Adsorption of Biopolymers, with Special Emphasis on Globular Proteins
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1. Introduction........................................................................................... 99 2. Flexible Polymers ............................................................................... 101 3. Globular Proteins ................................................................................ 104 3.1. Types of Interaction Involved in Protein Adsorption at a Smooth Surface .................................................................... 105 3.1.1 Interaction Between Electrical Double Layers ................. 106 3.1.2 Dispersion Interaction ...................................................... 107 3.1.3 Changes in the State of Hydration .................................... 109 3.1.4 Rearrangements in the Protein Structure .......................... 110 3.2. Protein Adsorption in Model Systems ........................................ 111 3.3. Morphology of the Sorbent Surface ........................................... 114 3.4. Adsorption-Induced Changes in the Structure and Biological Activity of Proteins. A Case Study ............................................. 116 References................................................................................................ 121
1 Introduction Adsorption of (bio)polymers occurs ubiquitously, and among the biopolymers, proteins are most surface active. Wherever and whenever a protein-containing (aqueous) solution is exposed to a (solid) surface, it results in the spontaneous accumulation of protein molecules at the solidwater interface, thereby altering the characteristics of the sorbent surface and, in most cases, of the protein molecules as well (Malmsten 2003). Therefore, the interaction between proteins and interfaces attracts attention from a wide variety of disciplines, ranging from environmental sciences to food processing and medical sciences.
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W. Norde
Proteins are biopolymers of some 22 different amino acids. Because of the variation in physical-chemical properties, mainly polarity and electrical charge, between the constituent amino acids, protein molecules are ampholytic (i.e., containing positively and negatively charged groups) and more or less amphiphilic (i.e. comprising polar and apolar domains). These properties, in turn, lead to the formation of complex three-dimensional (3D) structures. Soil systems are highly heterogeneous, containing particles of colloidal dimensions. Hence, soil represents a relatively large interfacial area per unit volume, and a large fraction of the surface-active components, e.g., proteins, present in soil are adsorbed at interfaces. This has a number of consequences. For instance, by being adsorbed at surfaces, the hydrolysis of proteins by proteases (from micro-organisms) may be affected and, therefore, their availability as a nutrient. Further, the structure of a protein molecule and, hence, its biological activity are influenced by changes in its environment (Haynes and Norde 1994; Norde et al. 2005), as occurring during adsorption. This would affect the biological functioning of extrac
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