Probe Diffusion in Concentrated Polyelectrolyte Solutions: Effect of Background Interactions on Competition between Electrostatic and Viscous Forces

Abstract

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 η from 1 to 50−100 cP, contrasted the behavior of high molecular weight (1 × 106 Da) and low molecular weight (7 × 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 hig...