TRACER DIFFUSION OF PROTEINS IN DNA SOLUTIONS

We have used fluorescence photobleaching recovery to measure the translational diffusion of bovine serum albumin (BSA) in DNA solutions as a function of DNA concentration (from 2 to 36 mg/mL) and added salt concentration (0.1 and 0.01 M NaCl). The BSA was labeled with fluorescein isothiocyanate; the DNA was 160 base pair mononucleosomal material, about 1 persistence length. Lowering the salt concentration decreases the BSA tracer diffusion coefficient, probably due to the expanded counterion atmosphere and consequently greater effective excluded volume of the polyions. The DNA-concentration dependence is fit well by an accessible volume fraction model and also by a stretched exponential function D/Do=exp(-ac(nu)). The data follow a nu = 1 dependence, rather than the nu = 1/2 seen in some other recent studies of rigid rods.