Capillary sodium dodecyl sulfate gel electrophoresis of proteins: Introducing the three dimensional Ferguson method

One of the most extensively utilized rapid characterization, release and stability testing methods of therapeutic proteins in the biopharmaceutical field today is capillary SDS gel electrophoresis using borate cross-linked high molecular weight dextran. In spite of its widespread use, however, the gel composition dependent separation characteristics of this very unique sieving matrix has not been investigated yet. Introduction of three dimensional (3D) Ferguson plots, based on simultaneous variation of the dextran (D) and borate (B) concentrations generating 16 different D/B ratio gels, allowed better understanding of the electromigration process of the SDS-protein complexes. As a result of this comprehensive study, non-linear 3D logarithmic mobility vs dextran and borate concentration surfaces were obtained. Both, the molecular weight protein standards and the new modality fusion protein etanercept resulted in concave 3D Ferguson plots. The interplay between the electroosmotic flow and the viscosity of the matrices played a key role in the resulting migration time and resolution. Selectivity values were defined and evaluated in 3D graph formats for the regular and de-N-glycosylated subunits of etanercept, as well as for the latter with the 10 kDa internal standard to understand both the dextran-borate complexation and sized based selectivities. K_R plots of the retardation coefficients as the function of the logarithmic molecular weights were used to more precisely assess the Mw of the samples and to obtain information about the molecular characteristics of the electromigrating SDS-protein complexes.