Additional conformational data for the mapping of the progestin binding site: crystal structures of 21-(phenylseleno)progesterone and 17 alpha-(phenylseleno)progesterone.

: The crystal structures of 21-(phenylseleno)progesterone (1), which binds with moderate affinity to the progestin receptor, and 17 alpha-(phenylseleno)progesterone (2), which binds hardly at all, have been studied in an attempt to explain these differences in affinity and to obtain further information on the location of the progestin receptor binding site with respect to the progesterone molecule. The crystal structures were refined by isotropic thermal approximation to R values of 0.082 for 1 and 0.084 for 2. The unusual 17 beta side-chain orientation of 2 with a C16-C17-C20-O20 torsion angle of +13 degrees compared to -7 degrees for progesterone would seem to preclude hydrogen bonding with the progestin receptor binding site and provides strong supporting evidence for the contention that this site is located above the beta face of the molecule. Any rotation of the C21 methyl group into a more appropriate position is furthermore impeded by the presence of the 17 alpha-phenylseleno substituent. On the other hand, some hydrogen bonding can occur in the case of 1 (C16-C17-C20-O20 = 31 degrees) despite the fact that the difference in torsion angle (24 degrees) with respect to progesterone is, in absolute values, greater than that for 2 (20 degrees). This is because the orientation of the 17 beta-acetyl side chain of 1 is directed above the beta face closer to the progestin binding site, as previously defined on the basis of data on a large number of molecules, and because the 21-phenylseleno substituent constitutes only limited steric hindrance to binding. Thus, the difference in affinity of these two compounds is entirely consistent with observations that the H-bond donor is located toward O20 in the beta region of C16.