Characterizing the Poly(A) tail in mRNA using Ion-Pairing Reversed-Phase LC/MS (IPRP-LC/MS) and Deconvolution Tools

Significance of the topic

The poly(A) tail is a critical structural element of eukaryotic messenger RNA (mRNA) that stabilizes transcripts and modulates translation efficiency. Accurate characterization of poly(A) tail length distributions is important for biopharmaceutical development, quality control of mRNA therapeutics and vaccines, and for understanding biological stability and potency of mRNA constructs. Robust analytical workflows that combine enzymatic trimming with liquid chromatography–mass spectrometry (LC/MS) provide sequence-resolved, length-distribution data that complement sequencing and orthogonal biochemical assays.

Aims and study overview

This work demonstrates an ion-pairing reversed-phase LC/MS (IPRP-LC/MS) workflow to measure poly(A) tail length distributions after targeted enzymatic digestion. The primary objectives were to: (1) produce defined cleavage products of an mRNA to isolate the poly(A) fragment, (2) optimize LC/MS conditions to maximize sensitivity and minimize metal adduction, and (3) apply spectral deconvolution to report the distribution of polyadenylation lengths for a firefly luciferase (FLuc) mRNA sample.

Methodology

The experimental workflow consisted of enzymatic digestion, chromatographic separation under ion-pairing conditions, and high-resolution mass detection with subsequent deconvolution. Key procedural points include:

• Enzymatic digestion: FLuc mRNA (from TriLink) was denatured at 90 °C then cooled; RNase 4 was used to digest the mRNA (37 °C, 45 min). RNase 4 was chosen for its preferential cleavage after uridine followed by purine, producing fragments favorable for isolating the poly(A) tail.
• Sample handling and quenching: Digests were quenched with a murine quencher and transferred to polypropylene LC/MS vials.
• Mobile phases and conditioning: Ion-pair reagents and HFIP-containing mobile phases were prepared fresh and sonicated each day. Plastic bottles and dedicated caps were used to reduce metal contamination. The LC system was conditioned with ion-pairing reagent for ~4 hours prior to analysis.
• Calibration and system suitability: A mixed RNA oligonucleotide standard (14-, 17-, 20-, 21-mers) was used to assess sodium adduction, retention time repeatability, and extracted-ion chromatogram (EIC) reproducibility before analyzing mRNA digests.

Instrumentation used

The analysis used Agilent instrumentation and software: an Agilent 1290 Infinity III Bio LC coupled to an Agilent 6230C LC/TOF operated in negative ESI mode. Deconvolution and data reporting utilized Agilent MassHunter BioConfirm. Key LC and MS parameters (selected highlights) included:

• Column: Altura Oligo HPH-C18, 2.7 µm, 2.1 × 150 mm.
• Mobile phases: 10 mM hexylamine (HA) with 50 mM HFIP in aqueous/organic mixtures; a high‑MeOH organic solvent and acetonitrile for reverse flushing were employed.
• On-column load: 5.5 µg (12 µL of 0.45 µg/µL) per injection; flow rate 0.25 mL/min; column temperature 60 °C.
• MS source: Agilent Jet Stream ESI in negative polarity; gas temperature 325 °C, sheath gas 275 °C; capillary 4500 V; fragmentor 250 V; scan range m/z 400–3200.
• Data processing: Time-domain spectra were deconvoluted to obtain neutral mass distributions of poly(A) fragments.

Results and discussion

The LC/MS workflow successfully detected and deconvoluted the poly(A) tail from RNase 4–digested FLuc mRNA. Principal findings were:

• Poly(A) length distribution: Deconvoluted masses corresponded to poly(A) tails of approximately 119–128 adenosine residues for the analyzed FLuc mRNA batch, indicating a relatively narrow distribution centered around ~122–125 A.
• Raw spectral features: The raw mass spectra showed a rise in signal beginning around m/z 800 consistent with the highly charged poly(A) fragments; these spectra provided the input for deconvolution to neutral mass space.
• Ion-pair reagent comparison and metal adduction: Using the RNA oligonucleotide standard, sodium (Na+) adduction was substantially lower with hexylamine (HA) + HFIP (1.44–3.78% Na+ adduction) compared with DIPEA + HFIP (10.4–13.1%). Signal intensity differences were noted (DIPEA produced higher intensity for some oligos), but HA provided a clear benefit for reduced metal adducts and cleaner spectral deconvolution.
• Reproducibility and system suitability: Retention time repeatability was excellent (RT relative standard deviation <0.2% across replicates) and EIC peak height RSDs for the RNA standard components were below 2%, supporting method robustness prior to mRNA digest analysis.
• Practical operational details: Metal-adduct mitigation included use of plastic mobile-phase bottles, GL45 adapter, and dedicated caps; extended ion-pair conditioning of the LC was necessary to stabilize MS response to oligonucleotides.

Benefits and practical applications of the method

The presented IPRP-LC/MS and deconvolution approach provides several advantages for mRNA analytical characterization:

• Direct mass-resolved measurement of poly(A) tail length distributions without requiring sequencing of the full transcript.
• High reproducibility and low metal adduction when using hexylamine/HFIP ion-pair conditions and careful system conditioning, enabling reliable deconvolution of large oligonucleotide fragments.
• Compatibility with standard bio‑LC and TOF instrumentation and established data analysis tools (MassHunter BioConfirm), facilitating adoption in analytical and QC laboratories for mRNA therapeutics.

Future trends and potential applications

Potential developments and broader uses of this workflow include:

• Integration with orthogonal characterization techniques (e.g., nanopore or short-read sequencing of poly(A) junctions) to cross-validate tail-length distributions and transcript isoforms.
• Automation and higher throughput sample preparation to support routine QC for mRNA manufacturing, lot release testing, and stability studies.
• Improvements in ion-pair chemistries or alternative adduct-suppression strategies to further lower metal adduction while maximizing sensitivity across a broader mass range of RNA fragments.
• Application to modified nucleotides and complex mRNA constructs, evaluating how base modifications or secondary structure influence RNase cleavage patterns and tail-length readouts.

Conclusions

The study demonstrates that targeted RNase 4 digestion combined with an ion-pairing reversed-phase LC/TOF workflow can measure poly(A) tail length distributions of mRNA with good reproducibility and limited metal adduction when using hexylamine/HFIP mobile phases. The method resolves a narrow polyadenylation range (119–128 A) for the tested FLuc mRNA and is supported by system-suitability testing using oligonucleotide standards. These attributes make the approach a practical option for analytical characterization and QC of mRNA-based therapeutics.

References

1. D’Ascenzo L, Popova AM, Abernathy S, Sheng S, Limbach PA, Williamson JR. Pytheas: a software package for the automated analysis of RNA sequences and modifications via tandem mass spectrometry. Nat Commun. 2022;13:2424. doi:10.1038/s41467-022-30057-5.
2. Gau BC, Dawdy AW, Wang HL, et al. Oligonucleotide mapping via mass spectrometry to enable comprehensive primary structure characterization of an mRNA vaccine against SARS-CoV-2. Sci Rep. 2023;13:9038. doi:10.1038/s41598-023-36193-2.