Soluble electrostatic complexes

Carboxyl-containing polysaccharides behave as polyanions at mild acidic and neutral pH values, which are typical of most foods. Electrostatic complexing between proteins and anionic polysaccharides generally occurs in the pH range between the pK value of the anionic groups (carboxyl groups) on the polysaccharide and the protein s lEP. Sulphated polysaccharides are capable of forming soluble complexes at pH values above the protein s lEP. 

Formation of electrostatic complexes means mutual neutralisation of the macro-molecular reactant. This mutual neutralization of opposite charges and formation of the concentrated complex coacervate phase, minimizes the electrostatic free energy and reduces both the hydrophilicity and the solubility of the resultant complex. The loss of entropy on complexing may be compensated by the enthalpy contribution from interactions between macro-ions and by liberation of counter-ions and water molecules. 

The formation of complexes affects both particle-solvent and particle-particle interactions. The solubility of proteins may be increased by their electrostatic complexing with anionic polysaccharides. Formation of titration-complexes may increase protein solubility and inhibit protein precipitation at the lEP. Anionic polysaccharides can act as protective hydrocoUoids inhibiting aggregation and precipitation of like-charged dispersed protein particles, for example, of denatured proteins. This protective action also can increase the stability of protein suspensions and oil-in-water emulsions stabilized by soluble protein-anionic polysaccharide complexes. 

More detailed discussion of food polymers and their functionality in food is now difficult because of the lack of the information available on thermodynamic properties of biopolymer mixtures. So far, the phase behaviour of many important model systems remains unstudied. This particularly relates to systems containing (i) more than two biopolymers, (ii) mixtures containing denatured proteins, (iii) partially hydrolyzed proteins, (iv) soluble electrostatic protein-polysaccharide complexes and conjugates, (v) enzymes (proteolytic and amylolytic) and their partition coefficient between the phases of protein-polysaccharide mixtures, (vi) phase behaviour of hydrolytic enzyme-exopolysaccharide mixtures, exopolysaccharide-cell wall polysaccharide mixtures and exopolysaccharide-exudative polysaccharide mixtures, (vii) biopolymer solutes in the gel networks of one or several of them, (viii) enzymes in the gel of their substrates, (ix) virus-exopolysaccharide, virus-mucopolysaccharides and virus-exudative gum mixtures, and so on. 

The effects of added electrolytes (salts) on protein solubility are complex. At low concentrations (typically with salt concentrations below about 0.5 mol dm ), electrostatic screening by the small ions in solution tends to reduce electrostatic interactions between the macromolecules, and the solubility of protein increases. Historically, this is known as the salting-in effect. However, at higher salt concentrations the solubility of protein tends to decrease due to the salting-out effect. 

Phytic acid R 0 I 0. -0 II y Y R R- -P. R, .L J-, HO OH V V R.O R Forms complexes with Ca, Fe, Mn, Zn and lower their bioavailability but also discussed to be cancer preventing Noncowdent interactions with proteins Forms insoluble electrostatic complexes with globulins and change their IP to low pH Seed dehuUing Pre-extraction at minimal protein solubility pH wdues Use of high ionic strenghts for protein isolation Ultrafiltration Phytase treatment... 

The complexation of two DHBCs has been recently described in the literature. The complex formation between a miktoarm star copolymer PEOn-PMAA and a quatemized PEO-PDMA diblock was studied upon changes in solution pH [54]. At elevated pH values, where PMAA blocks were ionized, the formation of electrostatic complexes was recorded between the oppositely charged PMAA and quatemized PDMAEMA blocks. However, the aggregates were soluble due to the presence of the PEG uncomplexed blocks originating from both of the copolymers. The situation was dramatically different at low pH values, where the PMAA block was not ionized. At the aforementioned pH values the development of hydrogen bonds between the PMAA and PEO blocks took place. In the later case, the aggregates were electrostatically stabilized by the quatemized PDMAEMA polyelectrolyte block. The micellization/complexation behavior is schematically illustrated in Figure 13. 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

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