Exploration and Optimization of Efficient Separation Conditions for Oligonucleotides by Reversed-Phase Ion-Pair Chromatography

Importance of the Topic

Oligonucleotide-based therapeutics such as antisense oligonucleotides demand high-resolution separation and purification to ensure safety and efficacy. Reversed-phase ion-pair chromatography (RP-IP) remains a cornerstone technique for profiling chain-length variants and impurities in synthetic nucleic acids, critical for quality control in pharmaceutical development.

Objectives and Study Overview

This study explored efficient chromatographic conditions for separating mixtures of unmodified single-stranded DNA oligonucleotides (dT(6), dT(10), dT(15)–dT(20), dT(25), dT(30), dT(40)) using RP-IP. The goals were to screen mobile phase compositions, optimize pH and temperature, and automate gradient design via AI to achieve a minimum resolution of 1.5 between adjacent chain-length species.

Methodology

The workflow comprised three phases:

• Initial mobile phase screening: Evaluation of ion-pair reagents triethylamine (TEA), dibutylamine (DBA), and hexylamine (HA) at various concentrations in acetic acid–buffered aqueous solutions, combined with acetonitrile or methanol as organic modifiers.
• pH and temperature optimization: Systematic variation of mobile phase pH (6.0–7.5) and column oven temperature (40–80 °C) under fixed gradient conditions to map regions achieving target resolution.
• Automatic gradient optimization: Application of LabSolutions MD’s AI algorithm to iteratively refine gradient profiles for two selected mobile phase conditions (50 mmol/L DBA pH 7.5 and 50 mmol/L HA pH 7.5), targeting resolution ≥ 1.5.

Used Instrumentation

The analysis was carried out on a Nexera XS inert UHPLC system equipped with:

• Shim-pack Scepter Claris C18-120 column (100 mm × 2.1 mm I.D., 3 µm)
• UV detection at 260 nm (SPD-M40, inert cell)
• Mobile phase A: Aqueous ion-pair reagent solutions (triethylamine, dibutylamine or hexylamine in acetic acid, pH adjusted)
• Mobile phase B: Acetonitrile or methanol
• Flow rates: 1.0 mL/min (screening) and 0.35 mL/min (pH/temperature optimization)
• Injection volume: 5 µL; column temperature varied between 40–80 °C

Main Results and Discussion

Screening revealed that TEA produced overlapping peaks across tested conditions, whereas DBA and HA improved separation with increasing concentration. Optimal screening conditions employed 50 mmol/L DBA or HA with either acetonitrile or methanol. Design-space plots showed that higher column temperatures consistently enhanced resolution, and optimal mobile phase pH shifted depending on organic solvent. AI-driven gradient optimization yielded tailored profiles for DBA and HA systems that achieved the resolution target within hours, with clear separation of eleven chain-length variants.

Benefits and Practical Applications

• Significant reduction in manual trial-and-error during method development.
• Automated AI-guided gradient design ensures consistent, high-resolution separations.
• Applicable for impurity profiling and quality control of synthetic oligonucleotides in pharmaceutical and research laboratories.

Future Trends and Applications

Advancements may include integration of multi-dimensional separations, expansion to modified and complex nucleic acid constructs, and coupling AI tools with mass spectrometry for enhanced identification. Wider adoption of automated method development platforms can accelerate analytical workflows in oligonucleotide therapeutics and other biopolymer analyses.

Conclusion

This work demonstrates the power of combining systematic RP-IP screening with AI-driven gradient optimization to rapidly develop robust separation methods for oligonucleotide mixtures. The approach delivers reproducible high-resolution chromatograms, streamlines method development, and supports stringent quality requirements in nucleic acid drug analysis.