Depletion interactions and protein crystallization

Depletion interactions induced by the addition of soluble, non-adsorbing po lymer significantly influence the phase behavior of protein solutions. Here we investigate the influence of polymer on the strength of interactions, p hase behavior and nucleation kinetics of protein solutions. The strength of protein interactions induced by the presence of polymer is characterized t hrough protein solution second virial coefficient measurements, Working wit h lysozyme and poly (ethylene glycol) (PEG), we find that the second virial coefficient, B-2 is a non-monotonic function of polymer concentration at m odest ionic strengths (where the protein molecules behave essentially as ha rd spheres). At high ionic strengths, B-2 and solubility are independent of polymer concentration. The effect of polymer on protein solution phase beh avior is studied by carrying out equilibrium or solubility measurements (fl uid-crystal boundary) and cloud point measurements (liquid-liquid boundary) . A scintillation cell setup is used to determine nucleation induction time s with and without the presence of polymer. We present a detailed experimen tal investigation of the influence of PEG on protein pair interactions, pha se behavior and nucleation kinetics. (C) 2001 Elsevier Science B.V. All rig hts reserved.