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In biopharmaceutical development, sequence variants (SV) are considered an inherent risk of producing complex proteins in living systems. Sequence variants are unintended changes to the amino acid sequence of a biotherapeutic and can be caused by errors in transcription or translation in the host cell, or cell culture and process conditions. Detailed analysis of SVs is important in process and product development to ensure the drug’s safety and efficacy. Even low‑level sequence variants can have significant implications for product quality, safety, and efficacy, making their accurate detection and characterization a critical requirement across development, process optimization, and regulatory submission.
As protein therapeutics increase in complexity, so does the challenge of confirming primary structure with confidence. Mass spectrometry‑based peptide mapping remains the foundation of sequence confirmation workflows. However, traditional LC-MS approaches relying on collision‑induced dissociation (CID) alone can fall short when tasked with identifying challenging or low‑abundance variants.
This is where electron-activated dissociation (EAD) provides an additional angle in characterizing these challenging product quality attributes (PQAs).
The challenge with CID‑only workflows
LC-MS workflows centered on CID fragmentation are a standard approach for peptide mapping. While effective for many applications, there are limitations for some of the more challenging post-translational modifications. For sequence variant analysis:
· CID cannot generate signature fragments for unambiguous differentiation of isomeric amino acids such as leucine (Leu) and isoleucine (Ile)
· CID does not provide efficient fragmentation of long peptides that may contain the amino acid substitutions in sequence variants
· CID preferentially cleaves labile post-translation modifications (PTMs), such as glycosylation, leading to the loss of site-specific information about these critical modifications
These challenges are particularly relevant in biopharma, where mass-neutral substitutions may arise during cell culture or manufacturing and must be confidently assigned. When CID cannot provide clear differentiation, sequence ambiguity remains.
How EAD overcomes these limitations
EAD provides a complementaryand additive fragmentation mechanism that preserves critical structural information while generating informative backbone fragments. By using radical‑driven fragmentation, EAD enables a level of sequence insight that is not achievable with CID alone.
Confident differentiation of Leu and Ile
One of the powerful advantages of EAD is its ability to differentiate amino acid isomers, a persistent analytical challenge. EAD generates diagnostic fragments from isomers like aspartic acid (D) vs. isoaspartic acid (isoD) or isoleucine (Leu) vs. isoleucine (Ile), enabling unambiguous identification of these isomeric residues within peptides. This capability removes a major source of uncertainty in sequence variant analysis.1
Differentiation of Leu and Ile residues using EAD. EAD can lead to secondary fragmentation of the side chains of Leu and Ile residues in the z ions, producing signature w ions (z-29 and z-43) from the neutral loss of C3H7 (43 Da) for Leu and C2H5 (29 Da) for Ile. The detection of these diagnostic fragments enables unambiguous differentiation of Leu vs. Ile.
Improved detection of low‑abundance sequence variants
EAD increases confidence in identifying sequence variants with its signature fragments. The speed and sensitivity of the ZenoTOF 7600+ system and ZenoTOF 8600 system further support confident characterization by improving the detection of sequence variants present at low levels. By providing clear fragment ion evidence, EAD enables reliable assignment of substitutions that might otherwise be missed or misidentified in CID‑only workflows.
EAD spectrum of a low-abundant V→Xle (Xle=Lue or Ile) sequence variant (<0.05%) of the VVSV peptide in NISTmAb. The detection of a z13-29 (w13) ion in this EAD spectrum shows that the Xle residue is an Ile instead of a Leu.
High sequence coverage in routine peptide mapping
EAD integrates seamlessly into data‑dependent acquisition (DDA) peptide mapping workflows, delivering comprehensive sequence coverage in a single injection. This makes advanced sequence variant analysis practical for routine use without adding unnecessary complexity to established methods.
EAD on the ZenoTOF systems: LC-MS for biopharma workflows
EAD is available on the ZenoTOF 7600 system, ZenoTOF 7600+ system and ZenoTOF 8600 system, making it accessible for both routine characterization and advanced biopharma applications, with the ability to run both CID and EAD in a single injection.
· ZenoTOF 7600 system and ZenoTOF 7600+ system
Enable streamlined EAD‑based peptide mapping workflows with high sensitivity and reproducibility, supporting confident differentiation of amino acid isomers and identification of low‑level sequence variants without extensive method optimization.
· ZenoTOF 8600 system
Delivers enhanced MS sensitivity to support routine detection and characterization of trace‑level PQAs, further extending the power of EAD for advanced biotherapeutic characterization.
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