Surface Plasmon Resonance

Overview

Surface plasmon resonance (SPR) measures biomolecular interactions in real time without labels. One binding partner is immobilized on a sensor chip, and solutions of the other partner flow across the surface. Binding changes the refractive index at the sensor, producing a response recorded continuously.

SPR is the gold standard for kinetic characterization of peptide-target interactions: it provides association rate (kon), dissociation rate (koff), and dissociation constant Kd — all from a single experiment. These kinetic parameters define binding affinity more completely than endpoint measurements from ELISA or fluorescence polarization.

Instrumentation

Sensor chip

• Gold-coated glass chip with a functionalized surface (typically carboxymethyl dextran, CM5 most common)
• Other chemistries: streptavidin-coated (SA), Ni-NTA (NTA), amine (C1), protein A (CAP)
• Surface density controls dynamic range

Flow system

• Microfluidic channels (~30 nL volume) allow precise delivery of analytes
• Typical flow rates: 5–100 μL/min
• Parallel channels enable reference subtraction

Optics

• Polarized light reflected off the chip's gold film at a specific angle
• SPR angle shifts with mass bound to the surface
• Response Unit (RU) = 0.0001° shift ≈ 1 pg/mm²

Platforms

• Biacore (Cytiva) — industry standard
• OpenSPR, Reichert, Sierra Sensors — academic alternatives
• MASS-1 — grating-coupled, microarray format

Experimental Formats

Direct binding

• Immobilize one partner (typically the larger or more stable one)
• Flow analyte across the surface
• Measure association, dissociation
• Extract kon, koff, Kd

Capture format

• Covalently attach a "capture" molecule (e.g., anti-Fc antibody, streptavidin)
• Capture tagged analyte (e.g., biotinylated peptide) non-covalently
• Re-use chip with fresh capture for each experiment

Especially useful for peptides because immobilization chemistry is gentler and orientation more controlled.

Competition and inhibition

• Pre-incubate analyte with competing peptide
• Measure reduced binding response
• Extract inhibitor Ki

Sample Preparation

Immobilization target

• For antibodies, streptavidin-biotin capture preserves orientation
• For His-tagged proteins, NTA chips work but check for avidity artifacts
• Amine coupling is common but orientation is random
• Keep surface density low (50–200 RU) for kinetic measurements

Analyte (peptide)

• Purify by HPLC to >95%
• Confirm identity by mass spec
• Dilute in running buffer with matched composition
• Centrifuge to remove aggregates (aggregates produce nonspecific or avidity artifacts)
• Typical concentration range: 0.1–100× expected Kd

Running buffer

• HBS-EP+ (HEPES-buffered saline with EDTA and surfactant) is the standard
• PBS-T (phosphate with Tween) for physiological conditions
• Include 0.005% P20 or Tween 20 to reduce nonspecific binding
• Match analyte buffer to running buffer — mismatch produces bulk refractive index artifacts

Experimental Workflow

1. Prime system and condition chip
2. Immobilize or capture target on sample flow cell; leave reference flow cell blank or capture inactive protein
3. Inject running buffer to establish baseline
4. Inject analyte dilution series (typically 5–7 concentrations, doubling or tripling dilutions)
5. Monitor association phase (30–120 s, until plateau for steady state) and dissociation phase (60–600 s, longer for tight binders)
6. Regenerate surface between injections (low pH glycine, high salt, or chaotrope — chosen to remove analyte without damaging target)
7. Run buffer-only control between samples to detect drift
8. Include a replicate of one concentration to check reproducibility

Data Analysis

Sensorgram fitting

Raw data are response (RU) vs. time. Fit to a binding model:

• 1:1 Langmuir — simplest; assumes bivalent target has independent sites
• Heterogeneous ligand — immobilized target has multiple binding sites
• Two-state — binding + conformational change
• Bivalent analyte — IgG or similar binders with two sites

Extract kon (M⁻¹s⁻¹), koff (s⁻¹), Kd = koff/kon.

Steady-state analysis

If dissociation is too fast for kinetic fitting (koff > 1 s⁻¹), fit equilibrium response vs. concentration to:

Req = Rmax × [C] / (Kd + [C])

Extract Kd and Rmax.

Quality checks

• Residuals should show no systematic trends
• χ² of fit should be small
• Dissociation phase should return to baseline (incomplete return signals mass transport, rebinding, or irreversible binding)
• Rmax consistent with immobilized amount and 1:1 stoichiometry

Common Issues

Mass transport limitation

When binding is faster than analyte delivery, apparent kon is underestimated. Mitigations:

• Increase flow rate
• Reduce immobilization density
• Use smaller injection volumes

Nonspecific binding

Analyte sticks to chip surface or to reference. Mitigations:

• Include surfactant in running buffer
• Use CM5 chip with minimal activation
• Pre-treat analyte surface with BSA

Avidity

Multivalent analytes bind with apparent higher affinity. For true 1:1 kinetics, use monomeric analyte and low-density immobilization.

Drift

Temperature or bulk refractive index changes produce drift. Control via temperature stabilization and matched buffer composition.

Applications

Ligand characterization

Primary tool for characterizing peptide ligand kinetics and affinity during lead optimization.

Structure-activity relationships

Compare kon/koff across peptide variants to identify which modifications tune binding rate vs. off-rate.

Selectivity profiling

Screen peptide against multiple related targets in parallel to quantify selectivity.

Competition screening

Identify small molecules or peptide fragments that compete with natural ligand — complementary to peptide library screening.

Biotherapeutic development

Full characterization of binding kinetics is required for regulatory filings of peptide and protein therapeutics.

Limitations

• Immobilized target may differ from solution target
• Sensitivity scales with molecular weight — very small analytes (<200 Da) challenging
• Kinetic range limited by diffusion and instrument response
• Requires pure, well-behaved material; aggregates and nonspecific binding obscure results

Complementary Methods

• Biolayer interferometry (BLI) — similar in concept, label-free kinetics
• Isothermal titration calorimetry (ITC) — thermodynamics of binding
• MicroScale Thermophoresis (MST) — solution-based, low sample requirement
• Fluorescence polarization — high-throughput binding

Summary

Surface plasmon resonance delivers real-time, label-free kinetic characterization of peptide-target binding. When properly executed — clean chip, good reference subtraction, thoughtful analyte series, appropriate fitting model — SPR yields kon, koff, and Kd with precision that supports every stage of peptide therapeutic development.