where [P ], [L] and [LP ] are the molar concentrations of the protein, ligand, and complex respectively and c is the standard state concentration, taken to be 1 M by convention for a ligand in solution. In the simple, and most common case, of one ligand binding in one binding site, we can calculate K D as the dimensionless ligand concentration, [L] c , at the point at which the concentration of the bound protein is equal to the concentration of the free protein, [LP ] = [P ].