The Fundamentals of Binding Kinetics
What is a Sensorgram?
In Surface Plasmon Resonance (SPR) and Bio-Layer Interferometry (BLI), the primary output is a Sensorgram. This is a real-time plot of the binding response (Y-axis, usually in Resonance Units or nm shift) over time (X-axis).
The Anatomy of a Sensorgram
Stable signal with running buffer only.
Analyte binds ligand. Signal rises (ka).
Analyte unbinds. Signal decays (kd).
Surface stripped for next cycle.
Dissociation begins the moment analyte injection stops and buffer flow resumes — the bound complex is no longer in equilibrium with free analyte, so it starts to fall apart. After dissociation, the surface is regenerated — briefly exposed to a regeneration solution that strips any remaining analyte from the ligand so the same surface can be reused for the next cycle.
The Rate Constants
Kinetics describes how fast an interaction happens. We define this with two key parameters:
Together, ka and kd govern the 1:1 binding rate law:
dR/dt = ka · C · (Rmax − R) − kd · R
R is the response, C is the analyte concentration, and Rmax is the maximum binding capacity of the surface. The slopes, plateaus, and decays you see in a sensorgram all fall out of this single equation.
ka (Association Rate)
Units: M-1s-1
How quickly the complex forms, scaled by analyte concentration. A higher ka means more productive binding events per unit time at a given concentration.
Fast ka = Steep slope upwards (at fixed concentration).
Slow ka = Shallow slope upwards (at fixed concentration).
kd (Dissociation Rate)
Units: s-1
How quickly the complex falls apart. This determines the Residence Time.
Fast kd = Quick drop to baseline.
Slow kd = Flat, stable holding.
Affinity and the Equilibrium Dissociation Constant (KD)
Equilibrium Dissociation Constant
KD describes the strength of the binding at equilibrium. It is the concentration of analyte where half the ligand sites are occupied. Affinity is the inverse of KD — a lower KD means higher affinity.
- Lower KD = Tighter binding (High Affinity).
- Higher KD = Weaker binding (Low Affinity).
Note: Two interactions can have the same KD (Affinity) but completely different kinetics (Fast On/Off vs Slow On/Off).
Interactive Simulator
Adjust the sliders below to see how changing ka and kd affects the shape of the sensorgram.
Interactive Lab: The 1:1 Model
Kinetics
Experimental Setup
Comma separated
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