kD

Kd, short for the dissociation constant, is a parameter used in chemistry and biochemistry to quantify the strength of the interaction between two species, typically a receptor and a ligand or a protein and a small molecule. For a binding reaction P + L ⇌ PL, the dissociation constant Kd is defined as [P][L]/[PL] at equilibrium. A smaller Kd indicates a higher affinity, meaning the complex is more stable and dissociates less readily.

Kd is the reciprocal of the association constant Ka (Ka = 1/Kd); sometimes the negative logarithm pKd = −log10(Kd) is used, analogous to pH or pKa scales. Kd is expressed in units of concentration, typically molarity (M).

Ranges of Kd vary widely: sub-picomolar to picomolar for extremely tight interactions, nanomolar to micromolar for moderate affinities, and higher Kd for weak interactions. Kd values depend on temperature, ionic strength, pH, and presence of cofactors. Comparisons of Kd values should consider identical experimental conditions.

Common methods to determine Kd include surface plasmon resonance, isothermal titration calorimetry, equilibrium dialysis, and fluorescence-based assays. These techniques yield binding curves from which Kd can be extracted by fitting to appropriate binding models.

Applications of Kd include drug discovery and affinity maturation, protein engineering, and mechanistic studies of signaling pathways. Kd provides a quantitative metric for comparing interactions and guiding optimization of ligands.

Outside biochemistry, the symbol KD may denote other concepts in different fields, such as potassium deuteride (chemical formula KD) or the kd-tree data structure in computer science. Context typically clarifies the intended meaning.