Probe diffusion in concentrated polyelectrolyte solutions: Effect of background interactions on competition between electrostatic and viscous forces

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 eta from 1 to 50-100 cP, contrasted the behavior of high molecular weight (1 x 10(6) Da) and low molecular weight (7 x 10(4) 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 eta decreased, indicating a competition between electrostatic force and viscous drag. Fitting of diffusion coefficients to the stretched exponential equation D/D-0 = exp(-alpha c(nu)) yielded values of nu 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.