Illustration of an SPR biosensor screen showing an orange sensorgram curve

SPR Binding Kinetics Services

Label-free surface plasmon resonance (SPR) measurement of association rate (ka), dissociation rate (kd) and equilibrium dissociation constant (KD), with assay design and data interpretation included.

Discuss Your Binding Assay

Surface plasmon resonance is widely used to measure how two biomolecules bind and release each other in real time, with no label on either partner. Boster's surface plasmon resonance services, including the SPR binding kinetics service described here and delivered through Boster and its partner labs, are built around three questions: how tightly your molecule binds its partner, how fast, and how specifically. They sit alongside Boster's other assay services. This page covers how the method works, the interaction types and surface chemistries available, the instrument formats and throughput options, what to send us, and what the report contains.

How SPR works

SPR is an optical, label-free way to watch molecular binding. One binding partner, the ligand, is immobilized on a thin gold film on a sensor chip. The other, the analyte, flows over that surface in solution. Polarized light reflected off the underside of the gold film excites a surface plasmon at a specific angle, and that angle depends on the refractive index within roughly 100–300 nm of the surface.

When analyte binds the ligand, mass builds up at the surface, the refractive index changes and the resonance angle shifts. The shift is recorded continuously as a sensorgram (response versus time), showing association while analyte flows and dissociation once buffer alone resumes. No fluorophore, radiolabel or enzyme tag is needed.

Because the readout follows how the resonance condition of the surface plasmon shifts as the surface changes, the technique is also called surface plasmon resonance spectroscopy. The response scales with the mass that binds, so a large protein produces a bigger signal than a small molecule at the same occupancy, which is one reason assay design starts with which partner is immobilized.

SPR data can report four things: kinetics (how fast binding and unbinding happen), affinity (equilibrium binding strength), specificity (binding to the intended partner versus reference or off-target surfaces) and concentration (an unknown analyte's concentration read against a calibration curve). One well-designed experiment often answers more than one.

Diagram of an SPR setup: light source, prism, gold sensor surface and detector

The surface plasmon resonance method, one cycle at a time

Every binding cycle in the surface plasmon resonance method follows the same sequence, and the sensorgram records each stage:

Baseline: running buffer flows over the ligand surface until the signal is stable.
Association: analyte is injected, and the response rises as it binds the ligand.
Dissociation: buffer resumes, and the response falls as the complex releases.
Regeneration: a short pulse strips remaining analyte so the next cycle starts from the same baseline.
Referencing and fitting: the signal from a reference surface is subtracted, and a binding model is fitted to the processed curves to give ka, kd and KD.

Interaction types

SPR detects any binding that adds mass to the surface, so it is not limited to one class of molecule. Common interaction types, their typical research uses and the discovery decisions they inform:

Interaction type	Typical research use	Decision it informs
Protein–protein	Confirming target engagement, mapping complex assembly, comparing wild-type and mutant binding	Whether a target pair truly engages, and how an engineered variant changes that binding
Antibody–antigen	Ranking hybridoma or phage-display hits, confirming target specificity before downstream work	Which antibody hits advance, ordered by affinity and off-rate
Antibody–receptor	Characterizing antibodies against cell-surface receptors, Fc receptor engagement studies	Receptor engagement strength and the effect of Fc or format changes
Protein–small molecule	Fragment and hit validation, structure–activity relationship (SAR) support, residence-time comparison of inhibitors	Which hits are genuine binders and which analogs to make next
Protein–nucleic acid	Transcription factor–DNA binding, aptamer characterization, RNA-binding protein studies	Sequence preference and which aptamer or binding construct to carry forward
Protein–lipid / membrane protein	Peripheral membrane protein binding, liposome-based studies of transmembrane targets	Whether a target binds as expected in a bilayer context, where a soluble form may not predict it

Each can be run as a multi-cycle titration, a single-cycle kinetic experiment or a qualitative yes/no binding check.

Kinetic and affinity measurements

A kinetics experiment produces a small set of values, each useful in a different way for ranking candidates:

ka (association rate constant): how quickly the analyte binds the immobilized ligand, in M⁻¹s⁻¹.
kd (dissociation rate constant): how quickly the complex falls apart once free analyte is removed, in s⁻¹. Its inverse, 1/kd, is the residence time, which can matter as much as affinity in lead optimization.
KD (equilibrium dissociation constant): the ratio kd/ka. Lower KD means tighter binding.
Binding specificity: checked by running the analyte over a reference surface with no ligand and, where relevant, over related off-target proteins.

These values come from fitting a binding model to the sensorgrams, usually a 1:1 Langmuir model, with two-state or heterogeneous-ligand models when the data show more than one binding mechanism. Fitted curves are reported next to the raw traces so you can see how well the model describes the data.

Ranking on KD alone can hide useful differences. Two candidates with the same KD may reach it by different routes, one binding fast and releasing fast, the other binding slowly and releasing slowly. Reporting ka and kd together with KD lets you rank candidates on the property your program cares about, whether that is speed of engagement, residence time or overall affinity.

Infographic of a sensorgram with association, plateau and dissociation phases, labeled ka, kd and KD

Assay development and experimental design

SPR data quality is mostly set before the first injection. Each project starts with a feasibility review of your ligand and analyte:

Orientation & Roles

Deciding which partner to immobilize as ligand and which to flow as analyte. The smaller, more stable or more plentiful molecule is usually the analyte.

Capture Strategy

Direct amine coupling or a capture system (biotin/streptavidin, His-tag/NTA, Fc capture with Protein A/G), chosen from your reagent's tags and stability.

Concentration Series

An analyte range that brackets the expected KD, typically about 0.1–10× that value.

Buffer & Reference Surfaces

A running buffer suited to the biology, plus a blank reference flow cell to subtract bulk refractive-index and non-specific signal.

Regeneration

A condition that strips bound analyte between cycles without damaging ligand activity.

A short pilot run to check capture level, surface stability and regeneration usually costs far less than finding a buffer problem partway through a full titration. If you do not yet have a purified ligand or binding partner, Boster can make one through custom antibody production or recombinant protein expression.

Sensor surfaces and immobilization methods

Surface chemistry is chosen to match how your ligand is best presented. Available surfaces:

Surface	Immobilization chemistry	Suited to
Carboxymethyl dextran (CM5-type)	Covalent amine coupling (thiol or aldehyde coupling as alternatives)	General covalent immobilization of proteins and peptides through free amine, thiol or aldehyde groups
Streptavidin capture (SA-type)	Capture of biotinylated ligand through near-irreversible streptavidin–biotin binding	Biotinylated proteins, peptides or nucleic acids that need a stable, oriented surface
NTA / His capture	Nickel-chelate capture of His-tagged ligand	His-tagged recombinant proteins; reversible, oriented capture without chemical modification
Protein A capture	Capture through the antibody Fc region	Antibodies of Protein A-compatible species and subclasses, oriented by Fc
Protein G capture	Capture through the antibody Fc region, with broader subclass coverage than Protein A	Human and a wider range of mammalian antibody subclasses
Lipid capture (L1-type)	Direct capture of liposomes and vesicles with the bilayer kept intact	Membrane-protein and protein–lipid studies that need a bilayer context

Amine coupling to carboxymethyl dextran is covalent and attaches the ligand at any accessible lysine or at the N-terminus, so it can reduce activity when an amine sits near the binding site. Capture surfaces (streptavidin, NTA/His, Protein A/G) usually give more uniformly oriented, active ligand and are the better choice when a suitable tag or Fc region is available.

Matching the surface to your reagent

Untagged, stable protein: amine coupling on a carboxymethyl dextran surface is the simplest route.
Biotinylated ligand: streptavidin capture gives a stable, oriented surface for proteins, peptides and nucleic acids.
His-tagged protein: NTA capture presents the ligand through its tag, and the surface can be stripped and reloaded between cycles.
Antibody as ligand: Protein A or Protein G capture holds the antibody by its Fc region so the antigen-binding sites face the flow.
Liposomes and membrane systems: lipid capture keeps the vesicle intact for protein–lipid and membrane-protein work.

Instrumentation and throughput

SPR instruments differ in how many interactions they can read at once, how sensitive they are to small analytes and how many samples they can run unattended. The platform for your project is matched to the question being asked and confirmed at quote. Three practical formats cover most requests:

Few-channel flow-cell systems

One to a few flow cells read in sequence. Well suited to feasibility checks, surface and buffer scouting and pilot runs, where flexibility matters more than sample volume.

Best for: exploratory work and assay scouting

Multi-channel automated systems

Several parallel flow channels with an automated sampler. Parallel reference surfaces and unattended cycling support full kinetic series and good sensitivity for smaller analytes.

Best for: fitted kinetics on a shortlist

Array and imaging systems

Surface plasmon resonance imaging reads many ligand spots on one chip at once. Multiplexing is high, but sensitivity per spot is generally lower than flow-cell systems, so it favors ranking and screening.

Best for: panel screening and ranking

How format affects fit for your study

Consideration	Exploratory study	Decision-critical study
Goal	Does binding occur, and under what conditions	A fitted ka, kd and KD you can use to select a candidate
Sensitivity	Adequate for large analytes; small molecules may need scouting	Matched to analyte size, with surface density tuned to the expected signal
Multiplexing	Many candidates, one condition, often a single concentration	Fewer candidates, full bracketed concentration series with replicates
Assay complexity	Simple capture, minimal optimization	Optimized capture, regeneration scouting and mass-transport checks
Reporting	Ranking or yes/no outcome	Fitted models, fit statistics and QC criteria for every condition

Throughput depends on more than the instrument. A multi-cycle kinetic series needs several analyte concentrations plus blank cycles, each followed by regeneration, so sample count and cycle time grow with assay complexity. Single-cycle kinetics removes the regeneration step between concentrations and suits ligands that do not tolerate harsh regeneration. All work is performed as non-GLP research studies, so these formats suit discovery and development research rather than regulated submissions.

What to send us

To scope a project we need to know what each molecule is and what buffer it is in. Amounts depend on the assay design and are confirmed at quote.

Molecule type	What we need to know	Amount
Protein (ligand or analyte)	Purity, molecular weight, tags (His, biotin, Fc), buffer composition, concentration	Confirmed at quote
Antibody	Species and subclass (for Protein A/G capture), purity, buffer composition, concentration	Confirmed at quote
Nucleic acid	Sequence and length, biotin or other labels, buffer composition	Confirmed at quote
Small molecule	Supplied as a DMSO stock; molecular weight, stock concentration, aqueous solubility	Confirmed at quote
Liposome or membrane protein	Lipid composition, vesicle preparation, any detergent present	Confirmed at quote

Flag these before you ship a sample. Several common additives interfere with immobilization or add bulk signal:

Tris buffer: its primary amine competes with amine coupling on carboxymethyl dextran, so a capture surface or a buffer exchange is used instead.
Sodium azide (NaN₃): can affect surface chemistry and detector readings at higher concentrations.
Imidazole: competes with His-tag/NTA capture and must be removed or reduced first.
Sucrose and glycerol: raise bulk refractive index and can look like binding if not referenced properly.
DMSO: small mismatches between sample and running buffer cause large bulk shifts, so DMSO levels are matched and a solvent correction is applied.
DTT and other reducing agents: can react with gold surfaces and thiol-based coupling over time.

None of these rule out a project. They change the surface chemistry, buffer exchange or reference strategy, which the feasibility review sorts out.

Remediation options

When a sample arrives in an incompatible buffer, the usual fixes are buffer exchange by desalting or dialysis into the running buffer, dilution below the level at which an additive interferes, removal of imidazole before NTA capture, or a switch to a capture chemistry that is not affected. Samples that are aggregated or heterogeneous may need an extra polishing step before injection. Tell us what is in your buffer and we will recommend the least disruptive route.

Deliverables and data interpretation

Turnaround is quoted per project. It depends mainly on how much assay development the ligand needs and on the size of the analyte panel.

Each project report includes:

Raw sensorgrams for every cycle, with reference-subtracted and blank-subtracted traces
The fitted binding model overlaid on the processed data
A table of ka, kd and KD with fit statistics (Chi²) for each condition
Assay conditions: surface chemistry, immobilization level, buffer, flow rate, temperature and regeneration cycle count
QC notes flagging any cycle that failed acceptance criteria, and why
A plain-language interpretation of the binding behavior, including non-ideal cases such as heterogeneous ligand, avidity effects or slow conformational change

Biological samples rarely give textbook 1:1 kinetics. A KD reported without its fit quality can mislead a project, so the report states how well each model fits and notes where a two-state or other model describes the data better.

Illustration of overlapping sensorgram curves for a series of analyte concentrations — Illustration of a concentration series.

How a project moves from sample to report

Feasibility review

We check tags, buffer and molecule sizes, then agree the surface chemistry and reference strategy.

Pilot run

A short test of capture level, surface stability and regeneration before committing the full panel.

Full run

Titrations or screening cycles with blanks, duplicates and controls interspersed.

Report

Processed data, fitted models, kinetic tables, QC notes and expert interpretation.

SPR screening and epitope binning

Early-stage work often needs a relative ranking more than full kinetics. Screening formats:

Hit Confirmation

Yes/no binding checks across a candidate panel to confirm hits from a primary screen before full kinetics.

Ranking

Many candidates compared against one target under identical conditions, ordered by apparent affinity or response.

Epitope Binning

Sequential or premixed injections of antibody pairs to sort a panel into groups that bind overlapping or distinct epitopes.

Competition Studies

Testing whether a candidate blocks a known binder, to check mechanism of action against a reference ligand.

Kinetic Screening

Single-cycle kinetics or a reduced concentration series across a wider panel, for an approximate ka, kd and KD ranking.

Multiplexed surface plasmon resonance imaging (SPRi) reads many ligand spots on one sensor surface at once and is available on request, for example to rank a panel of candidates against several targets in parallel.

Screening versus characterization. A screening ranking compares many candidates under identical conditions with less time per candidate. Full characterization runs a longer, bracketed concentration series on a shortlist to give a fitted KD. The two are reported as separate deliverables so a ranking is not read as a final kinetic result.

Data quality and troubleshooting

SPR also picks up signal that has nothing to do with binding. Common artifacts and how they are controlled:

Artifact	What it looks like	How it's controlled
Non-specific binding	Signal on a reference surface with no immobilized ligand	Blank/reference flow-cell subtraction; surface blocking
Mass-transport limitation	Association looks linear rather than exponential; kinetics depend on flow rate	Lower immobilization density; higher flow rate; check that rate constants do not change with flow rate
Bulk refractive-index shift	An instant, uniform jump in signal at injection start unrelated to binding	Buffer-matched reference subtraction; matched DMSO and excipient levels; solvent correction
Aggregation	Unusually high or erratic response, poor fit to any standard model	Sample QC (SEC or DLS) before injection; filtration
Carryover between cycles	Baseline drifts upward cycle over cycle	Extended buffer wash; verified regeneration step; blank cycles interspersed
Poor surface regeneration	Capture or response amplitude declines across the run	Regeneration scouting during assay development; surface activity checks between titrations

Before data go into a report, they are checked for reproducibility between duplicate injections, a stable reference baseline and a residuals pattern consistent with the chosen binding model. Data that fail these checks are flagged in the report with the reason, rather than forced into a fit.

Controls built into each run

Reference surface: a flow cell without ligand, subtracted from every active-surface trace.
Blank injections: buffer-only cycles spread through the run to correct drift and carryover.
Replicate injections: repeated concentrations to confirm that the response is reproducible.
Solvent correction: applied whenever small molecules are run in DMSO.
Surface activity checks: a repeated reference injection to show the ligand is still active late in the run.

Research applications

SPR binding data supports several stages of discovery research:

Hit-to-lead: confirming real binders from a primary screen and giving an early affinity rank order for follow-up.
Lead optimization: tracking how ka, kd and KD change across a chemical or antibody engineering series, often weighing residence time (1/kd) alongside affinity.
Antibody and bispecific characterization: confirming each arm's target binding alone and together, including simultaneous or sequential binding.
Conjugate binding comparison: a research-grade check of whether conjugating an antibody to a linker or payload (as in ADC research) changed its binding kinetics.
Biosimilar candidate comparison: research-grade, side-by-side kinetics of a candidate and a reference molecule under identical assay conditions.
Candidate selection: a fitted kinetic dataset across a shortlist to support internal program decisions.

Illustration of gloved hands loading a sample plate into a benchtop SPR instrument

Client-ready reportables

Depending on the study, the deliverables handed back for these applications include:

A kinetic table of ka, kd, KD and residence time for each candidate, with fit statistics
A ranked candidate list from screening, labeled as a ranking rather than a fitted result
An epitope bin assignment for each antibody in a panel, with the competition data behind it
Side-by-side sensorgram overlays and a kinetic comparison for conjugated versus unconjugated or candidate versus reference molecules
Assay conditions, QC notes and a written interpretation to support your internal candidate-selection discussion

Common questions

What sample purity does SPR require?

Higher purity gives cleaner sensorgrams and more reliable fits, since contaminating species can bind non-specifically or interfere with immobilization. Standard chromatography-grade purity is recommended for kinetic work, and extra polishing steps can be discussed during assay design if a sample shows aggregation or heterogeneity on initial QC.

Can weak or transient interactions be measured?

Yes, though very fast dissociation (high kd) or very weak affinity is harder to resolve. These cases often need higher analyte concentrations, careful mass-transport control and sometimes a different capture orientation. A feasibility discussion before you commit sample is worthwhile here.

How is SPR different from ELISA or other endpoint assays?

Endpoint assays such as ELISA report a single signal after a fixed incubation, which folds affinity and kinetics into one number. SPR follows the whole binding event, separating how fast a complex forms from how tightly it holds. If an endpoint assay suits your project better, see Boster's ELISA assay development services.

What if my sample contains an interfering buffer component?

Tell us in advance. A buffer exchange, a different capture chemistry or a modified reference strategy usually resolves it without you reformulating the sample.

Is SPR spectroscopy the same as SPR imaging?

They share the same physics but differ in format. Surface plasmon resonance spectroscopy usually refers to flow-cell measurements that follow the resonance shift on a few surfaces in detail, which suits full kinetics. Imaging reads many spots on one chip at once, which suits screening and ranking. The right choice depends on whether you need depth on a shortlist or breadth across a panel.

Can SPR screen a large panel of candidates?

Yes. Screening formats such as hit confirmation, ranking, epitope binning and single-cycle kinetic screening handle larger panels than full kinetic characterization. They give a relative order or an approximate kinetic ranking, and a shortlist can then be taken forward to a full concentration series for fitted values.

Ready to characterize your interaction?

Send your ligand and analyte details. We will go through surface chemistry, sample needs and turnaround before any sample ships.

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