AtaGenix Laboratories
AtaGenix Laboratories
Release time: 2026-04-03 View volume: 2
Surface plasmon resonance (SPR) is the gold standard for measuring real-time, label-free antibody–antigen binding kinetics. This guide explains how SPR works, what the key parameters mean (ka, kd, KD), and how to interpret SPR data for antibody characterization.
SPR measures changes in the refractive index at a sensor chip surface when molecules bind or dissociate. One binding partner (the ligand, typically the antigen) is immobilized on a gold-coated chip. The other partner (the analyte, typically the antibody) flows over the surface. When binding occurs, mass accumulates at the surface, shifting the SPR angle. This produces a real-time sensorgram showing association, equilibrium, and dissociation phases — all without labels or modifications that could alter binding behavior.
| Parameter | Name | Units | What It Tells You |
|---|---|---|---|
| ka (kon) | Association rate constant | M−¹s−¹ | How fast the antibody binds. Higher = faster binding. |
| kd (koff) | Dissociation rate constant | s−¹ | How fast it falls off. Lower = longer residence time on target. |
| KD | Equilibrium dissociation constant | M (molar) | Overall affinity = kd / ka. Lower KD = tighter binding. Typical therapeutic mAbs: 10−&sup9;–10−¹¹ M. |
Association phase (rising curve): Analyte flows over the chip and binds the immobilized ligand. Steeper rise = faster ka.
Equilibrium (plateau): Binding and dissociation rates are equal. The plateau height correlates with the amount of complex formed at that analyte concentration.
Dissociation phase (falling curve): Buffer replaces analyte; bound molecules dissociate. Slower decay = smaller kd = longer target residence time.
Regeneration: A brief acidic or high-salt wash removes remaining bound analyte, preparing the chip for the next concentration cycle.
SPR is most valuable for ranking antibody candidates by affinity (lead selection), confirming that humanization or engineering hasn’t compromised binding (KD comparison), measuring epitope competition (tandem injection experiments), and characterizing bispecific antibody dual-binding. ELISA provides endpoint affinity estimates but cannot resolve kinetic rates; SPR uniquely reveals whether an antibody binds fast but falls off quickly (high ka, high kd) versus binds slowly but stays on (low ka, low kd) — a distinction critical for therapeutic efficacy.
Need SPR affinity measurement for your antibody candidates? AtaGenix provides Biacore-based SPR characterization as a standalone service or integrated into antibody development projects.
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