Binding affinities of host-guest, protein-ligand, and protein-transition-state complexes

The affinities of hosts-ranging from small synthetic cavitands to large proteins-for organic molecules are well documented. The average association constants for the binding of organic molecules by cyclodextrins, synthetic hosts, and albumins in water, as well as of catalytic antibodies or enzymes for substrates are 10(3.5+/-2.5) M(-1). Binding affinities are elevated to 10(8+/-2) M(-1) for the complexation of transition states and biological antigens by antibodies or inhibitors by enzymes, and to 10(16+/-4) M(-1) for transition states with enzymes. The origins of the distributions of association constants observed for the broad range of host-guest systems are explored in this Review, and typical approaches to compute and analyze host-guest binding in solution are discussed. In many classes of complexes a rough correlation is found between the binding affinity and the surface area that is buried upon complexation. Enzymes transcend this effect and achieve transition-state binding much greater than is expected from the surface areas.