| Abstract
| - Proteins diffuse to their sites of action within cells in a crowded, strongly interactingenvironment of nucleic acids and other macromolecules. To investigate the dynamics of a typical globularprotein in such an environment, we used fluorescence photobleaching recovery to measure the probediffusion of green fluorescent protein (GFP) in dilute to highly concentrated aqueous solutions of glycerol,Ficoll 70, and persistence-length calf thymus DNA. In glycerol, GFP accurately obeyed the Stokes−Einsteinequation that relates diffusion coefficient to solution viscosity. In concentrated Ficoll 70, GFP diffusedmoderately faster than predicted from viscosity, demonstrating the phenomenon of microviscosity in amolecularly heterogeneous solution. In DNA, the diffusion coefficient of GFP was markedly greater thanpredicted from the Stokes−Einstein equation, with deviations increasing at lower ionic strength. Thisbehavior reflects microviscosity, Coulombic interactions, and the dynamics of probe diffusion in DNAsolutions that had undergone the ordinary−extraordinary transition, which we demonstrated by dynamiclight scattering.
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