Polyelectrolyte dynamics solvent

The use of computer simulations to study internal motions and thermodynamic properties is receiving increased attention. One important use of the method is to provide a more fundamental understanding of the molecular information contained in various kinds of experiments on these complex systems. In the first part of this paper we review recent work in our laboratory concerned with the use of computer simulations for the interpretation of experimental probes of molecular structure and dynamics of proteins and nucleic acids. The interplay between computer simulations and three experimental techniques is emphasized (1) nuclear magnetic resonance relaxation spectroscopy, (2) refinement of macro-molecular x-ray structures, and (3) vibrational spectroscopy. The treatment of solvent effects in biopolymer simulations is a difficult problem. It is not possible to study systematically the effect of solvent conditions, e.g. added salt concentration, on biopolymer properties by means of simulations alone. In the last part of the paper we review a more analytical approach we have developed to study polyelectrolyte properties of solvated biopolymers. The results are compared with computer simulations. 

In order to resolve these challenges, it is essential to account for chain connectivity, hydrodynamic interactions, electrostatic interactions, and distribution of counterions and their dynamics. It is possible to identify three distinct scenarios (a) polyelectrolyte solutions with high concentrations of added salt, (b) dilute polyelectrolyte solutions without added salt, and (c) polyelectrolyte solutions above overlap concentration and without added salt. If the salt concentration is high and if there is no macrophase separation, the polyelectrolyte solution behaves as a solution of neutral polymers in a good solvent, due to the screening of electrostatic interaction. Therefore for scenario... 

Dilute polyelectrolyte solutions, such as solutions of tobacco mosaic virus (TMV) in water and other solvents, are known to exhibit interesting dynamic properties, such as a plateau in viscosity against concentration curve at very low concentration [196]. It also shows a shear thinning at a shear strain rate which is inverse of the relaxation time obtained from the Cole-Cole plot of frequency dependence of the shear modulus, G(co). 

A new area that has been examined is that of molecular dynamics in polyelectrolytes,106 where the molecular motions responsible for the glass transition have been identified. Another interesting study is the effect of molecular motion upon the ingress of solvent into polymer material, as a function of cross-linking density.107 In the particular case studied, of dioxane... 

Some of the relevant questions primarily motivated by scientific interest are the following. How is the size of a polyelectrolyte affected by molecular weight, intrinsic stiffness, solvent quality, or ionic strength Which observables are well characterized by coarse-grained quantities such as a linear charge density, and which depend on chemical details How are dynamic quantities like viscosity or electrophoretic mobility related to static properties of poly electrolytes ... 

Micka U, Holm C, Kremer K. Strongly charged, flexible polyelectrolytes in poor solvents—a molecular dynamics study. Langmuir 1999 15 4033. 

Here we describe dynamic light scattering results on ionomers in a salt-free, nonaqueous, polar solvent such a solution has a stronger scattering power and is easier to handle than polyelectrolytes in water, as already discussed in the Static Scattering section. 

These results confirm the observation that polyelectrolyte aqueous solutions show two separate decay modes in the autocorrelation function and support our contention that ionic polymer systems generally behave similarly in polar solvents [23], To support this, it may be added that similar dynamic scattering behavior was recently reported for another type of ionomer, polyurethane ionomer, dissolved in a polar solvent, dimethylacetamide (e = 38) [92], Finally, it should be stressed that the explanation given above for light scattering (both static and dynamic) behavior of salt-free polyelectrolytes is based on the major role of intermolecular electrostatic interactions in causing characteristic behavior. No intramolecular interactions are explicitly included to explain the behavior. This is in accord with our contention that much of the polyelectrolyte behavior, especially structure-related aspects, is determined by intermolecular interactions [23]. 

Relatively little use has been made of the phase-space kinetic theory to study solvent elTects m polymer solution dynamics. Also much more can be done with regard to wall effects, flow of polymers m constrictions, behavior of polymers at interfaces, and the thermal and diffiisional properties of polyelectrolytes. 

There are many problems that would require so much computer time that their study by the previous method would not be possible. For example polyelectrolyte solutions, or motions of particles in membranes would not be susceptible to study because of wide separations in the time scales for different dynamic processes characterizing solute and solvent or because the property of interest evolves so slowly that an excessively long trajectory would be required. The study of these systems requires a different approach. A beginning was made many years ago by Simon,who studied the melting of DNA by solving the coupled set of stochastic Langevin equations on a computer. This required an assumption about the statistical distribution of random forces. The precise values of the forces were then sampled from this distribution. 

The adsorption of polyelectrolytes to surfaces is a problem of growing interest stimulated by many industrial applications. Explicit and implicit solvent models were used in studying this problem via computer simulation [39,40]. Using molecular and Brownian dynamics simulations and... 

Chang R, Yethiraj A (2006) Dilute solutions of strongly charged flexible polyelectrolytes in poor solvents molecular dynamics simulations with explicit solvent. Macromolecules 39 821-828. doi 10.1021/ma051095y... 

Reddy G, Chang R, Yethiraj A (2(X)6) Adsorption and dynamics of a single polyelectrolyte chain near a planar charged surfaces molecular dynamics simulation with explicit solvent. J Chem Theory Comput 2 630-636. doi 10.1021/ct050267u... 

Molecular dynamics simulations of partially charged polyelectrolytes in poor solvent conditions were performed by the Kremer group [153,154] for... 

For detailed dynamics description and analysis of the continuum theory of ionic polymeric gel the reader is referred to Segalman, Witkowski, Adolf and Shahinpoor. Since polyelectrolytes are for the most part three dimensional network of macromolecules cross-linked nonuniformly, the concentration of ionic charge groups are also nonuniform within the polymer matrix. Therefore the mechanism of swelling and contraction are intimately related to osmotic diffiision of solvent, ions and counterions into and out of the gel. One possible way to describe this mechanism is to model the system by the governing continuum mechanics equations and Neo-Hookean deformation theory. In the next section an analytical relation is presented as described by Segalman, Wi owski, Adolf and Shahinpoor. ... 

---