Synthesis, Biological Evaluation, and Automated Docking of Constrained Analogues of the Opioid Peptide H-Dmt-D-Ala-Phe-Gly-NH2 Using the 4- or 5-Methyl Substituted 4-Amino-1,2,4,5-tetrahydro-2-benzazepin-3-one Scaffold

The Phe(3) residue of the N-terminal tetrapeptide of dermorphin (H-Dmt-D-Ala-Phe-Gly-NH2) was conformationally constrained using 4- or 5-methyl-substituted 4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-one (Aba) stereoisomeric scaffolds. Several of the synthesized peptides were determined to be high affinity agonists for the mu opioid receptor (OPRM) with selectivity over the delta opioid receptor (OPRD). Interesting effects of the Aba configuration on ligand binding affinity were observed. H-Dmt-D-Ala-erythro-(4S,5S)-5-Me-Aba-Gly-NH2 9 and H-Dmt-threo-(4R,5S)-5-Me-Aba-Gly-NH2 12 exhibited subnanomolar affinity for OPRM, while they possess an opposite absolute configuration at position 4 of the Aba ring. However, in the 4-methyl substituted analogues, H-Dmt-D-Ala-(4R)-Me-Aba-Gly-NH2 14 was significantly more potent than the (4S)-derivative 13. These unexpected results were rationalized using the binding poses predicted by molecular docking simulations. Interestingly, H-Dmt-D-Ala-(4R)-Me-Aba-Gly-NH2 14 is proposed to bind in a different mode compared with the other analogues. Moreover, in contrast to Ac-4-Me-Aba-NH-Me, which adopts a beta-turn in solution and in the crystal structure, the binding mode of this analogue suggests an alternative receptor-bound conformation.