Calculator Inputs
Example Data Table
| Ref T (°C) | Target T (°C) | Kd,ref | ΔH (kJ/mol) | R0 | L0 | Kd,target | Fraction bound |
|---|---|---|---|---|---|---|---|
| 25 | 37 | 120 nM | -35 | 1 uM | 0.5 uM | 207.216 nM | 0.3754 |
| 20 | 40 | 5 uM | 25 | 1 uM | 10 uM | 2.597 uM | 0.7802 |
| 25 | 10 | 50 uM | -10 | 2 uM | 2 uM | 40.3795 uM | 0.0452 |
These examples are generated using the same equations implemented in this page.
Formula Used
How to Use This Calculator
- Select whether your reference constant is Kd or Ka.
- Enter the reference value, unit, and the reference temperature.
- Enter the target temperature and ΔH (kJ/mol) for the binding reaction.
- Optionally enter receptor and ligand totals to estimate fraction bound.
- Click Calculate to show results above the form.
- Use Download CSV or Download PDF to export the latest result.
Temperature-dependent affinity across assay conditions
Binding constants are typically reported at one temperature, yet chromatography rooms, incubators, and plate readers rarely match that setpoint. This calculator translates a reference Kd or Ka to a target temperature so you can compare screens and choose buffer conditions rationally. A 10 °C shift can change Kd by 1.2× to 5×, altering hit ranking for faster decisions.
Using ΔH to extrapolate Kd and Ka
The engine applies a constant-enthalpy van’t Hoff relationship to link two temperatures. With ΔH = −35 kJ/mol, Kd = 120 nM at 25 °C becomes about 207 nM at 37 °C, reflecting weaker binding at higher temperature for exothermic association. For endothermic cases (positive ΔH), affinity can strengthen with heating. Treat ΔH as an experimental input from ITC, global fitting, or literature averages.
Unit discipline and standard-state assumptions
Kd and Ka are reciprocals only when both use the same concentration standard. The calculator normalizes to the 1 M standard state, then converts to your chosen output unit (nM, µM, mM, or M). This is useful when mixing assay stocks, where errors often come from silent unit mismatches. Report the unit, the temperature, and whether Ka was entered as “per unit” so collaborators can reproduce the math.
Occupancy planning with finite concentrations
When receptor and ligand totals are supplied, the tool estimates complex formation using a 1:1 mass-balance solution, not the infinite-ligand approximation. This matters when L0 is comparable to Kd or to R0, such as tight binders or scarce targets. Use fraction bound to select dosing points, design competition assays, and predict whether a measured signal should saturate. If fraction bound is low, raise ligand, lower temperature, or reconsider buffer.
Communicating uncertainty and next-step experiments
Van’t Hoff extrapolation is a model, not a guarantee: ΔH may vary with temperature, and heat capacity changes (ΔCp) can bend ln K versus 1/T. Use outputs as planning estimates, then verify with at least one measurement near your operating temperature. Exported CSV and PDF summaries help capture assumptions, parameter values, and calculated Kd(T) for review. When results drive decisions, document sources, replicate, and update ΔH as new data arrives.
FAQs
1) What does a negative ΔH imply for temperature effects?
Negative ΔH indicates exothermic association. Under the constant-ΔH van’t Hoff model, increasing temperature often raises Kd and reduces affinity. Entropic contributions can counteract this, so confirm with at least one measurement near your assay temperature.
2) Can I enter Ka instead of Kd?
Yes. Select Ka, enter the value, and choose the unit basis shown in the dropdown. The calculator converts internally to 1/M, then reports Ka(T) and the corresponding Kd(T) in your selected output unit.
3) Why does the model use Kelvin?
Van’t Hoff relationships require absolute temperature. The tool converts °C to K automatically and blocks values at or below −273.15 °C. Always report the target temperature used for the final Kd(T) and ΔG° values.
4) How is the fraction bound calculated?
If you provide receptor and ligand totals, the calculator solves the 1:1 mass-balance quadratic to estimate complex concentration [RL]. Fraction bound is [RL]/R0, which is more accurate than assuming ligand is in large excess.
5) What are common input pitfalls?
The most frequent issues are unit mismatches, mixing stock and assay concentrations, and using a ΔH measured in a different buffer or pH. Also remember ΔH uncertainty propagates into Kd(T), especially over larger temperature gaps.
6) When should I avoid extrapolating?
Avoid extrapolation when the temperature range is wide, binding stoichiometry changes, or ΔH varies strongly with temperature (large ΔCp). In those cases, measure at multiple temperatures or fit a model that includes heat capacity effects.