Capillary zone electrophoresis of proteins in semidilute polymer solutions: Inter- and intra-polymer predictability of size-dependent retardation

The retardation of three "spherical" proteins with Stokes' radii of 2.0, 2. 4, and 3.0 nm (35-104 kDa) was studied in capillary zone electrophoresis (C ZE), using semidilute solutions of polyethylene glycol (PEG), linear polyac rylamide (PA), and polyvinyl alcohol (PVA). The purpose was to test the mod els predicting that the ratio of particle radius, R, to the mesh size of po lymer network (the correlation or screening length of a semidilute polymer solution), xi, directly governs the size-dependent retardation in the form: mu/mu(o) = exp (-R/xi). Here xi = kc(0.75), where c is polymer concentrati on and the numerical factor k can be calculated based on polymer molecular weight. In application to polymers in a "good solvent" (PA and PEG in the a queous buffer) and to proteins of 2.4 and 3.0 nm radius, that relation betw een relative mobility and R/xi was found to be obeyed for PA, while for PEG the value of k derived from retardation experiments significantly exceeded that which was theoretically calculated. Thus, the retardation appears to be polymer-specific, rather than universal, even for polymers in a "good so lvent". It is suggested that, in that case, retardation of proteins of R > 2 nm be quantitatively described in the form mu/mu(o) = exp[-p(R/xi], where p is a parameter depending on monomer type and/or polymer polydispersity. For PVA, the logarithm of mu/mu(o) was found to be linearly related to c (i n line with the prediction that the aqueous buffer is a "poor solvent" for this polymer) and to be near-independent of R.