Polyethyleneglycol–pepsin interaction and its relationship with protein partitioning in aqueous two-phase systems

The interaction between the acidic protein, pepsin, and the non-charged polyethyleneglycol polymer was studied by dynamic light scattering, fluorescence spectroscopy and measurements of the protein thermal stability at neutral pH. Polyethyleneglycol of average molecular mass 1450 showed a higher interaction capacity with the protein than polyethyleneglycol of average molecular mass 8000. Polyethyleneglycol of average molecular mass 1450 showed a molecular mechanism where the interpolymer interaction led to the complex formation. This fact can be explained taking into account that the extended form on this polymer molecule favours the interaction with the protein, which is highly dependent of the polymer total concentration. Polyethyleneglycol of average molecular mass 8000 showed a cooperative interaction between the polymer and protein molecules which was independent of the PEG concentration.