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 (CZE), using semidilute solutions of polyethylene glycol (PEG), linear polyacrylamide (PA), and polyvinyl alcohol (PVA). The purpose was to test the models predicting that the ratio of particle radius, R, to the mesh size of polymer network (the correlation or screening length of a semidilute polymer solution), xi, directly governs the size-dependent retardation in the form: mu/muo = exp (-R/xi). Here xi = kc-0.75, where c is polymer concentration and the numerical factor kcan be calculated based on polymer molecular weight. In application to polymers in a "good solvent" (PA and PEG in the aqueous buffer) and to proteins of 2.4 and 3.0 nm radius, that relation between 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 solvent". It is suggested that, in that case, retardation of proteins of R > 2 nm be quantitatively described in the form mu/muo = exp[-p(R/xi], where p is a parameter depending on monomer type and/or polymer polydispersity. For PVA, the logarithm of mu/muo was found to be linearly related to c (in line with the prediction that the aqueous buffer is a "poor solvent" for this polymer) and to be near-independent of R.