Synthesis of a spin-labeled anti-estrogen as a dynamic motion probe for the estrogen receptor ligand binding domain.

The preparation and characterization of a novel nitroxide spin probe based on a steroidal anti-estrogen is described. The probe 5 demonstrated very high binding affinity for both the alpha and beta isoforms of the estrogen receptor-ligand binding domain. EPR spectrometric studies demonstrate conformational constraints for the ligand, consistent with the nitroxyl moiety occupying a position just beyond the receptor-solvent interface.