Optimized quantitative XL-MS Analysis on an Orbitrap Tribrid Apex mass spectrometer

Significance of the Topic

Cross-linking mass spectrometry (XL-MS) enables distance-constrained mapping of protein tertiary structure and protein–protein interaction networks at proteome scale. Quantitative XL-MS, particularly when combined with isobaric tags such as TMT, adds the ability to compare crosslink abundances across conditions, increasing biological interpretability. However, reporter ion yield, acquisition speed and sensitivity remain technical bottlenecks. The study summarized here addresses these limitations by optimizing quantitative XL-MS workflows for DSSO crosslinks using the Thermo Scientific Orbitrap Tribrid Apex mass spectrometer, demonstrating substantive gains in reporter ion intensity, identification depth and quantitative completeness.

Goals and Overview of the Study

• Benchmark and optimize a TMT-based quantitative XL-MS workflow for the Orbitrap Tribrid Apex MS platform using the MS-cleavable crosslinker DSSO.
• Systematically evaluate acquisition strategies (MS2–MS2 and MS2–MS3), activation schemes (HCD vs IRMPD for MS3), FAIMS compensation voltages and AGC/injection time settings to maximize crosslink identification and reporter ion signal.
• Implement data analysis with Proteome Discoverer (XlinkX node) and visualize crosslink networks.

Materials and Methods

• Crosslinking: DSSO (disuccinimidyl sulfoxide) applied to purified proteins and HEK293 cell lysates to create MS-cleavable lysine-to-lysine crosslinks.
• Quantitation: TMT 11-plex labeling used for relative quantification across conditions.
• Chromatography: NanoLC separation on a Vanquish Neo system with Thermo Optispray PepMap Neo 75 µm × 50 cm cartridge using 1–3 hour gradients.
• Mass spectrometry: Analyses performed on the Thermo Scientific Orbitrap Tribrid Apex mass spectrometer with the OptiSpray ion source; experiments run with and without FAIMS. Acquisition approaches compared included MS2–MS2 and MS2–MS3 workflows; FAIMS CVs such as −50, −60 and −70 were tested.
• Data processing: Proteome Discoverer 3.3 SP1 with the XlinkX node for open XL search and Sequest for unmodified/mono/looplinks. Static carbamidomethylation on cysteine; variable oxidation on methionine and appropriate crosslink mass shifts for DSSO on lysine/N-terminus. Searches performed against the human proteome database. FDR controlled at 1% for CSMs and crosslinks.

Used Instrumentation

• Thermo Scientific Orbitrap Tribrid Apex mass spectrometer with OptiSpray ion source.
• FAIMS interface (tested with multiple compensation voltages).
• Thermo Scientific Vanquish Neo liquid chromatography system.
• Thermo Optispray PepMap Neo 75 µm × 50 cm nanoLC cartridge.
• Software: Thermo Scientific Proteome Discoverer 3.3 SP1 with XlinkX and Sequest nodes; Cytoscape for network visualization.

Main Results and Discussion

• Reporter ion enhancement: Implementation of an infrared multiphoton dissociation (IRMPD) step using the new IR laser on the Apex instrument produced approximately threefold higher TMT reporter ion signal compared with conventional methods, substantially reducing missing channels in multiplexed quantification.
• Acquisition optimization: Tuning of parallel ion injection times, AGC targets and collision strategies increased identification rates of crosslinked spectral matches (CSMs) with quantifiable reporter ions. The optimized workflow produced a marked increase in crosslinks carrying quantitative information relative to baseline settings.
• MS2–MS3 comparisons: Both MS2–MS2 and MS2–MS3 strategies were evaluated; IRMPD-based MS3 spectra yielded superior reporter ion intensity versus HCD MS3 for DSSO crosslinked peptides, improving quantitative accuracy for TMT-labeled crosslinks.
• FAIMS impact: Application of FAIMS with stepped compensation voltages enhanced selectivity and overall crosslink identification, contributing to deeper coverage when combined with optimized MS parameters.
• Data quality: Stringent search and FDR filtering (1% at CSM and crosslink levels) and combined use of XlinkX/Sequest enabled confident assignment of crosslinks and linked peptides. Visualized interactome maps illustrated quantitative changes across samples (example shown for HEK293-derived data).

Key Benefits and Practical Applications

• Increased reporter ion yield reduces missing values and improves quantitation precision in multiplexed XL-MS experiments, enabling more reliable cross-condition comparisons.
• Higher identification depth of quantitative crosslinks permits more comprehensive mapping of interaction interfaces and structural constraints at proteome scale.
• The optimized Apex workflow supports experiments that require both structural insight (via crosslinks) and comparative quantitation (via TMT), applicable to studies of conformational dynamics, interactome remodeling, and drug–target engagement.

Future Trends and Possible Applications

• Further integration of tailored activation methods (IRMPD, UVPD, stepped HCD) with advanced acquisition schemes to maximize reporter ion yield for higher-plex isobaric tags.
• Improved search algorithms and machine-learning approaches for crosslink identification and site localization, reducing false positives and accelerating analysis of large-scale datasets.
• Combining FAIMS or other ion mobility separations with faster, higher-capacity analyzers to scale quantitative XL-MS to larger cohorts and deeper proteome coverage.
• Adoption of more diverse MS-cleavable crosslink chemistries and higher-plex TMT reagents to expand throughput and biological contrast in comparative structural proteomics.
• Broader application fields: structural biology, systems-level interactomics, biomarker discovery, and assessing conformational effects of therapeutics.

Conclusions

The optimized quantitative XL-MS workflow on the Orbitrap Tribrid Apex demonstrated clear methodological advantages for TMT-based crosslink quantification. Key improvements include a threefold enhancement in TMT reporter ion signal using IRMPD, better identification of quantifiable crosslinks owing to optimized injection/AGC settings and the beneficial use of FAIMS. Collectively these advances improve data completeness and sensitivity for comparative crosslinking studies, enabling more robust structural and interaction analyses at scale.

Acknowledgements

We thank the contributor of biological samples acknowledged in the original work for enabling method validation and benchmarking.

References

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2. Shannon P, Markiel A, Ozier O, Baliga NS, Wang JT, Ramage D, Amin N, Schwikowski B, Ideker T. Cytoscape: a software environment for integrated models of biomolecular interaction networks. Genome Research. 2003 Nov;13(11):2498–2504.