The binding of fluorescent 4,6,8(14)-triene-3-one steroids to cyclodextrins as a model for steroid-protein interactions.

The 4,6,8(14)-triene-3-one steroids, highly fluorescent in aqueous solutions, lose their fluorescence power when binding occurs to hydrophobic regions of other molecules, such as the hydrophobic cavity in the ring system of cyclodextrins. The fluorescence intensity decreases almost completely when beta- and gamma-cyclodextrins are present in the solution. Scatchard plots derived from fluorescence titrations show that one or two molecules of steroid bind to one cyclodextrin molecule with KD,F-values of about 10(-4)-10(-5) mol/liter. Temperature-jump experiments show a single relaxation process, with rate constants for the decay of the beta-cyclodextrin-steroid complexes of about 10(4)-10(5) per s. For alpha- and gamma-cyclodextrins such relaxation processes are not observed.