The rate of access to the organic ligand-binding region of serum albumin is entropy controlled.

The technique of real time dielectric relaxation measurement coupled with a conventional stopped-flow device has made it possible to measure the rates of association and dissociation of the complex of human serum albumin with its most prevalent ligands, the long-chain fatty acids. This association was previously shown to proceed in two steps: a fast, probably diffusion-controlled, nonspecific association, followed by a slower (approximately 3 sec--1) rearrangement of the intermediate protein--ligand configuration, whose kinetics is first order. By use of the Arrhenius relation and standard theory of rate processes it is determined that there is virtually no activation enthalpy in the forward binding reaction and that the rate of access to the interior hydrophobic binding region of serum albumin is controlled by a negative entropy of activation, reflecting a high degree of ordering in the transition state. A complete thermodynamic and kinetic profile of the association reaction is given.