THE 55-PERCENT SOLUTION - A FORMULA FOR MOLECULAR RECOGNITION IN THE LIQUID-STATE

Evidence is presented that molecular recognition through encapsulation processes is largely determined by the volumes of the guest and host. Binding of molecules of suitable dimensions in the internal cavity of a molecular receptor in solution can be expected when the packing coe fficient, the ratio of the guest volume to the host volume, is in the range of 0.55 +/- 0.09. Larger packing coefficients, up to 0.70, can b e reached if the complex is stabilized by strong intermolecular forces such as hydrogen bonds. These considerations also apply to situations in which more than one molecule is encapsulated. Organic liquids are generally characterized by the same packing factors as encapsulation c omplexes, and it is proposed that the short-range structure of liquids and the complexes resulting from encapsulation are two aspects of the same phenomenon. Similar volume considerations are expected to apply to the binding of substrates in biological receptors.