Oxygen Attachment Dissociation for the Structural Characterisation of Lipids Involved in Pancreatic Cancer

Importance of the topic

The structural characterization of lipids at double-bond positional resolution is critical for understanding lipid metabolism alterations in disease and for reliable biomarker discovery. Conventional CID-MS/MS often identifies lipid class, acyl chain lengths and degrees of unsaturation but cannot unambiguously assign C=C positions. Oxygen Attachment Dissociation (OAD) is a radical-induced MS/MS technique that selectively cleaves C=C bonds, enabling positional annotation of double bonds and therefore a higher structural level of lipid identification. Applied to lipids implicated in pancreatic ductal adenocarcinoma (PDAC), OAD provides improved confidence in identifying disease-associated changes in the lipidome.

Objectives and overview of the study

The study aimed to apply OAD-MS/MS on a Shimadzu LCMS-9050 QTOF to structurally characterize unsaturated lipids previously flagged as potential PDAC serum biomarkers by untargeted metabolomics. Key goals were to assign C=C positions in those lipids, compare results with conventional CID fragmentation, and demonstrate simultaneous acquisition of OAD-MS/MS and CID-MS/MS in both positive and negative electrospray ionisation (ESI) modes for improved identification confidence.

Materials and methods

The work analysed human serum extracts from 30 PDAC patients and 30 matched healthy controls (pooled QC samples for structural work). Lipids shown to be significantly reduced in PDAC profiles by prior untargeted UHPLC-CID-MS/MS were re-analysed with simultaneous OAD/CID DDA-MS/MS. Data processing used LabSolutions Insight Explore and MS-DIAL (with an OAD spectral database) to assign double-bond positions using omega and delta nomenclature.

• Chromatography: Nexera X2 UHPLC, C18 column (2.1 × 100 mm, 1.7 μm), 50 °C; flow 0.4 mL/min; gradient from 2% to 100% B (A: water + 0.1% FA; B: ACN + 0.1% FA); 1 μL injection (ESI+) and 2 μL (ESI−); autosampler at 4 °C.
• Radical source: OAD Radical Source I generating atomic oxygen (O) and hydroxyl radicals (OH•) from microwave discharge of water vapor; radicals introduced to the collision cell.
• Mass spectrometry: LCMS-9050 QTOF; ESI interface 300 °C, 4.0 kV (positive) / −3.0 kV (negative); heat block 400 °C; DL 250 °C; gas flows: nebulizing 3 L/min, heating 10 L/min, drying 15 L/min; collision gas pressure 17 kPa.
• Acquisition: TOF-MS m/z 60–1250; DDA-MS/MS with simultaneous OAD and CID: 5 DDA events (ESI+) or 3 DDA events (ESI−), precursor isolation 0.8 Da (ESI+) / 3 Da (ESI−), CE spread 6–30 V, 1 s cycle, MS/MS m/z 40–1250.

Used instrumentation

• UHPLC: Shimadzu Nexera X2 with C18 column (2.1 × 100 mm, 1.7 μm).
• Mass spectrometer: Shimadzu LCMS-9050 Quadrupole Time-of-Flight with OAD Radical Source I (attachable radical source producing O and OH• from water vapor).
• Software: LabSolutions Insight Explore and MS-DIAL (with OAD spectral support) for automated processing and matching.

Main results and discussion

The simultaneous acquisition of OAD-MS/MS and CID-MS/MS enabled confident structural annotation of multiple unsaturated lipid species that were found to be decreased in PDAC serum relative to healthy controls. Key findings:

• OAD fragmentation produces double-bond-specific neutral loss fragment pairs that map to specific C=C positions. Interpretation of these fragment pairs allowed assignment of double bonds in omega (n‑) and delta notation.
• For LPC 20:5 (initially annotated by CID as PC 20:5/0:0 in one example), OAD revealed double bonds at n-3, n-6, n-9, n-12 and n-15 (equivalent to delta positions 5,8,11,14,17), enabling full positional characterisation of the pentacosaenoic acyl chain.
• Phospholipids containing linoleic acid (18:2) were consistently identified and shown to carry double bonds at n-6 and n-9, confirming the presence of omega‑6 linoleic acid in multiple reduced PDAC lipids (e.g., PC(18:2/0:0), PE(18:2/0:0), PC(18:1_18:2), PC(18:2_20:4)).
• A broader set of lipid classes (LPC, LPE, PC, SM) and individual species were structurally resolved and are compiled in the study table; many showed log2 fold reductions in PDAC vs controls (examples: LPC 18:2, LPC 20:5, PC 18:2_18:2, PC 16:0_20:5).

Discussion points and caveats:

• OAD provides complementary fragments to CID, improving structural resolution without resorting to hazardous reagents or complex setups required by some alternative C=C localization methods.
• Fragment intensities in OAD spectra depend on lipid subclass, acyl chain type, charge state and adduct; reliable annotation requires detection of the main fragment pair for each double bond.
• Simultaneous OAD/CID acquisition in positive and negative modes consolidates headgroup and acyl chain information with positional C=C data in a single experiment, streamlining structural identification workflows.

Benefits and practical applications of the method

• Enables positional assignment of C=C bonds in complex lipids, elevating identifications from class/elemental composition to structural level (omega/delta positions).
• Compatible with routine UHPLC-ESI-QTOF platforms when equipped with an attachable OAD radical source; acquisition can run concurrently with standard CID, minimizing additional runtime.
• Improves confidence in biomarker characterization for clinical lipidomics, which supports mechanistic insights and targeted follow-up studies.
• Less hazardous and operationally simpler than some alternative radical or derivatization strategies while providing high fragmentation efficiency for singly charged precursors.

Future trends and potential applications

• Broader adoption of OAD-enabled workflows in lipidomics and clinical metabolomics could standardize C=C positional annotation across laboratories, improving comparability of biomarker studies.
• Integration of OAD spectral libraries into open-source identification tools (e.g., MS-DIAL) and development of automated scoring routines will accelerate high-throughput positional lipidomics.
• Combining OAD with ion mobility, higher-resolution instruments, or targeted quantitation workflows could extend positional lipidomics to isomer-resolved quantitation and spatial lipidomics.
• Application to other biomedical contexts (cardiometabolic disease, neurodegeneration, inflammation) will clarify biological roles of specific double-bond isomers and support translation to diagnostics or therapeutic monitoring.

Conclusion

OAD-MS/MS is a practical, efficient radical-induced fragmentation technique that complements CID to enable unambiguous C=C positional assignment in unsaturated lipids. When implemented on an LCMS-9050 QTOF with the OAD Radical Source I and combined with MS-DIAL and LabSolutions processing, it provides higher-confidence structural identifications for lipids implicated in PDAC. The approach strengthens the analytical basis for lipid biomarker discovery and offers a scalable pathway toward routine positional lipidomics in clinical research.

References

• Uchino H., Tsugawa H., Takahashi H., Arita M. Computational mass spectrometry accelerates C = C position-resolved untargeted lipidomics using oxygen attachment dissociation. Communications Chemistry 2022;5:1.