Mass spectrometry-based structural analysis of oligonucleotide DNA duplexes

Significance of the Topic

Oligonucleotide duplexes are central to modern therapeutic and diagnostic applications, including gene editing, RNA interference and antisense approaches. Detailed structural characterization of these duplexes in their native-like states is crucial to predict stability, binding interactions and functionality. Ion mobility–mass spectrometry (IM-MS) coupled with non-denaturing separations offers a rapid route to assess conformational landscapes and confirm duplex integrity without extensive sample preparation.

Aims and Study Overview

This work aimed to demonstrate a proof-of-principle workflow for the gas-phase structural analysis of synthetic DNA duplexes of varying lengths (24 to 72 base pairs). Two preparation protocols—simple mixing of single strands versus a controlled annealing procedure—were compared to determine their impact on duplex formation and compactness in the gas phase. Key performance metrics included collision cross sections (CCS), charge state distributions and chromatographic purity.

Methodology

Samples of DNA sense and antisense strands were prepared at 200 pmol/µL in a non-denaturing buffer (10 mM Tris-HCl, 50 mM NaCl, 1 mM EDTA, pH 7.5). Duplexes were generated either by immediate mixing or by a gradual annealing protocol (95 °C to room temperature over 1 hour). After dilution to 100 pmol/µL, samples were analyzed by size-exclusion chromatography (SEC) under native conditions, followed by electrospray ionization into a cyclic ion mobility–time-of-flight mass spectrometer.

Used Instrumentation

• ACQUITY Premier UPLC system with TUV detector (260 nm)
• BEH SEC 200 Å, 4.6 × 300 mm, 1.7 µm column
• SELECT SERIES Cyclic IMS mass spectrometer in ESI+ Mobility TOF mode
• QuanRecovery vials and nanoESI source for CCS calibration

Results and Discussion

Non-denaturing SEC chromatograms showed high duplex purity under both preparation methods, with minor residual single-strand peaks. Mass spectra confirmed narrow charge distributions increasing with duplex length and no observable gas-phase dissociation. Ion mobility arrival time distributions revealed that annealed samples consistently favored more compact conformations, reflected in smaller CCS values. For example, ds24mer duplexes exhibited predominant CCS around 1400–1700 Å2 for low charge states, while ds72mer duplexes ranged from 5500 to 6000 Å2. Mixed samples displayed broader conformer populations with a shift toward higher CCS.

Benefits and Practical Applications

Combining native SEC with IM-MS provides:

• Rapid assessment of duplex formation and purity without denaturants
• Quantitative CCS measurements to infer structural compactness
• Minimal sample consumption and high sensitivity for low-pmol quantities
• A workflow compatible with routine QA/QC in oligonucleotide production

Future Trends and Possibilities

Advancements may include high-resolution cyclic IMS separations to resolve subtle structural isomers, integration of on-line fragmentation for site-specific mapping, and extension to modified nucleic acids (locked nucleic acids, peptide conjugates). Machine learning–guided CCS prediction could further streamline the interpretation of complex mobility data.

Conclusion

This study validates an SEC-IM-MS platform for probing the gas-phase structures of synthetic DNA duplexes. The annealing protocol yields more compact assemblies, closely reflecting solution-phase conformations. The approach offers a practical tool for structural quality control in oligonucleotide research and manufacturing.

Reference

No external references were provided in the original document.