Directly Visualize Intact Complexes — Stoichiometry, Subunit Composition & Ligand Binding in a Single Experiment
Creative Proteomics offers native electrospray ionization mass spectrometry (native ESI-MS) services to characterize noncovalent biomolecular complexes under near-physiological conditions. No labels, no immobilization — just a direct mass-resolved readout of who binds to whom, how many, and in what assembly state.
What You Can Learn:
See your complex. Understand your interaction. Design the next experiment with confidence.
Native electrospray ionization mass spectrometry (native ESI-MS) is a structural mass spectrometry technique that transfers intact, noncovalent biomolecular complexes from solution into the gas phase while preserving their quaternary architecture. By using volatile, non-denaturing buffers (typically ammonium acetate at neutral pH) and gentle nano-electrospray ionization, the technique retains the hydrogen bonds, hydrophobic contacts, and electrostatic interactions that hold complexes together.
Unlike conventional denaturing MS — where proteins are stripped of their binding partners and detected as unfolded monomers — native ESI-MS measures the intact complex mass directly. This yields a complete picture of assembly state, stoichiometry, and bound ligands from a single spectrum. The technique is applicable to protein–protein complexes, protein–small molecule interactions, protein–DNA/RNA assemblies, antibody–antigen binding, and multimeric protein machines. Complexes from ~10 kDa to several MDa can be routinely analyzed.
Researchers familiar with conventional intact mass analysis often ask how native MS differs. The distinction is fundamental — and it determines what biological information you can extract:
| Parameter | Native ESI-MS | Denaturing Intact MS |
| Spray Buffer | Neutral ammonium acetate (pH 6.8–7.5) | Acidic water/acetonitrile + formic acid/TFA |
| Protein State | Folded; noncovalent interactions preserved | Unfolded; all noncovalent contacts disrupted |
| What You See | Intact complex mass (e.g., tetramer + 2 ligands) | Individual subunit masses only |
| Charge States | Narrow distribution, high m/z (low charge density) | Broad distribution, low m/z (high charge density) |
| Information Obtained | Stoichiometry, ligand occupancy, oligomeric state, assembly heterogeneity | Subunit molecular weight, PTM mass shifts, sequence confirmation |
In short: denaturing MS tells you what each subunit weighs. Native MS tells you what they build together — and what cargo they carry.
If understanding the composition and binding architecture of your complex is critical to your research, native ESI-MS provides answers no other technique can deliver in a single experiment.
Direct Stoichiometry Readout — No Labels, No Immobilization
Unlike SPR, BLI, or fluorescence-based methods, native ESI-MS detects the complex itself — not a proxy signal. You see the actual mass of the intact assembly, providing unambiguous stoichiometry and ligand occupancy data without fluorophores, surface chemistry, or reporter molecules.
Resolve Heterogeneous Assembly States Simultaneously
Ensemble techniques average over all species present. Native ESI-MS resolves multiple coexisting oligomeric states, ligand-bound subpopulations, and heterogeneous assemblies in a single spectrum — detect monomer, dimer, tetramer, and ligand-bound forms all in one acquisition.
| Parameter | Typical Range |
| Complex Mass Range | ~10 kDa to >2 MDa |
| Accessible Kd | 10-9 to 10-3 M |
| Mass Accuracy | < 50 ppm (Orbitrap); < 100 ppm (Q-TOF) |
Low Sample Consumption — 1–5 µg Per Injection
Nano-ESI requires only 1–5 µL of sample at low micromolar concentration per acquisition, conserving precious protein for downstream experiments. Typical consumption is 10–50 µg total for a full characterization across multiple conditions and replicates.
End-to-End Support: Buffer Exchange Through Expert Interpretation
We handle buffer exchange into ammonium acetate, concentration optimization, and QC assessment. You receive not just spectra, but a fully interpreted report — annotated mass assignments, stoichiometry models, and actionable recommendations written by the scientist who performed the analysis.
Native ESI-MS is a versatile platform for characterizing a wide range of noncovalent biomolecular interactions. Each service below leverages the technique's unique ability to provide direct mass-based evidence of complex formation, stoichiometry, and binding state distribution — information that is difficult or impossible to obtain from ensemble-averaging methods.
Determine the exact subunit composition and oligomeric state of homo- and hetero-protein assemblies. Detect coexisting oligomeric forms and monitor assembly changes upon mutation, buffer condition, or post-translational modification.
Confirm small molecule binding, determine ligand occupancy (how many ligands per protein), and assess binding specificity. Ideal for hit validation in drug discovery — see which compounds genuinely bind and at what stoichiometry, without labels or immobilization artifacts.
Characterize transcription factor–DNA binding, ribosomal protein–RNA assembly, and small molecule–nucleic acid interactions. Direct mass measurement reveals binding stoichiometry and discriminates between specific and non-specific nucleic acid binding.
Measure intact antibody–antigen complex mass to confirm binding stoichiometry (1:1, 2:1, etc.). Assess aggregation, mispairing in bispecific antibodies, and the effect of formulation conditions on complex integrity.
Detect endogenously bound metal ions, cofactors (NADH, FAD, heme), and nucleotides — often retained through native ESI and visible as discrete mass shifts. Identify what your purified complex actually carries, without assuming it's stripped by purification.
Compare complex formation between wild-type and mutant proteins, across buffer/pH conditions, or in the presence/absence of ligands. Identify conditions that stabilize or disrupt the interaction — critical for construct design, formulation screening, and assay development.
Every project is customized. Our scientists work with you to design optimal experimental conditions — buffer composition, concentration, ionization parameters — based on your specific complex, expected mass range, and research goals.
Sample Conditioning & Buffer Exchange
Your sample is buffer-exchanged into volatile ammonium acetate (typically 50–500 mM, pH 6.8–7.5) using desalting spin columns, ultrafiltration, or microdialysis. This removes non-volatile salts, glycerol, and detergents incompatible with electrospray ionization while maintaining native complex structure.
Concentration Optimization & Quality Assessment
Protein concentration is measured (A280) and adjusted to the optimal range (typically 1–10 µM). Sample homogeneity is assessed to confirm the complex is well-behaved before MS acquisition.
Nano-ESI Source Parameter Optimization
Spray voltage, capillary temperature, and in-source trapping voltages are systematically optimized to achieve gentle desolvation while preserving noncovalent interactions. Low-energy conditions are verified by monitoring charge state distributions — folded complexes produce narrow, high-m/z charge envelopes.
High-Resolution Mass Acquisition
Intact mass spectra are acquired on Q-TOF or Orbitrap platforms with extended mass range. Multiple technical replicates ensure reproducibility. For ligand-binding studies, titration series or competition experiments may be performed.
Deconvolution & Stoichiometry Assignment
Raw charge state series are deconvolved to zero-charge mass distributions using Bayesian deconvolution (UniDec). Observed masses are compared to calculated masses of all possible subunit/ligand combinations. Stoichiometry models are assigned and verified against replicate data.
Expert Interpretation & Report Delivery
You receive annotated spectra, deconvolved mass tables, stoichiometry models, and a written interpretation of complex composition, heterogeneity, and ligand occupancy — ready for publication or project decision-making.
Thermo Scientific Q Exactive UHMR (Ultra-High Mass Range) Orbitrap
– Extended mass range up to m/z 80,000 for large complex analysis
– Resolution up to 200,000 for precise mass determination
– Surface-induced dissociation (SID) capability for subunit connectivity mapping
– Ideal for complexes from ~50 kDa to >2 MDa with high mass accuracy (<50 ppm)
Waters Q-TOF with High-Mass Modification
– Time-of-flight analyzer with theoretically unlimited mass range
– Higher-pressure ion guide for improved transmission of large macromolecular ions
– Excellent for very large assemblies and heterogeneous complexes where broad mass range is critical
Our dual-platform approach ensures that every project is run on the instrument best suited to the specific complex, mass range, and resolution requirements. For projects requiring ion mobility separation (collision cross-section measurement), IM-MS capability is available upon consultation.
| Parameter | Recommended Specifications |
| Sample Types | Purified proteins, protein complexes, protein–ligand mixtures, protein–DNA/RNA assemblies, antibodies, membrane proteins (in compatible detergent, amphipol, or nanodisc) |
| Purity | ≥ 90% by SDS-PAGE; free from non-volatile salts, glycerol, and high concentrations of detergent |
| Concentration | 1–10 µM (protein or complex); optimized during feasibility assessment based on expected complex mass and stability |
| Volume Required | ≥ 20 µL per condition (minimum); 50–100 µL recommended for replicates and optimization |
| Amount Required | ~1–5 µg protein per injection; ~10–50 µg total for a full characterization (multiple conditions, replicates) |
| MS Buffer (Required) | Ammonium acetate (20–500 mM), pH 6.8–7.5. We perform buffer exchange from your storage buffer at no additional charge |
| Storage Buffer (Your Sample) | Any standard protein buffer acceptable; non-volatile components (NaCl, Tris, phosphate) will be removed during buffer exchange |
| Additives to Avoid | Non-volatile salts (NaCl >10 mM, phosphate, Tris), glycerol >1%, PEG, high concentrations of detergents (≥2× CMC) |
| Compatible Additives (Limited) | DMSO ≤2% (for hydrophobic ligands); low concentrations of volatile reducing agents; non-ionic detergents at ≤1× CMC |
| Membrane Proteins | Compatible when solubilized in low-CMC detergents, amphipols, or nanodiscs — consult our team for specific recommendations |
| Shipping | Ship on dry ice or ice packs. Include buffer composition and concentration information with the sample submission form |
Not sure if your sample is compatible? We offer a complimentary feasibility assessment — submit your sample details and our team will evaluate buffer compatibility, concentration requirements, and expected data quality before the project begins.
Researchers studying noncovalent complexes face a critical decision: which structural mass spectrometry technique answers my specific question? The table below compares native ESI-MS with complementary approaches — each linked to its detailed service page.
| Feature | Native ESI-MS | XL-MS | HDX-MS | Cryo-EM |
| Primary Information | Stoichiometry, subunit composition, ligand occupancy, assembly states | Residue-level distance restraints, protein–protein contact maps | Conformational dynamics, solvent accessibility, binding-induced changes | High-resolution 3D structure (near-atomic resolution) |
| Spatial Resolution | Subunit / complex level | Residue-level (pairwise crosslinks) | Peptide-level (5–15 residues) | Near-atomic (2–4 Å typical) |
| Stoichiometry | ||||
| Ligand / Cofactor Detection | ||||
| Conformational Dynamics | ||||
| Label / Modification | ||||
| Environment | Gas phase (native-like retention validated) | Solution labeling, gas-phase readout | Solution labeling, gas-phase readout | Cryogenic (frozen-hydrated) |
| Size Range | ~10 kDa to >2 MDa (up to 18 MDa) | No practical size limit | No practical size limit | > ~50 kDa (practical lower limit) |
| Sample Amount | ~1–5 µg per injection | ~10–50 µg | ~10–100 µg | ~10–50 µg (sample-dependent) |
| Best Used For | Stoichiometry, ligand occupancy, assembly state profiling | Protein–protein contact mapping, integrative structural modeling | Conformational changes, epitope mapping, protein dynamics | High-resolution 3D structure determination |
Native MS excels at stoichiometry and composition questions, but it is not a universal solution. Being transparent about its limitations helps you make the right choice — and often, the best answer is a complementary combination of techniques:
| If Your Question Is... | Native ESI-MS Can't Answer It Well | Consider Instead |
| What are the on/off rates (ka, kd) of this interaction? | Native MS provides occupancy, not real-time kinetics | Surface Plasmon Resonance (SPR) or Bio-Layer Interferometry (BLI) |
| What are the thermodynamic parameters (ΔH, ΔS, ΔG) of binding? | Native MS does not measure heat; Kd estimation from MS is semi-quantitative | Isothermal Titration Calorimetry (ITC) |
| Which residues are at the binding interface? | Native MS reports on mass changes, not atomic contacts | Crosslinking MS (XL-MS) or HDX-MS |
| I need a high-resolution 3D structure. | Native MS provides mass and stoichiometry, not atomic coordinates | Cryo-EM, X-ray Crystallography, or NMR Spectroscopy |
| My complex dissociates in ammonium acetate buffer. | Some complexes require specific salts or co-solutes that are incompatible with ESI | Chemical crosslinking stabilization + native MS, or switch to XL-MS |
When in doubt, talk to us. We routinely combine native ESI-MS with complementary techniques to provide a complete picture — from stoichiometry to structure to dynamics. Contact our team to design a multi-technique interaction analysis strategy.
Publication-Ready Data Packages with Expert Interpretation
When you partner with us for native ESI-MS analysis, you receive a complete, interpreted results package. Every deliverable is designed to support both scientific insight and downstream decision-making.
Annotated Raw m/z Spectra
Full m/z spectra with charge state assignments labeled. Multiple acquisitions shown for reproducibility assessment.
Deconvolved Zero-Charge Mass Distributions
Mass spectra transformed to the zero-charge domain via Bayesian deconvolution (UniDec) for direct molecular weight determination and stoichiometry interpretation.
Ligand Occupancy & Binding State Distribution
Quantitative distribution of apo, singly-bound, doubly-bound (etc.) species. For titration experiments, binding curves and estimated Kd values are provided.
What types of noncovalent complexes can native ESI-MS analyze?
Native ESI-MS can analyze protein–protein complexes, protein–ligand complexes, protein–DNA/RNA complexes, antibody–antigen assemblies, multimeric protein machines, and protein–cofactor/metal ion complexes. The key requirement is that the complex remains intact in ammonium acetate buffer at neutral pH.
How do I know my complex will survive the transition to the gas phase?
Extensive published evidence demonstrates that noncovalent interactions — including hydrogen bonds, hydrophobic contacts, and electrostatic interactions — are preserved under gentle nano-ESI conditions. Our protocols systematically optimize source voltages and monitor charge state distributions to confirm complexes are not artificially dissociated.
Can native ESI-MS measure binding affinity (Kd)?
What if my protein storage buffer contains NaCl, Tris, or glycerol?
No problem — buffer exchange into ammonium acetate is a standard part of our workflow. We use desalting spin columns, ultrafiltration, or microdialysis to remove non-volatile components before MS analysis. Simply provide your sample in its normal storage buffer and we handle the rest.
How does native ESI-MS differ from denaturing intact mass analysis?
Denaturing intact mass analysis uses acidic conditions and organic solvents that unfold proteins and dissociate noncovalent complexes — you see only the individual subunits. Native ESI-MS uses neutral pH ammonium acetate and gentle ionization to preserve the folded state and noncovalent interactions — you see the intact complex mass. In short: denaturing MS tells you the mass of each subunit; native MS tells you what they assemble into and what cargo they carry.
Can you analyze membrane protein complexes by native ESI-MS?
Yes, with appropriate sample preparation. Membrane proteins can be analyzed when solubilized in low-CMC detergents, amphipols, or nanodiscs. Our team will advise on the optimal solubilization and buffer exchange strategy for your specific target.
Can I detect multiple coexisting assembly states in one experiment?
Yes — and this is one of native ESI-MS's greatest strengths. Unlike ensemble-averaging techniques, native MS resolves all coexisting oligomeric states in a single spectrum. You can see the relative abundance of each assembly state and monitor how conditions (concentration, pH, ligand addition) shift the distribution.
What if I only have a very small amount of sample?
Native ESI-MS is relatively low-consumption — each injection requires only 1–5 µL of sample at 1–10 µM. For a full characterization including replicates and multiple conditions, we typically need 10–50 µg total protein. If your sample is extremely limited, we can design a minimized acquisition plan.
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