Turn binding data into clear metrics. Compare clones with Ka, Kd, and free-energy changes fast. Export tables and reports for labs, teams, and reviewers.
| Clone | kon (M-1·s-1) | koff (s-1) | KD (nM) | ΔG° (kJ/mol) at 25°C |
|---|---|---|---|---|
| A1 | 1.0e5 | 1.0e-3 | 10 | -45.6 |
| B7 | 2.5e5 | 5.0e-4 | 2 | -50.1 |
| C3 | 8.0e4 | 2.4e-3 | 30 | -43.0 |
This calculator focuses on equilibrium binding strength between an antibody and its antigen. The dissociation constant Kd describes the analyte concentration where half of binding sites are occupied under a 1:1 model. Lower Kd means tighter binding. Typical therapeutic antibodies often fall in the low nM to pM range, while discovery hits may start in the uM range. Because labs report Kd across wide scales, consistent unit handling and scientific notation are essential for reliable comparisons.
Association constant Ka is the reciprocal of Kd after converting Kd into molar units. Reporting both helps when comparing literature that prefers Ka. For example, a 10 nM Kd equals 1×10^8 1/M Ka, and a 100 pM Kd equals 1×10^10 1/M. The tool performs the conversion automatically, preserves significant digits, and keeps the displayed unit you selected for readability and reporting.
If temperature is provided, the calculator estimates standard Gibbs free energy change: ΔG° = RT ln(Kd). At 298.15 K, each tenfold decrease in Kd improves ΔG° by about 5.7 kJ/mol (≈1.36 kcal/mol). ΔG° links affinity to the net balance of enthalpic contacts and entropic costs, yet it does not reveal kinetics, epitope accessibility, glycan effects, or conformational penalties that influence function.
Many experiments care about fractional occupancy theta at a chosen antibody concentration. Under a simple Langmuir isotherm, theta = [Ab]/([Ab]+Kd) when antigen is limiting and binding is monovalent. This provides intuition for dosing or assay design: using ten times Kd yields 91% occupancy, three times Kd yields 75%, and equal to Kd yields 50%. If antigen is in excess or multivalent, occupancy can deviate, so treat theta as a planning estimate rather than a prediction.
Affinity estimates depend on the measurement method and model assumptions. Surface plasmon resonance and biolayer interferometry can separate kon and koff, while equilibrium titration and competitive binding often return an apparent Kd. ELISA-derived values may reflect avidity, coating density, and mass transport limits. Bivalent antibodies can appear stronger than true 1:1 affinity, especially on multivalent antigens or clustered receptors. Always report buffer composition, temperature, and fitting model alongside Kd for reproducible interpretation.
Affinity describes a single binding site interaction under a defined model. Avidity reflects combined multivalent interactions, often appearing stronger when antibodies bind clustered or repeating epitopes on a surface.
Kd must be in molar units to compute Ka and ΔG° consistently. Unit conversion prevents mistakes when comparing results reported in nM, uM, or pM.
Not always. Function depends on epitope, kinetics, effector mechanisms, and context. Extremely tight binding can increase off-target risks or reduce tissue penetration in some scenarios.
Yes. Under a 1:1 interaction, Kd = koff/kon. Make sure your rates share compatible units and reflect the same experimental conditions.
Use the temperature of your binding experiment, typically 20–25°C for many in vitro assays or 37°C for physiological modeling. ΔG° changes with temperature through the RT term.
ELISA often measures an apparent value influenced by immobilization, avidity, and transport limits. SPR or BLI can better control binding geometry and may separate kinetic contributions.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.