SEQUENCE CONFIRMATION OF OLIGONUCLEOTIDES VIA AUTOMATED TOP-DOWN SPECTRAL ANNOTATION

Significance of the Topic

Top-down mass spectrometry for oligonucleotide sequencing is critical in pharmaceutical development and regulatory compliance. Automated workflows that confirm full-length sequences of modified and unmodified oligonucleotides accelerate therapeutic discovery, ensure product quality, and meet stringent characterization requirements.

Objectives and Overview

This study introduces three complementary analytical workflows—full MS, data-dependent MS/MS, and data-independent MSE—to generate top-down spectra of a mixture of oligonucleotides. A software tool called Spectrum automates annotation of fragment ions, calculates sequence coverage, and reports mass accuracy.

Methodology and Instrumentation

Oligonucleotide Standards and Sample Preparation:

• 20-mer RNA and 20-mer DNA with defined backbone and nucleotide modifications.
• Reconstituted in RNase-free water and mixed at 10 µM total concentration.

Chromatography and Mass Spectrometry:

• UPLC: Waters ACQUITY I-Class system, Oligonucleotide BEH C18 column (2.1 × 50 mm, 1.7 µm) at 60 °C, 200 µL/min.
• Mobile phases: Solvent A – 10 mM TEA/50 mM HFIP in water; Solvent B – 5 mM TEA/25 mM HFIP in 50% methanol.
• Gradient to separate three oligos over 15 minutes.
• MS: Waters Xevo G2 XS QTof, negative ESI, alternating low/high energy acquisition, mass range m/z 300–3000.

Main Results and Discussion

Sequence Coverage and Mass Accuracy:

• High charge states (−8, −9) delivered up to 90% sequence coverage at collision energies 20–55 V.
• Data-dependent MS/MS and MSE achieved comparable coverage for unmodified RNA and modified DNA; MS/MS slightly outperformed for modified RNA.
• Mass errors consistently below 8 ppm across workflows.

Automated Annotation:

• Spectrum tool matched observed fragment ions to theoretical lists for both 5′ and 3′ cleavages.
• Rapid report generation including percent coverage, observed vs. theoretical mass, and mass error.

Benefits and Practical Applications

• Streamlined verification of oligonucleotide therapeutics and research molecules.
• Automated spectral annotation reduces manual interpretation time.
• Supports both backbone and nucleobase modifications for quality control and method development.

Future Trends and Applications

• Integration of machine learning to predict optimal collision energies and charge states.
• Expansion of software libraries to cover novel modified nucleotides.
• High-throughput implementations for large oligonucleotide libraries.
• Real-time data processing for online QC in manufacturing.

Conclusion

The combination of top-down MS workflows with automated annotation via Spectrum enables fast, reliable sequence confirmation of complex oligonucleotides. These methods offer high coverage, accurate mass measurement, and scalable throughput to support research and regulatory needs.

Instrumentation

• Waters ACQUITY UPLC I-Class system
• Waters Xevo G2 XS QTof mass spectrometer
• Oligonucleotide BEH C18 column (2.1 × 50 mm, 1.7 µm)
• MassLynx software and Spectrum data analysis tool