IEX-MALS - a method for protein separation and characterization

Importance of the Topic

Ion-exchange chromatography coupled to multi-angle light scattering (IEX-MALS) offers absolute molar mass measurements and structural insights for proteins, complexes and peptides in heterogeneous samples, overcoming size-based separation limits of standard SEC-MALS. This capability is critical for quality control, biotherapeutic characterization and analysis of charge variants and aggregates that co-elute in conventional methods.

Study Objectives and Overview

The white paper introduces IEX-MALS as an advanced technique to:

• Provide absolute molar mass and size measurements independent of column calibration.
• Resolve charge variants, modified proteins and mixed-protein samples inaccessible by SEC.
• Demonstrate reproducibility and accuracy using model proteins (BSA) and monoclonal antibodies (mAb1, mAb2).

Methodology and Instrumentation

Analyses were performed on an AKTA Pure FPLC connected to a miniDAWN MALS detector with embedded QELS (DLS) and an Optilab differential refractive index detector. UV absorbance at 280 nm provided concentration measurements in salt gradient elution; dRI was noted for broader applications. IEX separations used anion-exchange (MonoQ) and cation-exchange (SCX-NP5) analytical columns with tailored salt gradients (linear, sharp, stepwise) in buffers at defined pH and ionic strength. Data processing and molar mass calculations employed ASTRA software with a dn/dc of 0.185 mL/g.

Main Results and Discussion

• BSA reproducibility: Linear gradients (30 CV) separated monomer, dimer and fragments with consistent monomer mass (~67 kDa). Sharper gradients (15 CV) improved signal sensitivity for main peak analysis. Stepwise gradients (35% followed by linear) enhanced separation of fragments and oligomers.
• Monoclonal antibodies: SEC-MALS showed single homogeneous peaks for mAb1 and mAb2, masking charge variants. CIEX-MALS resolved acidic and basic variants with similar hydrodynamic radii but distinct elution profiles. mAb1 variants displayed slight molar mass differences indicative of glycosylation or sequence changes; mAb2 variants confirmed as true charge variants with identical mass.

Benefits and Practical Applications

IEX-MALS complements SEC-MALS by exploiting charge-based separation, enabling absolute molar mass and size characterization in quality assessment of biotherapeutics, analysis of native oligomers and detection of charge heterogeneity. A single IEX column accommodates a wide range of biomolecules (proteins, peptides, nucleic acids, membrane complexes) that may not be resolved by SEC.

Future Trends and Potential Applications

• Integration of pH and multi-step gradient profiles for tailored separations.
• Expansion to preparative IEX for real-time purification monitoring.
• Online coupling of MALS and DLS to diverse biomacromolecules including vesicles and virus-like particles.

Conclusion

IEX-MALS provides a powerful, complementary platform for absolute protein characterization, addressing limitations of size-exclusion methods. It is particularly valuable for resolving charge variants and heterogeneous samples in analytical and bioprocess development contexts.

References

1. Wyatt PJ. Light scattering and the absolute characterization of macromolecules. Anal Chim Acta. 1993;272:1–40.
2. Folta-Stogniew E, Williams KR. Determination of molecular masses of proteins in solution: Implementation of an HPLC size exclusion chromatography and laser light scattering service in a core laboratory. J Biomol Tech. 1999;10(1):51–63.
3. Amartely H, Avraham O, Friedler A, Livnah O, Lebendiker M. Coupling multiangle light scattering to ion exchange chromatography (IEX-MALS) for protein characterization. Sci Rep. 2018;8:6907.
4. Khawli LA, et al. Charge variants in IgG1. MAbs. 2010;2(6):613–624.
5. Du Y, et al. Chromatographic analysis of the acidic and basic species of recombinant monoclonal antibodies. MAbs. 2012;4(5):578–585.