Binding constant
Consider reaction：R + L ⇌ RL， we haveKon[R][L]=Koff[R][L]
then equilibrium association constant Ka = Kon/Koff，the reciprocal of Ka is equilibrium dissociation constant(Kd)。Binding affinity is typically measured and reported by the equilibrium dissociation constant。
Kd has a quantitative relationship with ΔG(molar Gibbs free energy)：ΔG=RTlnKd
the relation between ΔG and Kd at 298K（25℃）
| ΔG(kcal/mol) | Kd(mol/L) |
| -12 | 1.56E-09 |
| -11.5 | 3.64E-09 |
| -11 | 8.47E-09 |
| -10.5 | 1.97E-08 |
| -10 | 4.59E-08 |
| -9.5 | 1.07E-07 |
| -9 | 2.49E-07 |
| -8.5 | 5.79E-07 |
| -8 | 1.35E-06 |
| -7.5 | 3.14E-06 |
| -7 | 7.30E-06 |
| -6.5 | 1.70E-05 |
| -6 | 3.95E-05 |
| -5.5 | 9.20E-05 |
| -5 | 2.14E-04 |
| -4.5 | 4.99E-04 |
| -4 | 1.16E-03 |