Effect of electrostatic interactions on the structure and dynamics of a model polyelectrolyte. I. Diffusion

Abstract

The dynamics of a 20 base pair oligonucleotide is studied by dynamic light scattering-photon correlation spectroscopy and depolarized Fabry-Perot interferometry. The 20 base pair oligonucleotide is a well-defined, albeit short, rigid rod molecule that serves as a model for polyelectrolyte solution dynamics. The effects of added salt on the solution rotational and translational dynamics are examined in detail as functions of the 20-mer concentration. Coupled mode theory together with counterion condensation theory gives good predictions for the effects of salt on the translational diffusion of the 20-mer at the relatively low oligonucleotide concentrations studied. Comparison of the experimental results with these theories shows that the effective charge density of the polyion in solution is approximately equal to the reciprocal of the product of the Bjerrum length and the counterion charge, eff Х1/N B. Calculation shows that the numerical solution of the coupled mode theory matrix gives a better fit of our measured polyion diffusion coefficients than the approximate equation derived by Lin, Lee, and Schurr. Simple approximations for the effective rod length, L eff ϭLϩ Ϫ1 , and effective rod diameter, d eff ϭdϩ Ϫ1 , are used to model the thermodynamic-hydrodynamic interactions for charged rodlike molecules and to make predictions for the diffusion second virial coefficient as a function of added salt concentration. This alternative to the coupled mode theory also gives good agreement with experiment. The rotational diffusion constants of the oligonucleotide measured by depolarized Fabry-Perot interferometry show a slowing down of the rotation at low added salt concentrations as the oligonucleotide concentration is increased.

Denaturation and aggregation of hen egg lysozyme in aqueous ethanol solution studied by dynamic light scattering

“…We consider that any large aggregates cannot be formed in the low ethanol solution because of the strong repulsion between the lysozyme molecules. A similar slow mode was often observed when dynamic light scattering was used for a salt-free polyelectrolyte solution, [17][18][19][20] where strong repulsion exists between polyelectrolytes. The origin of the slow mode in these cases is thought to be "temporary clusters," proposed recently 19,20 to form via strong repulsion between polyelectrolytes.…”

Rouse Dynamics of Polyelectrolyte Solutions: Molecular Dynamics Study

“…2,8,[10][11][12] Unfortunately, experimental data 13,[14][15][16][17][18][19][20][21][22][23] alone cannot provide a detailed test of the microscopic foundations of these models. Computer simulations [24][25][26][27][28][29][30][31] play an important role in our understanding the static properties of polyelectrolyte solutions by providing a valuable insight into molecular origin of the phenomena and by direct testing the assumptions of the theoretical models.…”

Interparticle correlations due to electrostatic interactions: A small angle x-ray and dynamic light scattering study. I. Apoferritin

“…Two additional modes appear: a fast mode which is related to the diffusion of small ions and the charge fluctuations on the polyion 42,43 and a slow mode which is related to the collective dynamics of correlated polyions ͑paracrystalline structure, long range order͒. 1,2,4,44 The slow mode was observed also in semidilute solutions of semiflexible linear polyelectrolytes, 44 but in this case the molecular interpretation is not straightforward. In the case of semiflexible linear polyelectrolytes of a size comparable with 1/q, the internal modes of the chain contribute to the DLS correlation function both for neutral and charged polymers.…”

“…There is, however, mounting evidence that in the case of dilute solutions of compact polyelectrolytes the slow mode can be directly related to the peak observed in the SAXS data and to the dynamics of ordered polyion regions. 1,2,4 Usually, for charged colloidal suspensions the most interesting region for scattering studies is the combination of scattering vector and concentration ranges for which both structure and form factors can be measured in static measurements and, simultaneously, dynamic measurements reveal the diffusion coefficient values in the same q range. Theoretical models based on the Poisson-Boltzmann equation are used to interconnect these results and calculate other properties like hydrodynamic interactions responsible for additional change in the particles diffusivity.…”

“…1,2,4,44 Such evidence of liquid-like order stays in contrast to the crystalline order in very dense hard sphere systems, where small aggregates are formed. 55 The main difference lies in packing: densely packed crystalline aggregates ͑as in colloid crystal͒ are surrounded by lower concentration solution, while the average distance between small polyions in a low ionic strength solution is the same in the entire sample.…”