Theoretical conformational analysis of the opioid δ antagonist H‐Tyr‐Tic‐Phe‐OH and the μ agonist H‐Tyr‐D‐Tic‐Phe‐NH2

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Abstract
A molecular mechanics study (grid search and energy minimization) of the highly δ receptor‐selective δ opioid antagonist H‐Tyr‐Tic‐Phe‐OH (TIP; Tic: tetrahydroisoquinoline‐3‐car‐boxylic acid) resulted in four low energy conformers with energies within 2 kcal/mol of that of the lowest energy structure. These four conformers contain trans peptide bonds only and represent compact structures showing various patterns of aromatic ring stacking. The centrally located Tic residue imposes several conformational constraints on the N‐terminal dipeptide segment; however, the results of molecular dynamics simulations indicated that this tripeptide still shows some structural flexibility, particularly at the Phe3 residue. Analogous studies performed with the structurally related μ receptor‐selective μ agonist H‐Tyr‐D‐Tic‐Phe‐NH2 resulted in low energy structures that were also compact but showed patterns of ring stacking different from those obtained with TIP. Superim‐position of low energy conformers of TIP and H‐Tyr‐D‐Tic‐Phe‐NH2 revealed that the Phe3 residues of the L‐Tic‐ and the D‐Tic peptide were always located on opposite sides of the plane defined by the Tic residue, thus providing an explanation for the distinct activity profiles of the two compounds in structural terms. Attempts to demonstrate spatial overlap between the pharmacophoric moieties of low energy conformers of TIP and the nonpeptide δ antagonist naltrindole were made by superimposing either the Tyr1 and Tic2 aromatic rings and the N‐terminal amino group or the Tyr1 and Phe3 aromatic rings and the N‐terminal amino group of the peptide with the corresponding aromatic rings and nitrogen atom in the alkaloid structure. In each case a low energy structure of TIP was found that showed good spatial overlap of all three specified pharmacophoric groups. These two conformers may represent candidate structures for the δ receptor‐bound conformation of TIP. © 1994 John Wiley & Sons, Inc.