Fundamentals of Capillary Electrophoretic Migration and Separation of SDS Proteins in Borate Cross-Linked Dextran Gels

Recent progress in the development and production of new, innovative protein therapeutics require rapid and adjustable high-resolution bioseparation techniques. Sodium dodecyl sulfate capillary gel electrophoresis (SDS-CGE) using a borate (B) cross-linked dextran (D) separation matrix is widely employed today for rapid consistency analysis of therapeutic proteins in manufacturing and release testing. Transient borate cross-linking of the semirigid dextran polymer chains leads to a high-resolution separation gel for SDS–protein complexes. To understand the migration and separation basis of the D/B gel, the present work explores various gel formulations of dextran monomer (2, 5, 7.5, and 10%) and borate cross-linker (2 and 4%) concentrations. Ferguson plots were analyzed for a mixture of protein standards with molecular weights ranging from 20 to 225 kDa, and the resulting nonlinear concave curves pointed to nonclassical sieving behavior. While the 2% D/4% B gel resulted in the fastest analysis time, the 10% D/2% B gel was found to produce the greatest separation window, even higher than with the 10% D/4% B gel, due to a significant increase in the electroosmotic flow of the former composition in the direction opposite to SDS–protein complex migration. The study then focused on SDS-CGE separation of a therapeutic monoclonal antibody and its subunits. A combination of molecular weight and shape selectivity as well as, to a lesser extent, surface charge density differences (due to glycosylation on the heavy chain) influenced migration. Greater molecular weight selectivity occurred for the higher monomer concentration gels, while improved glycoselectivity was obtained using a more dilute gel, even as low as 2% D/2% B. This latter gel took advantage of the dextran–borate–glycoprotein complexation. The study revealed that by modulating the dextran (monomer) and borate (cross-linker) concentration ratios of the sieving matrix, one can optimize the separation for specific biopharmaceutical modalities with excellent column-to-column, run-to-run, and gel-to-gel migration time reproducibilities (<0.96% relative standard deviation (RSD)). The widely used 10% dextran/4% borate gel represents a good screening option, which can then be followed by a modified composition, optimized for a specific separation as necessary.