Affinity is the thermodynamic strength of the non-covalent interaction between a ligand and its target, typically quantified by the equilibrium dissociation constant Kd. Kd corresponds to the ligand concentration at which half of the target sites are occupied; a lower Kd therefore denotes higher affinity.
In competition-binding experiments, affinity is often reported as an inhibition constant Kᵢ, derived from the measured IC50 using the Cheng-Prusoff equation. Affinity is independent of efficacy: a high-affinity ligand may be an agonist, an antagonist or an inverse agonist depending on the receptor conformation it stabilises.
Affinity determines the concentration range at which a research peptide can act at its target and is a core input to selectivity profiling across receptor subtypes.