Electrolyte polyampholytes

Proteins that are polyampholytes, upon addition of electrolyte, first undergo some salting in up to a certain ionic strength, since electrostatic interactions between ions are shielded by the additional simple ions of both sign. Adding more salt will then cause salting out as the added ions compete for water that would otherwise solvate the protein. 

A kind of colloidal electrolyte consisting of a macromolecule that, when dissolved, dissociates to yield a polyionic parent macromolecule and its corresponding counterions. Also termed polyion , polycation , or polyanion . Similarly, a polyelectrolyte can be referred to in certain circumstances as a polyacid , polybase , polysalt , or polyampholyte (Ref. [978]). Example carboxymethylcel-lulose. 

Polyampholytes. Ampholytic copolymers (i, 30-35) exhibit interesting viscosity behavior in electrolyte solutions that is quite opposite to the behavior of conventional polyelectrolytes. Recently we prepared high charge density copolymers from the matched, nonhydrolyzable comonomer pairs sodium 2-acrylamide-2-methylpropanesulfonate (NaAMPS) and 2-acrylam-ide-2-methylpropanedimethylammonium chloride (AMPDAC) (1, 33) see structures). 

The sensitivity of stoichiometrically balanced polyampholytes to addition of simple electrolytes is also a function of chain stifihess, counterion binding, and side-chain length. In semidilute solutions, physical entanglements. 

In polyampholytes the interaction between charged macromolecules and surrounding low-molecular-weight electrolytes is important because the nature of these interactions plays an essential role in understanding the structure of polyampholytes on a molecular level. Authors [61] have reported the Na, Cl, and NMR relaxation rates and chemical shifts of amphoteric gel MAA-co-DMAMA under a variety of water contents and 1 M NaCl and 1 M KCl solution contents. 

Figure 3 represents the effect of added electrolyte concentration on the [nl obtained from the modified Huggins plot for poly(4VMP/pSS) and poly(MPTMA/AMPS), and the usual Huggins plot for poly(METMA/MES). The intrinsic viscosity increases with increasing salt concentration for all three ampholytic systems. Similar results are also reported for other polyampholyte-salt systems (6,13,27,28). This behavior may be rationalized on the basis of chain expansion which results in increased solute-solvent interaction. The [ri] is related to the hydrodynamic volume of macromolecules in solution (29). An expansion of the chain results in the viscosity increase due to an increase in effective hydrodynamic volume of the solute in the given solvent. It is expected that the added electrolyte would disrupt the intramolecular and intermolecular interactions and allow the polymers to behave more freely. Thus, the increase in [n] may be related to extended chain conformations resulting from the increased polymer-solvent interactions. 

This study confirms the concept that polyampholytes take an expanded conformation in aqueous salt solutions which is in contrast to typical polyelectrolyte behavior. Viscosity determinations in conjunction with light scattering studies has provided a general confirmation of the polyampholyte effect in the polymers derived from the ion-pair comonomer in aqueous salt solutions. This effect is related to the ion-binding capabilities of the added electrolytes. 

FIGURE 17.12 Electroviscous effects for an aqueous solution of a polyampholyte (e.g., a protein). Electrolyte — NaCl --BaClj, ... Na2S04. 

Solution Properties. The aqueous solution behavior of polyampholytes is dictated by coulombic interactions between the basic and acidic residues. Polyampholytes have the ability to exhibit both polyelectrolyte and antipolylelectrolyte behavior in aqueous media. Which type of behavior is exhibited depends on factors such as solution pH, copolymer composition, the relative strengths of the acidic and basic residues, and the presence/absence of low molecular weight electrolyte (239). A feature of polyampholytes—in particular those comprised of weak acidic and basic residues—is the so-called isoelectric point, or lEP. This is simply defined as the solution pH at which the polyampholyte is electrically neutral. Statistical polyampholytes often remain soluble at and around the lEP whereas block polyampholytes tend to be soluble above and below but insoluble at this critical pH. The lEP may be determined either by titration or by measuring the reduced viscosity as a function of pH—the lEP also represents the point at which the polyampholyte chain is in its most compact conformation and thus corresponds to the minimum in reduced viscosity (239,266). With a knowledge of the respective piiLa s and copolymer composition it is also possible to predict the lEP (267). 

Solution Properties. Zwitterionic polymers show interesting aqueous solution behavior. As a general rule, they are insoluble in pure water due to the formation on intra- and interchain ion contacts resulting in an ionically cross-linked network-type structure. Polyampholytes and polybetaines which are not soluble become soluble upon the addition of low molecular weight electrolytes, such as NaCl (Fig. 51). This dissolution process can best be understood in terms of the low molecular weight electrolyte penetrating the ionically cross-linked network whereupon the ions screen the net attractive interactions between the polymer chains and hence promote solubility. The addition of the salt also results in an-tipolylelectrolyte behavior, ie chain expansion upon the addition of the salt. 

Neyret and Vincent [30] have developed such an approach for the formation of microgel particles, named inverse microemulsion polymerisation. The oil phase consisted of anionic 2-acrylamido-2-methylpropanesulfonate (AMPS) and cationic (2-(methacryloyloxy)ethyl) trimethylammonium (MADQUAT) monomers in addition to a BA cross-linker. The co-polymerisation was initiated using UV irradiation and the product isolated and re-dispersed in aqueous electrolyte solution to yield polyampholyte microgel particles. The particles became swollen in the presence of high electrolyte concentrations as a result of screening of the attractive electrostatic interactions between neighbouring chains. 

A characteristic that distinguishes polyelectrolytes from polyampholytes is behavior in aqueous media. Hydrophilic polyelectrolytes exhibit extended conformations in water at low concentrations due to repulsive coulombic interactions and the associated osmotic effects. A reduction in charge by pH adjustment or addition of electrolyte allows the chain to assume a less extended, random coil conformation as evidenced by a decrease in hydrodynamic volume. In contrast, structure-behavioral relationships of hydrophilic polyampholytes are governed by coulombic attractions between anionic md cationic fimctional... 

Our research group has been intensively studying the behavior of polyampholytes in aqueous media of moderate electrolyte content for applications in petroleum recovery, drag reduction, water remediation, and formulation of pharmaceuticals, coatings, and cosmetics. For each application, selection of appropriate comonomers and synthetic techniques can lead to desired conformational behavior under external conditions of pH, temperature, shear stress, and electrolyte concentration. 

Salt-tolerant polyampholytes with potential for viscosity maintenance (or increase) at low concentration in the presence of simple electrolytes such as NaCl include polymers formed by equamolar incorpomtion of sulfonate and quaternary ammonium mer units (Type A) or those formed by the copolymerization of a zwitterionic sulfobetaine monomer with a water-soluble monomer (Type B) as shown in Scheme 1. Usually, a water-soluble mer unit, W, is included for adequate hydration. Key features of polyampholytes from sulfonated quaternary ammonium monomers (Types A and B) are discussed here, although corresponding carboxylate (4-9), phosphonate (10-14), or tertiary ammonium derivatives have been syndiesized and are responsive to pH. 

Figure 1 illustrates the effect of NaCl concentration on intinsic viscosities for copolymers of series I with incrementally increasing composition of sulfobetaine mer units. Copolymers with 10,26, and 38 mole percent of 5 (MQ, t 26. and 1-38) are soluble in deionized water and show increases in viscosity across the NaCl concentration range. I-IO and 1-28 may be aggregated (multimers) in deionized water, explaining the initial decrease upon addition of salt. The homopolymer of 5 (MOO) and copolymers MZ and h69 are insoluble in deionized water and require a critical concentration of NaCl for dissolution. In deionized water, the intramolecular dipole-dipole interactions in these copolymers dominate, allowing little hydration. Addition of electrolyte induces the globule-to-random coil transition typical of polyampholyte hydration. 

In this chapto, we have illustrated the effects of molecular architecture of polysulfobetaines on solution behavior under specilBc environmental conditions of pH, added electrolytes, and polymer concentration. The nature of the comonomer and amount of incorporation of the sulfobetaine within the polymer chain dictate the polymer solubility and solution behavior. Polyampholyte behavior is realized for acrylamide-based systems containing the sulfobetaine moiety. Polyelectrolyte behavior is coupled with polyampholyte behavior for cyclopolymers containing >40mol% sulfobetaine. Incorporation of the sulfobetaine monomer hinders hydrophobic association for the pH responsive copolymers of series TV at low degrees of ionization. 

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