The Chemical Interaction between the Estrogen Receptor and Monohydroxybenzo[a]pyrene Derivatives Studied by Fluorescence Line-Narrowing Spectroscopy

A novel approach is presented for studying the chemical interaction between receptor binding sites and ligands. Monohydroxylated polyaromatic compounds were found to be environmentally sensitive ligands when applying a special mode of fluorescence: fluorescence line-narrowing spectroscopy (FLNS). With this technique, solvent dependencies and ligand−receptor interactions can be studied in great detail, due to the high spectral resolution and the fact that at cryogenic temperatures (4 K), no solvent reorientation effects complicate the interpretation. The FLN spectrum of a ligand bound to the receptor is compared to the spectra of the free ligand in solvent mixtures that mimic the functionalities present within the receptor's binding site. It is shown that for the well-known estrogen receptor (ER), the orientations of two xenoestrogenic ligands 3- and 9-hydroxybenzo[a]pyrene (3- and 9-OH-BaP) can be determined. The FLN results clearly indicate that an H-bond accepted by HIS524 plays a major role in the binding of these ligands to the ER. Furthermore, the spectra indicated a π−π stacking aromatic interaction for 9-OH-BaP with PHE404. These results are in line with molecular modeling studies published earlier.