Interaction Models Explained
Choosing the right kinetic model is crucial. A good fit doesn't always mean the model is physically correct. Learn the assumptions behind the math.
How to choose a model
Always start with 1:1 Langmuir. If systematic residuals remain, consider whether mass-transport artefacts are distorting the on-rate — add the Mass Transport model before escalating complexity. If residuals still show curvature, a Two-State or Heterogeneous Ligand model may be warranted — but each adds parameters, so escalate only when the data demand it. Use the Model Selection Guide for an interactive walkthrough.
If your analyte is bivalent (e.g. intact IgG) and the immobilised partner is monovalent, expect avidity — see the Bivalent Analyte page rather than defaulting to heterogeneous ligand.
1:1 Langmuir
The gold standard. A simple 1:1 interaction between analyte and ligand — the foundation of kinetic analysis.
Mass Transport
When analyte delivery is rate-limiting, a transport rate constant (kt) competes with the intrinsic kon — lowering the apparent on-rate and causing rebinding during dissociation.
Two-State / Conformational Change
Two-step binding: an initial encounter complex isomerises to a tighter state — consistent with multiple two-step mechanisms (induced fit or conformational selection).
Heterogeneous Ligand
Two ligand populations on the surface with distinct kinetics — most often from random-orientation coupling, but also from aggregates, polyclonals, or surface decay. Powerful but easy to overfit.
Bivalent AnalyteComing soon
A bivalent analyte (e.g. intact IgG) binding a monovalent immobilised partner — avidity tightens apparent KD and produces biphasic dissociation easily mistaken for heterogeneous ligand.
Fitting Best Practices
How to evaluate fit quality: χ², residuals, global vs local fitting, and common pitfalls to avoid.
Model Selection Guide
Interactive decision tree to help you choose the right kinetic model for your data.
Steady-State / Equilibrium Analysis
When kinetics are too fast to resolve, fit the equilibrium plateau vs. concentration to extract KD directly.