Abstracts of 2005 Publications
Probe Diffusion in Concentrated Polyelectrolyte Solutions: Effect of Background Interactions on Competition between Electrostatic and Viscous Forces
S. S. Jena and V.A. Bloomfield (2005), Macromolecules 38, 10551 - 10556.
We have used fluorescence recovery after photobleaching (FRAP) to measure the diffusion coefficient D of a small probe protein, green fluorescent protein (GFP), in solutions of the polyelectrolyte sodium polystyrenesulfonate (NaPSS) over a wide range of conditions. We covered a range of polyelectrolyte concentrations that resulted in solution viscosities from 1 to 50-100 cP, contrasted the behavior of high molecular weight (1 x 106 Da) and low molecular weight (7 x 104 Da) NaPSS, and explored the effects of low and high salt concentrations. We worked at a solution pH of 5.5, slightly higher than the isoelectric point of the GFP, which therefore had a small net negative charge. We observed positive deviations as large as 10-fold from Stokes-Einstein (S-E) behavior in high molecular weight NaPSS at low ionic strength. However, in low molecular weight NaPSS, approximately the same molecular weight as the DNA from our previous studies, deviations from S-E behavior were more modest, less than 2-fold. For high molecular weight NaPSS at high concentration, D increased with increasing salt concentration while decreased, indicating a competition between electrostatic force and viscous drag. Fitting of diffusion coefficients to the stretched exponential equation D/D0 = exp(-acv) yielded values of v near 1.0 and 0.68 for high and low molecular weight NaPSS solutions, respectively. These observations are consistent with mainly hydrodynamic influences on GFP diffusion in low molecular weight polyelectrolyte, but with increasing importance of electrostatic interactions in high molecular weight NaPSS. Comparisons with previous results show that polyelectrolyte size and flexibility, not just charge and concentration, play major roles in diffusion of probe molecules.
Probe Diffusion in Concentrated Polyelectrolyte Solutions: Effect of Probe Charge on Large Deviations from Stokes-Einstein Behavior
S. S. Jena and V.A. Bloomfield (2005), Macromolecules 38, 10557 - 10560.
The probe diffusion of green fluorescent protein (GFP) in a strongly interacting host polyelectrolyte solution of sodium polystyrenesulfonate (NaPSS) was studied using fluorescence recovery after photobleaching (FRAP). Along with providing a wide range of background polyelectrolyte conditions for probe molecules by varying the NaPSS concentration from dilute to highly concentrated and using two different NaPSS molecular weights, 1 x 106 and 7 x 104 Da, we also varied the net negative charge of the GFP by varying the pH. The probe diffusion coefficient D was significantly greater (up to 15-fold) than expected from the Stokes-Einstein (S-E) relation in concentrated solutions of high molecular weight NaPSS, but only moderately greater (2-fold or less) for lower molecular weight polymer. The deviations from S-E behavior increased with increasing pH, i.e., increasing negative GFP charge. We conjecture that the strong deviation from S-E behavior is due not only to microviscosity and electrostatic effects but also (for high molecular weight NaPSS) to the viscoelasticity of the concentrated polymer solution. D was fitted to the stretched exponential equation D/D0 = exp(-acv) at different pHs. In high molecular weight NaPSS solution the exponent decreased with increasing pH, whereas for low molecular weight NaPSS solution it increased. Comparison with previous results shows that along with the concentration, size, flexibility, and charge of the polyelectrolyte background, probe charge plays a significant role in the diffusion of probe molecules.
Last updated 12/11/05
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