Physical cross-links ionic interaction

Fig. 1 Representative methods of hydrogel formation. (A) Chemically cross-linked hydrogels are prepared from monomers, oligomers, or polymers in the presence of cross-linking agents. The chemical cross-linking proceeds via radical polymerization or polycondensation reaction. (B) Physically cross-linked hydrogels can be formed by ionic interactions, hydrophobic interaction, or hydrogen bonding.

NA isolation and molecular characterization will be important to define the origin and functions of these proteins. At this time, infected cell nuclei offer the only source of these proteins, and NA have proved resistant to classic nuclear extraction methods (Yao and Jasmer, 1998). NA can be solubilized under conditions that co-extract nuclear lamins a/c and b (4 M urea, pH 8.0). Despite these similar physical properties, NA do not co-localize with lamins in the nucleoskeleton. However, both disulphide bonds and ionic interactions appear to contribute to nuclear complexes containing NA. In addition, NA can be cross-linked within host nuclei with protein cross-linking reagents. The foregoing properties represent current information available for the development of strategies to isolate and characterize these proteins and to investigate host proteins with which NA interact. 

Mode of immobilization. Immobilization can be effected either chemically, by covalent bonding of the biocatalyst on a surface (Figure 5.6, option 1), by adsorption, or by ionic interactions between catalyst and surface (option 2), as well as by cross-linking of biocatalyst molecules for the purpose of enlargement (option 3), or physically by encapsulation in matrices or by embedding in a membrane (option 4). 

Preparation of active, insoluble enzymes by physical adsorption to matrices has already been discussed (see p. 362), and, in the case of polysaccharide matrices, most of the enzyme derivatives formed in this way depend upon ionic interaction between charged groups on the polysaccharide and enzyme (see Table II). The instability of these enzyme derivatives may be lessened by cross-linking the coupled enzyme molecules.A few enzymes have been insolubilized by inclusion in microcapsules of collodion, or by entrapment in a polysaccharide gel. - A partition eflFect, in which an enzyme is held in the aqueous phase of a cellulose column while the substrate flows through in an organic phase, has also been used. 

Fig. 2 (a-c) Physical polymer-network cross-linking provided by mixed micelles in hydrogels formed via hydrophobic interactions in surfactant solutions. Mixed micelles are formed by aggregation of hydrophobic blocks of per-se hydrophilic polymers and surfactant alkyl tails, (b) Nonionic polymer and ionic surfactant gel system at the state of preparation. For clarity, charges are not shown, (c) Ionic polymer and oppositely charged surfactant gel system after extraction of free micelles, (d) Structure of the hydrophobic monomers used in the micellar polymerization... 

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