Abstract
The constitutional similarity with different secondary structure preference between the Aba-Gly and the spiro-Aba-Gly scaffolds were exploited to design the novel endomorphin-2 analogs Tyr-spiro-(R/S)-Aba-Gly-Phe-NH2 (1 and 2) and Tyr-(R/S)-Aba-Gly-Phe-NH2 (3 and 4). The (R)-spiro analog 1 was found to be a potent and selective μ-opioid agonist/partial agonist (Kiμ = 29.3 nM, IC50 = 50 nM, Ke = 0.57). NMR experiments and molecular modeling indicated that its backbone adopts mainly a β-turn in aqueous solution.