In Migratio Noncovalent Fluorophore Labeling of Proteins by Propidium Iodide in Sodium Dodecyl Sulfate Capillary Gel Electrophoresis

Abstract

Sodium dodecyl sulfate capillary gel electrophoresis is one of the frequently used methods for size-based protein separation in molecular biology laboratories and the biopharmaceutical industry. To increase throughput, quite a few multicapillary electrophoresis systems have been recently developed, but most of them only support fluorescence detection, requiring fluorophore labeling of the sample proteins. To avoid the time-consuming derivatization reaction, we developed an on-column labeling approach utilizing propidium iodide for the first time in SDS-CGE of proteins, a dye only used before for nucleic acid analysis. As a key ingredient of the gel–buffer system, the oppositely migrating positively charged propidium ligand in migratio complexes with the SDS-proteins, therefore, supports in situ labeling during the electrophoretic separation process, not requiring any extra pre- or postcolumn derivatization step. A theoretical treatment is given to shed light on the basic principles of this novel online labeling process, also addressing the influence of propidium iodide on the electroosmotic flow, resulting in reduced retardation. The concept of propidium labeling in SDS-CGE was first demonstrated using a commercially available protein sizing ladder ranging from 6.5 to 200 kDa with different isoelectric points and post-translational modifications. Considering the increasing number of protein therapeutics on the market next, we focused on the labeling optimization of a therapeutic monoclonal antibody and its subunits, including the addition of the nonglycosylated heavy chain. Peak efficiency and resolution were compared between noncovalent and covalent labeling. The effect of ligand concentration on the effective and apparent electrophoretic mobility, the resulting peak area, and the resolution were all evaluated in view of the theoretical considerations. The best detection sensitivity for the intact monoclonal antibody was obtained by using 200 μg/mL propidium iodide in the separation medium (LOD 2 μg/mL, 1.35 × 10^–8 M) with excellent detection linearity over 3 orders of magnitude. On the other hand, the resolution between the biopharmaceutical protein test mixture components containing the intact and subunit fragments of the therapeutic monoclonal antibody was very good in the ligand concentration range of 50–200 μg/mL, but using the local maximum at 100 μg/mL for the nonglycosylated/glycosylated heavy chain pair is recommended. The figures of merit, including precision, sensitivity, detection linear range, and resolution for a sample mixture in hand, can be optimized by varying the propidium iodide concentration in the gel–buffer system, as demonstrated in this paper.