AR-SARM complexes seen in multiple orientations. (a) Overlay of S-4 (purple), S-21 (salmon), and S-24 (yellow) demonstrating interactions between the B-ring para substituent and residues His874, Ile898, Val903, and the backbone of Gln738. Notice that each of these ligands hydrogen bonds to a conserved water molecule in this region. The acetamido group on S-4 forms close contacts with the side chains of Ile898 and Val903 inducing a slight displacement of helix 12. (b) Overlay of S-1 (black, PDB code 2AXA 18), C-31 (orange), and C-23 (green) demonstrating differences in B-ring orientations with respect to Trp741. Substitution of a fluorine on the meta position of the B-ring in C-31 causes the B-ring to turn relative to S-1. This orientation of the B-ring appears to result in an edge-to-face aromatic ring interaction with Trp741 possibly due to the increased acidity of the remaining hydrogen atom on the meta position. C-23 contains a completely fluorine substituted B-ring, which orients nearly parallel to the Trp741 indole ring creating a π-π interaction and results in displacement of the Met745 side chain. Additionally, the fluorine on the ortho position seems to be acting as a hydrogen bond acceptor pulling the B-ring of C-23 towards the amide nitrogen of the compound.

Comparison of SARM binding conformations to the AR. (a) S-4 (carbons - purple), (b) S-24 (carbons - yellow), (c) S-21 (carbons - salmon), (d) C-31 (carbons - orange), (e) C-23 (carbons - green) fit into Fo-Fc similated annealing omit maps. AR carbons – gray; nitrogens – blue; oxygens – red; sulfurs – orange; fluorines – magenta; chlorine - green. Possible hydrogen bonds within 3.5 Å depicted as dashed lines. Each of the compounds forms hydrogen bonds to Arg752, Leu704, and Asn705. SARMs with a nitro substituent on the A-ring (i.e., S-4, S-21, and C-23) also form a hydrogen bond to Gln711. SARMs with a hydrophilic B-ring substituent (i.e. S-4, C-21, and C31) form a hydrogen bond to a water molecule that is stabilized by His874 and backbone residues of helices 4 and 5. (f) Overlap of the bound conformations of these SARMs portrays the different B-ring orientations.