5-Ethyl-2'-hydroxy-2-[(1-hydroxycyclopropyl)methyl]-9,9-dimethyl-6,7-benzomorphan hydrochloride (bremazocine), C20H29NO2.HCl

Opioid receptor three-dimensional structures from distance geometry calculations with hydrogen bonding constraints

Three-dimensional structures of the transmembrane, seven alpha-helical domains and extracellular loops of delta, mu, and kappa opioid receptors, were calculated using the distance geometry algorithm, with hydrogen bonding constraints based on the previously developed general model of the transmembrane alpha-bundle for rhodopsin-like G-protein coupled receptors (Biophys. J. 1997. 70:1963). Each calculated opioid receptor structure has an extensive network of interhelical hydrogen bonds and a ligand-binding crevice that is partially covered by a beta-hairpin formed by the second extracellular loop. The binding cavities consist of an inner "conserved region" composed of 18 residues that are identical in delta, mu, and kappa opioid receptors, and a peripheral "variable region," composed of 19 residues that are different in delta, mu, and kappa subtypes and are responsible for the subtype specificity of various ligands. Sixteen delta-, mu-, or kappa-selective, conformationally constrained peptide and nonpeptide opioid agonists and antagonists and affinity labels were fit into the binding pockets of the opioid receptors. All ligands considered have a similar spatial arrangement in the receptors, with the tyramine moiety of alkaloids or Tyr1 of opioid peptides interacting with conserved residues in the bottom of the pocket and the tyramine N+ and OH groups forming ionic interactions or H-bonds with a conserved aspartate from helix III and a conserved histidine from helix VI, respectively. The central, conformationally constrained fragments of the opioids (the disulfide-bridged cycles of the peptides and various ring structures in the nonpeptide ligands) are oriented approximately perpendicular to the tyramine and directed toward the extracellular surface. The results obtained are qualitatively consistent with ligand affinities, cross-linking studies, and mutagenesis data.

X-ray molecular structures and theoretical conformational studies of narcotic analgesics alpha-(-)-N-cis-3-chloroallyl-normetazocine, ethylketazocine, and ketazocine

The X-ray molecular structures of the narcotic analgesics alpha-(-)-2-cis-3-chlorallyl-2'-hydroxy-5,9-dimethyl-6,7-benzomorp han (1) and alpha-(+-)-2-cyclopropylmethyl-2'-hydroxy-5-ethyl-9-methyl-8-oxo-6,7- benzomorphan (ethylketazocine, 2) were determined. The structures and conformations in the crystal were compared and discussed with respect to that of alpha-(+-)-2-cyclopropylmethyl-2'-hydroxy-5,9-dimethyl-8- oxo-6,7-benzomorfan (ketazocine, 3) and those of 15 analogous compounds of the 2'-hydroxy-6,7-benzomorphan series whose structures were previously determined by X-ray analysis. Molecular modeling routines for 1, 2, and 3 produced configurations (N-equatorial) and conformations (distorted chair) of the piperidine ring that were in agreement with those found in the solids. Theoretical studies of the conformations and the rotational energetics of 1, 2, and 3 as cationic species were performed by both the force field (MM2) and the semiquantitative (AM1) methods. The latter method predicted three low energy conformations about N--C(12) and C(12)--C(13) bonds, one of these being more significantly populated (60-68%). The AM1 results were not reproduced by the MM2 method, which predicted four low energy conformations. An interesting common feature of 1, 2, and 3 that was noted with both methods was the restricted interconversion route from the conformational state to another through rotations about the C(12)--C(13) bond. The conformational results were discussed in terms of a working hypothesis for regulation of relative mu and kappa analgesic activities of benzomorphans.