Kd Kd – is the equilibrium dissociation constant for a drug. It describes the affinity between a ligand (L) (such as a drug) and a protein (P). Ligand-protein affinities are influenced by non-covalent intermolecular interactions between the two molecules such as hydrogen bonding, electrostatic interactions, hydrophobic and Van der Waals forces. Kd = ([P] [L]) / [PL] The dissociation constant has molar units (M), which corresponds to the concentration of ligand [L] at which the binding site on a particular protein is half occupied, i.e. the concentration of ligand, at which the concentration of protein with ligand bound [PL], equals the concentration of protein with no ligand bound [P]. The smaller the dissociation constant, the more tightly bound the ligand is, or the higher the affinity between ligand and protein. http://www.graphpad.com/curvefit/law_of_mass_action.htm binding constant