Quantitative measure of the strength of an interaction between two molecules at equilibrium, commonly expressed by an equilibrium dissociation constant (Kd), association constant (Ka) or related free energy (ΔG).

Principle

Binding affinity is governed by the balance of molecular interactions (hydrogen bonds, electrostatics, hydrophobic forces) and entropy, and is formalized by the law of mass action relating bound and unbound species at equilibrium.

Demonstration

Determine Kd by techniques such as surface plasmon resonance (sensorgrams giving kon and koff), isothermal titration calorimetry (direct binding isotherm), or equilibrium binding assays; report values with experimental conditions (temperature, buffer) and, where relevant, stoichiometry.

Misapplication

Interpreting a low Kd (high affinity) as proof of biological specificity or efficacy without considering concentration, kinetics, avidity or cellular context; or comparing affinities measured under different ionic strengths or temperatures as if equivalent.

Consequence

Accurate affinity measurements predict the fraction of complex formation at given concentrations, inform drug potency, receptor occupancy models and the design of ligands with desired on/off rates.

Reversal

Low affinity or weak binding: interactions with high Kd, leading to transient or negligible complex formation under physiological concentrations.

Boundary

Refers to equilibrium thermodynamic strength between defined binding partners; excludes purely kinetic descriptions unless kon/koff are reported, and must be distinguished from avidity in multivalent systems and from nonspecific aggregation.

Semantic Tension

Tension between affinity (single‑site thermodynamic strength) and avidity (multivalent cumulative binding): high avidity can compensate for low per‑site affinity, complicating interpretation.

Synthesis

Binding affinity is the equilibrium measure of how tightly two defined molecular partners interact; reported as Kd/Ka/ΔG under specified conditions, it predicts complex prevalence but must be interpreted alongside kinetics, stoichiometry and biological context.