Donor-Acceptor Tetrahydrochrysenes, Inherently Fluorescent, High-Affinity Ligands for the Estrogen Receptor: Binding and Fluorescence Characteristics and Fluorometric Assay of Receptort

We have examined the binding behavior and fluorescence characteristics of a series of novel ligands for the estrogen receptor (ER). These ligands are derivatives of 5, 6, 11, 12-tetrahydrochrysene (THC), a structure that embodies a stilbene chromophore, found in many nonsteroidal estrogens, within a rigid tetracyclic system where it cannot easily be distorted from planarity, thus providing the conjugation and rigidity required for efficient fluorescence. Additional steric bulk, as trans-disposed ethyl substituents at the internal C-5 and C-11 positions, is required for the highest relative binding affinity (RBA), and the trans-5, 11-diethyl-2, 8-dihydroxy-THC derivative binds to ER with an affinity greater than that of estradiol. The replacement of one of the phenolic hydroxyl groups of this THC derivative with an electron-withdrawing group (COMe, COOMe, CONH₂, CN, or NO₂) yields unsymmetrical THCs with binding affinities 1540% that of estradiol (E₂). The fluorescence emission shifts from about 380 nm for the dihydroxy THC to 475-688 nm for the donor-acceptor THCs. The emission of these donor-acceptor THCs is highly solvatochromic and shifts to longer wavelengths as the solvent polarity increases. In ethanol, the fluorescence quantum yield of the first four of these compounds is high (Φ_f = 0.43-0.69), but the fifth compound, the nitro-THC, is almost nonemissive in protic solvents. When they are incubated with protein solutions containing ER 10^-9M), the emission from the donor-acceptor THCs bound specifically to ER is in the 500-570-nm range, whereas fluorescence from non-receptor-bound fluorophores is in the 425-460-nm range. Thus, fluorescence from these probes bound specifically to ER could be measured under equilibrium conditions as well as after the removal of free and non-receptor-bound material by treatment with charcoal-dextran. This is one of the first demonstrations of ligands whose fluorescence is distinctly different when free, when bound to ER, or when bound to non-receptor proteins. It is also the first demonstration of ER assay by fluorescence under equilibrium conditions.