Modifying last layer in polyelectrolyte multilayer coatings for capillary electrophoresis of proteins

A simple and efficient approach for preparing cationic coating with tunable electroosmotic flow for capillary zone electrophoresis-mass spectrometry-based top-down proteomics

BACKGROUND

Capillary zone electrophoresis-tandem mass spectrometry (CZE-MS/MS) has become a valuable analytical technique in top-down proteomics (TDP). CZE-MS/MS-based TDP typically employs separation capillaries with neutral coatings (i.e., linear polyacrylamide, LPA). However, issues related to separation resolution and reproducibility remain with the LPA-coated capillaries due to the unavoidable non-specific protein adsorption onto the capillary wall. Cationic coatings can be critical alternatives to LPA coating for CZE-MS/MS-based TDP due to the electrostatic repulsion between the positively charged capillary inner wall and proteoform molecules in the acidic separation buffer. Unfortunately, there are only very few studies using cationic coating-based CZE-MS/MS for TDP studies.

RESULTS

In this work, we aimed to develop a simple and efficient approach for preparing separation capillaries with a cationic coating, i.e., poly (acrylamide-co-(3-acrylamidopropyl) trimethylammonium chloride [PAMAPTAC]) for CZE-MS/MS-based TDP. The PAMAPTAC coating-based CZE-MS produced significantly better separation resolution of proteoforms compared to the traditionally used LPA-coated approach. It achieved reproducible separation and measurement of a simple proteoform mixture and a complex proteome sample (i.e., a yeast cell lysate) regarding migration time, proteoform intensity, and the number of proteoform identifications. The PAMAPTAC coating-based CZE-MS enabled the detection of large proteoforms (≥30 kDa) from the yeast cell lysate reproducibly without any size-based prefractionation. Interestingly, the mobility of proteoforms using the PAMAPTAC coating can be predicted accurately using a simple semi-empirical model.

SIGNIFICANCE

The results render the PAMAPTAC coating as a valuable alternative to the LPA coating to advance CZE-MS-based TDP towards high-resolution separation and highly reproducible measurement of proteoforms in complex samples.

Dual Contributions of Analyte Adsorption and Electroosmotic Inhomogeneity to Separation Efficiency in Capillary Electrophoresis of Proteins

Improving separation efficiency in capillary electrophoresis (CE) requires systematic study of the influence of the electric field (or solute linear velocity) on plate height for a better understanding of the critical parameters controlling peak broadening. Even for poly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium styrenesulfonate) (PSS) successive multiple ionic-polymer layer (SMIL) coatings, which lead to efficient and reproducible separations of proteins, plate height increases with migration velocity, limiting the use of high electric fields in CE. Solute adsorption onto the capillary wall was generally considered as the main source of peak dispersion, explaining this plate height increase. However, experiments done with Taylor dispersion analysis and CE in the same conditions indicate that other phenomena may come into play. Protein adsorption with slow kinetics and few adsorption sites was established as a source of peak broadening for specific proteins. Surface charge inhomogeneity was also identified as a contribution to plate height due to local electroosmotic fluctuations. A model was proposed and applied to partial PDADMAC/poly(ethylene oxide) capillary coatings as well as PDADMAC/PSS SMIL coatings. Atomic force microscopy with topography and recognition imaging enabled the determination of roughness and charge distribution of the PDADMAC/PSS SMIL surface.

Investigating cationic and zwitterionic successive multiple ionic-polymer layer coatings for protein separation by capillary electrophoresis

Successive multiple ionic-polymer layers (SMILs) have long since proved their worth in capillary electrophoresis as they ensure stable electroosmotic flow (EOF) and relatively high separation efficiency. Recently, we demonstrated that plotting the plate height (H) against the solute migration velocity (u) enabled a reliable quantitative evaluation of the coating performances in terms of separation efficiency. In this work, various physicochemical and chemical parameters of the SMIL coating were studied and optimized in order to decrease the slope of the ascending part of the H vs u curve, which is known to be controlled by the homogeneity in charge of the coating surface and by the possible residual solute adsorption onto the coating surface. SMILs based on poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium styrene sulfonate) (PSS) were formed and the effect of each polyelectrolyte molar mass and of the number of polyelectrolyte layers (up to 21 layers) was studied. The use of polyethylene imine as an anchoring first layer was considered. More polyelectrolyte couples based on PDADMAC, polybrene, PSS, poly(vinyl sulfate), and poly(acrylic acid) were tested. Finally, zwitterionic polymers based on the poly(α-l-lysine) scaffold were synthesized and used as the last layer of SMILs, illustrating their ability to finetune the EOF, while maintaining good separation efficiency.

Size-based characterization of dendrigraft poly(L-lysine) by free solution capillary electrophoresis using polyelectrolyte multilayer coatings

Dendrigraft poly(L-lysine) (DGL) constitutes a promising dendritic-like drug vehicle with high biocompatibility and straightforward access via ring-opening polymerization of N-carboxyanhydride in water. The characterization of the different generations of DGL is however challenging due to their heterogeneity in molar mass and branching ratio. In this work, free solution capillary electrophoresis was used to perform selective separation of the three first generations of DGL, and optimized conditions were developed to maximize inter-generation resolution. To reduce solute adsorption on the capillary wall, successive multiple ionic polymer layer coatings terminated with a polycation were deposited onto the inner wall surface. PEGylated polycation was also used as the last layer for the control of the electroosmotic flow (EOF), depending on the PEGylation degree and the methyl-polyethylene glycol (mPEG) chain length. 1 kDa mPEG chains and low grafting densities were found to be the best experimental conditions for a fine tuning of the EOF leading to high peak resolution. Molar mass polydispersity and polydispersity in effective electrophoretic mobility were successfully determined for the three first generations of DGL.

Recent developments in capillary and microchip electroseparations of peptides (2021-mid-2023)

This review article brings a comprehensive survey of developments and applications of high-performance capillary and microchip electromigration methods (zone electrophoresis in a free solution or in sieving media, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography) for analysis, micropreparation, and physicochemical characterization of peptides in the period from 2021 up to ca. the middle of 2023. Progress in the study of electromigration properties of peptides and various aspects of their analysis, such as sample preparation, adsorption suppression, electroosmotic flow regulation, and detection, are presented. New developments in the particular capillary electromigration methods are demonstrated, and several types of their applications are reported. They cover qualitative and quantitative analysis of synthetic or isolated peptides and determination of peptides in complex biomatrices, peptide profiling of biofluids and tissues, and monitoring of chemical and enzymatic reactions and physicochemical changes of peptides. They include also amino acid and sequence analysis of peptides, peptide mapping of proteins, separation of stereoisomers of peptides, and their chiral analyses. In addition, micropreparative separations and physicochemical characterization of peptides and their interactions with other (bio)molecules by the above CE methods are described.