMR and other methods used in the book. This text presents a comprehensive description of a large array of hard and soft materials, allowing the analysis of the structure-property relationships and generalities on the interfacial behavior of materials and adsorbates. 

In contrast to relatively small proteins of the respiratory chain, metalloenzymes represent more complex systems. Their catalytic reaction center is usually hidden inside the protein moiety. The adsorption of metalloproteins on the electrode surfaces is also irreversible, which results in their denaturation and dissociation of the catalytic center from the molecule. A typical protein which exhibits such interfacial behavior is glucose oxidase (GO). This flavoenzyme catalyzes oxidation of glucose to gluconolactone. The interest in studies of electrochemical and other interfacial behavior of GO is motivated by an effect to prepare a sensor for a rapid and simple monitoring of glucose level in patients suffering from diabetes. 

The subsequent chapters will discuss the following areas in greater detail structure-function of proteins, with specific reference to their surface property/interfacial behavior, amino acid surfactants, both chemically and enzymatically synthesized peptide surfactants and potential applications and the market assessment of PBS. 

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]. 

Constructing bio.sensors with greater reliability requires a better understanding of the basic principles of enzyme catalysis and immunochemical reactions, protein structures, pathways for receptor-based signal amplification, and the interfacial behavior of biocompounds at the artificial transducer surface. Biosen.sor research must therefore be directed toward the following points ... 

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