Coiled conformation, polyelectrolytes

The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et al., 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al., 1980). 

Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry of polyelectrolyte cements. 

Random Coil Conformations of Polyelectrolyte Chains in Aqueous Solution ... 

A discussion of random coil conformations is difficult because it perforce includes many deductions which involve excluded volume theories. This is a very controversial subject and extremely difficult to discuss in an easily grasped way. It must be emphasized however that this secticHi is not concerned with excluded volume theory, but rather with the problem of how unperturbed dimensions may be related to polyelectrolyte structure. 

To achieve the latter, spectroscopists have exploited the fact that when in a coiled conformation in aqueous solution, certain polyelectrolytes can solubilize [6,12,17-21] low molar organic molecules. On changing the pH, the chain expands releasing this material into the aqueous phase. Analysis of the resultant emission from the probe can reveal information concerning its environment and thence, the conformation of the polymer [6,12,17-21],... 

The addition of a positively charged peptide with a random-coil conformation, JR2K, will force the polyelectrolyte to adopt a nonplanar conformation with separated polyelectrolyte chains, observed as a blue shift and an increased intensity of the emitted Kght. Upon exposure to a negatively charged... 

The increase of the hydrodynamic volume of PVA molecules owing to the interaction with NaDS can be interpreted by the formation of a polyelectrolyte-type complex, i.e. the polymer coil expands, as a result of the electrostatic repulsion between the charged surfactant subaggregates. The increase in viscosity also suggests that the complex molecules are not compact, but the polymer keeps its random coil conformation. 

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... 

All the scaling features are explicitly shown in Fig. 2 in chapter Polyelectrolyte Science and Application . The ratio Rg/Rn is indicative of three dimensional conformation of the polymer molecule and this ratio is exactly 1.8 for a random coil [105]. For the HEC samples used in this study (M3 = 1. 3 X 10 Da), Rg/Rn = 1.78 0.06 that attributed random coil conformation to this biopolymer molecule in water at room temperature. 

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