Conformational Determinants of Agonist versus Antagonist Properties of [d-Pen²,d-Pen⁵]Enkephalin (DPDPE) Analogs at Opioid Receptors. Comparison of X-ray Crystallographic Structure, Solution ¹H NMR Data, and Molecular Dynamic Simulations of [l-Ala³]DPDPE and [d-Ala³]DPDPE

c-[d-Pen²,d-Pen⁵]enkephalin (DPDPE, 1) is a cyclic, constrained, highly potent, δ opioid receptor selective peptide agonist. Substitution of Gly³ with l-Ala in DPDPE to give [l-Ala³]DPDPE (2) has been shown to produce a peptide with much greater δ receptor binding selectivity than DPDPE itself. However [l-Ala³]DPDPE is only a partial agonist in in vivo antinociception and actually was found to potently antagonize the antinociceptive effects of DPDPE at δ receptors in the brain. In comparison, [d-Ala³]DPDPE (3) is a weak and poorly selective δ agonist. In an effort to correlate the biological profiles of these peptides with secondary structure, [l-Ala³]DPDPE and [d-Ala³]DPDPE were studied by X-ray crystallography and ¹H and ¹³C NMR in DMSO solution. Crystals of both peptides were obtained using vapor diffusion techniques. [l-Ala³]DPDPE crystallizes in the monoclinic space group C2 with cell dimensions a = 36.35(1) Å, b = 19.737(4) Å, c = 28.16(1) Å, β = 129.07(2)°, and V = 15688(9) ų. The asymmetric unit contains four peptide molecules and approximately 20 water molecules, giving a calculated density of 1.274 g cm^-3. The conformation of all four independent [l-Ala³]DPDPE molecules is essentially the same. [d-Ala³]DPDPE crystallizes in the monoclinic space group P2₁ with cell dimensions a = 12.271(2) Å, b = 9.600(a) Å, c = 18.750(4) Å, β = 103.56(2)°, and V = 2147.2(7) ų. The asymmetric unit contains one peptide molecule and 10 molecules of water, giving a calculated density of 1.298 g cm^-3. Comparison of these X-ray structures with the crystal structure previously reported for DPDPE indicates that there are differences in the disulfide bond region for all three peptides. ROEs determined about the disulfide regions of 1−3 in solution are indicative of a high degree of conformational interconversion, while heteronuclear coupling constants between the d-Pen^2,5 Hα and Cγ,γ‘ carbons indicate a strong preference for a gauche (+) χ¹ angle in 2. The backbone conformations of DPDPE and [d-Ala³]DPDPE in the X-ray structures are virtually identical, while in [l-Ala³]DPDPE, there is a rotation of approximately 160° about both ψ² and φ³ compared to DPDPE which has the effect of rotating this amide group approximately 180° relative to DPDPE. The solution NMR data for the peptide backbone conformations of 2 and 3 are mainly consistent with their X-ray structures. However, MD simulation of all three compounds, starting with the geometries of their X-ray structures, indicates that by comparison of observed and predicted ROE intensities an equilibrium between these conformations is likely in solution. The “DPDPE-like” conformation for [l-Ala³]DPDPE is however significantly higher in energy than the X-ray structure reported here and, thus, is predicted to be less populated in solution and in receptor binding. It is concluded that the X-ray structure of DPDPE represents an agonist conformation for this peptide at the δ opioid receptor and that the corresponding X-ray structure of [l-Ala³]DPDPE represents an antagonist conformation due to the differences in conformation between positions 2 and 3. Considerations on the structural implications of this conformational difference on receptor binding are discussed.