Protein intermolecular depletion interactions induced by polyethylene glycol (PEG) depend largely on its concentration and molecular weight. Herein, we investigated the effects of various concentrations and molecular weights of PEG on lysozyme interactions through the analysis of protein rotational diffusion, which is susceptible to intermolecular interactions at short range. To this end, we measured fluorescence anisotropy of fluorescein-tagged lysozyme added as a tracer in concentrated native lysozyme solutions and introduced a protein concentration-dependent interaction parameter, krot. The results show the nonmonotonic changes in krot as the concentrations of PEG10000 and 6000 are increased. The depletion attractions are characterized by the decrease in krot, indicating an increase of a degree at which protein rotational diffusion slows down. The influences of temperature on the lysozyme rotational diffusion and krot were also measured, and the validity of this approach was checked through comparison with the colloidal theory.
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