Understanding Binding Free Energy
Binding free energy describes the strength of an interaction. In physics, it links molecular affinity with thermodynamic work. A negative value usually means favorable binding under the chosen standard state. A more negative value often means a tighter complex. The calculator uses affinity constants, temperature, and optional thermodynamic terms. It then returns ΔG in joules, kilojoules, and kilocalories per mole.
Why Temperature Matters
Temperature changes the scale of thermal energy. The factor RT sets how strongly an equilibrium constant affects ΔG. At higher temperature, the same affinity shift can create a different energy difference. This is why the temperature field is important. Enter the experimental temperature when available. Use room temperature only when a study does not provide a better value.
Affinity Inputs and Modes
Most users enter Kd, Ki, IC50, or Ka. Kd and Ki act like dissociation constants. Smaller values indicate tighter binding. Ka is the association constant, so larger values indicate tighter binding. For IC50, the tool can apply the Cheng-Prusoff correction. That correction needs substrate concentration and Km. If those fields are zero, IC50 is treated as an approximate Ki. The tool also accepts direct ΔG input. It can reverse the equation and estimate Kd from energy.
Interpreting Results
The pKd value is a compact affinity score. Each unit increase means a tenfold lower Kd. The occupancy estimate uses ligand concentration and Kd. It predicts the bound fraction for a simple one site model. The ΔΔG field compares your case with a reference affinity. This is useful when ranking variants, ligands, or conditions. A fold change greater than one means stronger binding than the reference. Uncertainty is estimated from the relative affinity error. It is only a first order guide.
Practical Notes
Binding calculations assume equilibrium behavior. They also assume a defined standard concentration. Most reports use one molar standard state. Real systems may include activity effects, proton coupling, salt effects, and conformational change. Therefore, results should support judgment, not replace it. Record all inputs with units. Compare only experiments made under similar conditions. Use exported files to keep a clear audit trail for reports. Check assumptions before sharing values. Small unit mistakes can change energy by several kilocalories per mole quickly.