Transition from double to single diffusive behavior with increase in polymer concentration for oppositely charged...

The effect of crowding, background and probe charges and chain flexibility on probe dynamics of Green Fluorescent Protein in polyelectrolyte solutions of poly-l-lysine was investigated using Fluorescence Recovery After Photobleaching (FRAP). An interesting double diffusive behavior in FRAP recovery curve was observed at low polymer concentration resulting in two relaxation modes, which disappears with rise in polymer concentration. The fast relaxation mode attributes to diffusion of free protein molecules alone, where as slow mode is credited to polymer adsorbed protein molecules. Absence of double diffusive behavior at higher polymer concentration is argued in terms of varying host chain conformation and the only relaxation mode present is due to movement of free probes alone rather than that of adsorbed proteins. We noticed only a marginal decrease in diffusion coefficient with rise in salt concentration and the trend is reversed when variation in sample viscosity with salt is taken into account. In addition a small but systematic decrease in diffusion coefficient is seen with increase in magnitude of probe charge. Comparison of results with ideal Stoke - Einstein relation brings out the importance of polyelectrolyte effect and indicates ∼200 - fold positive deviations from predicted value for both variation in ionic strength and solution pH.