Proteins thermodynamic compatibility

Nowadays it is well established that the interactions between different macromolecular ingredients (i.e., protein + protein, polysaccharide + polysaccharide, and protein + polysaccharide) are of great importance in determining the texture and shelf-life of multicomponent food colloids. These interactions affect the structure-forming properties of biopolymers in the bulk and at interfaces thermodynamic activity, self-assembly, sin-face loading, thermodynamic compatibility/incompatibility, phase separation, complexation and rheological behaviour. Therefore, one may infer that a knowledge of the key physico-chemical features of such biopolymer-biopolymer interactions, and their impact on stability properties of food colloids, is essential in order to be able to understand and predict the functional properties of mixed biopolymers in product formulations. 

Pectate, alginate, and CMC have held proteins dispersed under conditions that might otherwise have caused precipitation (Imeson et al., 1977). Polysaccharide stabilizers, in the order of decreasing thermodynamic compatibility with proteins, are pectin > CMC > alginate > gum arabic > dextran (Tolstoguzov, 1986). 

The thermodynamic compatibility of biological and synthetic polymers is a common question.993 Beijerinck,994 and Ostwald and Hertel,995 studied the thermodynamic compatibility of proteins and polysaccharides, and the latter authors evaluated the role of the source of starch. In contrast to cereal starches, potato starch is compatible with proteins in both acidic and basic media, whereas, Dahle991 reported that wheat starch sorbs proteins mainly in acidic and neutral solutions. 

On mixing solutions of a protein and a polysaccharide, four kinds of mixed solutions can be obtained. Figure 3.1 shows that two single-phase systems (1 and 3) and two-types of biphase systems (2 and 4) can be produced. The two-phase liquid systems 2 and 4 differ in the distribution of biopolymers between the co-existing phases. The biopolymers are concentrated either in the concentrated phase of system 2 because of interbiopolymer complexing, or within separated phases of system 4 because of incompatibility of the biopolymers. The term biopolymer compatibility implies miscibility of different biopolymers on a molecular level. The terms incompatibility or limited thermodynamic compatibility cover both limited miscibility or limited cosolubility of biopolymers (i.e., system 2) and demixing or phase separation... 

Han, X. and Damodaran, S. (1996). Thermodynamic compatibility of substrate proteins affects their cross-linking by transglutaminase. J. Agric. Food Chem. 44(5), 1211-1217. Herman, E. and Larkins, B. A. (1999). Protein storage bodies and vacuoles. Plant Cell 11, 601-613. 

When (S-lg adsorbs at the air-water interface in the presence of PS three phenomena can occur (i) the polysaccharide adsorbs at the interface on its own in competition with the protein for the interface (competitive adsorption) (ii) the polysaccharide complexates with the adsorbed protein mainly by electrostatic interactions or hydrogen bonding (Dickinson, 2003), and (iii) because of a limited thermodynamic compatibility between the protein and polysaccharide, the polysaccharide concentrates the adsorbed protein. In a previous work we have shown that the existence of competitive or cooperative adsorption between the (3-lg and the PS could be deduced from the comparison of rr-time curves for the single biopolymers and for the mixtures (Baeza et al., 2005b). 

The results revealed a significant effect of surface-active and nonsurface active polysaccharides on the properties of adsorbed protein films at the air-water interface. To explain the observed effects on the dynamics of adsorption, the rates of diffusion and rearrangement and the surface dilatational modulus were taken into accoimt (i) the competitive adsorption, (ii) the complexation, and (iii) the existence of a limited thermodynamic compatibility between protein and polysaccharide at the air-water interface and in the aqueous bulk phase. 

A. Effects of Thermodynamic Compatibility of Protein Substrates on Crosslinkage... 

For those samples that are not compatible with GC, the first question to ask involves the size (molecular weight) of the solute molecules. Their size should be compared to the pores of the packing materials that can be used in LC. If the size of the molecules is not negligible relative to the (average) pore size, then part of the pores and hence part of the stationary phase present in the column will not be accessible to the solute molecules. Hence, the simple relationship between chromatographic retention and thermodynamic distribution (eqn.l. 6) loses its significance. To avoid that, wide pore materials can be used for the separation of large molecules (e.g., proteins) based on their distribution over the two phases [202]. 

All life processes are the result of enzyme activity. In fact, life itself, whether plant or animal, involves a complex network of enzymatic reactions. An enzyme is a protein that is synthesized in a living cell. It catalyzes a thermodynamically possible reaction so that the rate of the reaction is compatible with the numerous biochemical processes essential for the growth and maintenance of a cell. The synthesis of an enzyme thus is under tight metabolic regulations and controls that can be genetically or environmentally manipulated sometimes to cause the overproduction of an enzyme by the cell. An enzyme, like chemical catalysts, in no way modifies the equilibrium constant or the free energy change of a reaction. 

For instance, denaturation and partial hydrolysis of proteins oppositely influence their incompatibility with other biopolymers (Tolstoguzov 1991). Most biopolymers are polyelectrolytes. Factors such as pH and salt concentration affect their interactions with one another, with the solvent and their compatibility. For instance, when the pH is shifted to their isoelectric point (lEP), the thermodynamic incompatibility of proteins is usually enhanced by self-association of the protein molecules. Generally, protein-neutral polysaccharide mixtures separate into two phases when the salt concentration exceeds 0.15 M. 

Many thermodynamic variables, such as temperature, pH, ionic strength, and other compositions in the solution system, affect protein solubility and compatibility with other macromolecule components of the system. At con-... 

---