Biopolymers ionic interactions

Polyelectrolytes are long chain molecules bearing ionisable sites. It is not always possible to predict with confidence the extent to which polyelectrolytes behaviour is exhibited. Thus, polyacrylic acid in water is only weakly ionised and in dioxan it behaves as a typical non-electrolyte. It is usual to overcome the complications imposed by ionic interactions by the inclusion of simple salts and LS studies in salt-free solutions are rather rare. The problems have been discussed recently by Kratochvil137), whilst the review of Nagasawa and Takahashi138 constitutes one of the few devoted exclusively to LS from polyelectrolyte solutions. LS from many biopolymers such as proteins is, of course, extremely relevant in this context. 

Ion bridging is a specific type of Coulombic interaction involving the simultaneous binding of polyvalent cations (e.g., Ca, Fe, Cu ) to two different anionic functional groups on biopolymer molecules. This type of ionic interaction is commonly involved in associative self-assembly of biopolymers. As a consequence it is also an important contributory factor in the flocculation (via bridging or depletion) of colloidal particles or emulsion droplets in aqueous media containing adsorbed or non-adsorbed biopolymers (Dickinson and McClements, 1995). 

Finally, we note here that the dehydration of biopolymers associated with their attractive non-ionic interactions (dipole-dipole, hydrophobic) can increase the contribution of the mixing entropy. This in turn can lead to a tendency towards enhancement of the thermodynamically favourable interactions between them (Appelqvist and Debet, 1997 Cai and Arnt-field, 1997 Semenova a/., 1991b). 

Binding enzymes to solid supports can be achieved via covalent bonds, ionic interactions, or physical adsorption, although the last two options are prone to leaching. Enzymes are easily bound to several types of synthetic polymers, such as acrylic resins, as well as biopolymers, e.g., starch, cellulose [52], or chitosan [53,54]. Degussa s Eupergit resins, for example, are used as enzyme carriers in the production of semisynthetic antibiotics and chiral pharmaceuticals [55], Typically, these copolymers contain an acrylamide/methacrylate backbone, with epoxide side groups... 

It is well known that a particular conformation of a biopolymer maintains its stability only in aqueous solutions. Addition of, say, 20% alcohol causes a conformational change which eventually leads to the process of denaturation. The latter is a very complex process and Involves the combination of many factors such as hydrogen bonding, ionic interaction, and van der Waals interaction between the various residues of the polymer. It has been conjectured that the tendency of the nonpolar groups (such as methyl or ethyl groups attached to the amino acids) to avoid the aqueous environment is one of the major reasons for the stabilization of the native conformation of the biopolymer. This is shown schematically in the first process depicted in Fig. 8.1. Here, we stress an extreme example where the polymer is folded in such a way that the side-chain nonpolar groups are completely removed from the aqueous medium and transferred to the interior of the polymer, where they are exposed to an environment similar to that of a typical nonpolar solvent. 

Film and coating fonnation occurs when biopolymer molecules interact through cohesive forces, named H-bonding, ionic bonds and covalent bonds (disulfide bonds). Factors affecting film strength are the chemical nature of the biopolymer and the rest of the components of the formulation (plasticizer type and amount and food additives), and the film forming process. 

The interactive character of a molecule can be very complex and a molecule can have many interactive sites. These sites will comprise the three basic types of interaction, i.e., dispersive, polar and ionic. Some molecules (for example, large molecules such as biopolymers) can have many different interactive sites dispersed throughout the entire molecule. The interactive character of the molecule as a whole will be... 

The ionic strength dependence of intrinsic viscosity is function of molecular structure and protein folding, ft is well known that the conformational and rheological properties of charged biopolymer solutions are dependent not only upon electrostatic interactions between macromolecules but also upon interactions between biopolymer chains and mobile ions. Due electrostatic interactions the specific viscosity of extremely dilute solutions seems to increase infinitely with decreasing ionic concentration. Variations of the intrinsic viscosity of a charged polyampholite with ionic strength have problems of characterization. 

Under the conditions of screening of electrostatic interactions between polyions, as occurs at high ionic strength (say, / > 0.1 mol dm- ), or in solutions containing neutral (non-ionic) polymers, the excluded volume term is the leading term in the theoretical equation for the second virial coefficient. In this latter type of situation, the sizes and conformation/ architecture of the biopolymer molecules/particles become of substantial importance. 

Thermodynamically unfavourable interactions between two biopolymers may produce a significant increase in the surface shear viscosity (rf) of the adsorbed protein layer. This change in surface rheological behaviour is a consequence of the greater surface concentration of adsorbed protein. For instance, with p-casein + pectin at pH = 5.5 and ionic strength = 0.01 M (Ay = 2.6 x 10 m3 mol kg-2), the surface shear viscosity at the oil-water interface was found to increase by 20-30%, i.e., rp = 750 75 and 590 60 mN s m-1 in the presence and absence of polysaccharide. These values of rp refer to data taken some 24 hours following initial protein layer formation (Dickinson et al., 1998 Semenova et al., 1999a). 

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