Comprehending COVID-19: Reversed-Phase Liquid Chromatography (RPLC) of Intact SARS-CoV-2 Spike Protein

Significance of the Topic

The SARS-CoV-2 spike protein is central to virus entry and represents a key vaccine target. Characterizing its intact form provides crucial insights into structural heterogeneity and proteoform distribution, informing therapeutic design and quality control.

Objectives and Study Overview

This study evaluates the effect of using difluoroacetic acid (DFA) versus formic acid (FA) as mobile phase modifiers in reversed-phase liquid chromatography (RPLC) of the intact SARS-CoV-2 spike protein. The goal is to enhance chromatographic resolution and maintain compatibility with subsequent mass spectrometric analysis.

Methodology

A gradient RPLC method was applied using 0.1% DFA or FA in water (mobile phase A) and acetonitrile (mobile phase B). The gradient ranged from 15% to 55% B over 20 minutes at a flow rate of 0.2 mL/min. Fluorescence detection (280 nm emission, 320 nm excitation) monitored protein elution, while mass spectra were recorded from m/z 1500 to 4000 in positive electrospray ionization mode.

Used Instrumentation

• Liquid Chromatograph: ACQUITY UPLC I-Class system
• Column: BioResolve RP mAb Polyphenyl, 2.7 µm, 450 Å, 2.1 × 50 mm at 80 °C
• Detector: FLR (280 nm emission / 320 nm excitation)
• Mass Spectrometer: Vion IMS QToF with ESI+ (capillary voltage 2.25 kV; cone voltage 140 V; collision energy 6 V)

Main Results and Discussion

Substituting FA with DFA increased gradient peak capacity more than three-fold and revealed better-resolved peaks for low-abundance spike protein proteoforms in both fluorescence and total ion chromatograms. Enhanced chromatographic resolution with DFA supports deeper intact proteoform profiling, particularly when combined with N- and O-glycosidase treatments.

Benefits and Practical Applications

• Improved separation of minor proteoforms in intact protein analysis
• Increased gradient peak capacity for higher resolution
• Retention of full MS compatibility for downstream characterization

Future Trends and Applications

Coupling DFA-based RPLC with targeted glycosidase digestion and advanced mass spectrometric techniques may enable comprehensive intact spike protein mapping. Scaling this approach for high-throughput workflows and extending it to other viral glycoproteins can accelerate vaccine and therapeutic development.

Conclusion

The use of difluoroacetic acid as a mobile phase modifier in intact RPLC of the SARS-CoV-2 spike protein significantly enhances chromatographic resolution while preserving MS compatibility, thus supporting robust proteoform characterization essential for COVID-19 therapeutic research.

References

• Pinto D. et al. Structural and Functional Analysis of a Potent Sarbecovirus Neutralizing Antibody. bioRxiv 2020.
• Stawiski E.W. et al. Human ACE2 Receptor Polymorphisms Predict SARS-CoV-2 Susceptibility. bioRxiv 2020.
• Liu X., Lauber M. Rapid and Sensitive Characterization of N-Glycans from SARS-CoV-2 Spike Protein. Waters Application Highlight.
• Novokmet M. et al. Understanding Glycans in COVID-19 Drug Design. GenEngNews.