Ultra-High-Resolution Separation of Oligonucleotides on Pellicular Anion-Exchange UHPLC Columns

Importance of the topic

The rapid and high-resolution separation of oligonucleotides is critical for the development, quality control, and regulatory compliance of therapeutic and diagnostic nucleic acids. Oligonucleotide drugs such as antisense, aptamers, and siRNA rely on precise sequence integrity and chemical modifications to enhance stability and efficacy. Analytical challenges arise from sequence length heterogeneity, incomplete syntheses, varied backbone and sugar modifications, and diastereomer formation. Effective chromatographic methods are therefore crucial to ensure purity, identify failure products, and characterize structural variants.

Objectives and overview

This study demonstrates the selectivity and resolving power of the Thermo Scientific DNAPac PA200 RS pellicular anion-exchange UHPLC column for a range of oligonucleotide biopharmaceuticals and related derivatives. Key goals include separation of homopolymer failure sequences, resolution of positional 2 ,5 linkages in a mixed-base 21-mer, and discrimination of phosphorothioate diastereoisomers.

Methodology and Instrumentation

• Column chemistry: DNAPac PA200 RS, 4 µm pellicular strong anion-exchange on nonporous polymer particles (4.6×150 mm and 4.6×250 mm); comparison with DNAPac PA200, 8 µm (4.0×250 mm).
• Mobile phases: Buffer A (40 mM Tris pH 8.0 or 40 mM 2-amino-2-methyl-1-propanol pH 9.5); Buffer B (Buffer A plus 1.0–1.25 M NaCl).
• Gradient: Linear and concave salt gradients tailored to improve resolution of longer oligonucleotides.
• UHPLC system: Thermo Scientific UltiMate Dionex 3000 RSLC with SRD degasser, DGP-3600RS pump, TCC-3000RS column compartment, WPS-3000 autosampler, DAD3000RS detector (260 nm), Chromeleon 6.8 data system.
• Sample set: Deoxythymidine homopolymers (12–60 nt), mixed-base eGFP antisense 21-mer variants with defined 2 ,5 linkages, and eGFP sense strands with two phosphorothioate linkages producing Rp and Sp diastereomers.

Main results and discussion

• Homopolymer separation: A 4.6×150 mm RS column resolved 49 full-length deoxythymidine oligonucleotides and 26 failure/modified products (incomplete deprotection or synthesis errors) in under 10 minutes using a concave 59→35→20 % Buffer A gradient at 1.0 mL/min, 30 °C.
• Positional 2 ,5 linkage resolution: Six 21-mer antisense strands with identical length and sequence but single aberrant 2 ,5 linkages at positions 1, 5, 10, 15, or 20 were baseline separated on a 4.6×250 mm RS column within ten minutes, indicating sensitivity to subtle conformational differences.
• Phosphorothioate diastereoisomers: A mixed-linkage eGFP sense strand harboring two PS bonds at positions 6 and 14 produced four peaks corresponding to Rp/Sp pairs. Comparison between the 8 µm (4.0×250 mm) and 4 µm RS (4.6×150 mm) columns showed a 28–42% improvement in resolution on the RS phase despite 60% of the column length.

Benefits and practical applications

This pellicular anion-exchange UHPLC method delivers:

• High throughput analysis of oligonucleotide quality and purity, enabling rapid QC in therapeutic oligonucleotide production.
• Resolution of closely related failure sequences and minor chemical variants without MS detection.
• Discrimination of structural isomers and diastereomers critical for in vivo stability and activity.
• Short run times and robust performance suitable for routine laboratory workflows.

Future trends and opportunities

• Integration with high-resolution mass spectrometry to combine separation and mass confirmation for complete characterization.
• Development of tailored pellicular phases for emerging oligonucleotide chemistries such as locked nucleic acids and morpholinos.
• Advanced gradient optimization and software-driven method scouting to further enhance resolution of challenging sequences.
• Automated high-throughput platforms for comprehensive profiling of oligonucleotide libraries in drug discovery.

Conclusion

The Thermo Scientific DNAPac PA200 RS pellicular anion-exchange UHPLC column offers industry-leading resolution and speed for the analysis of a broad spectrum of therapeutic and diagnostic oligonucleotides. Its ability to resolve homopolymer failure products, positional backbone variants, and phosphorothioate diastereoisomers in short gradients makes it a powerful tool for research, development, and quality control of next-generation nucleic acid drugs.

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