DESIGN AND COMPREHENSIVE CONFORMATIONAL STUDIES OF TYR(1)-CYCLO(D-PEN(2)-GLY(3)-PHE(4)-L-3-MPT(5)) AND TYR(1)-CYCLO(D-PEN(2)-GLY(3)-PHE(4)-D-3-MPT(5)) - NOVEL CONFORMATIONALLY CONSTRAINED OPIOID-PEPTIDES

Two compounds, Tyr(1)-cyclo(D-Pen(2)-Gly(3)-Phe(4)-L-3-Mpt(5)) (DPMPT; 3-Mpt is trans-3-mercaptoproline) and Tyr(1)-cyclo(D-Pen(2)-Gly(3)-Ph e(4)-D-3-Mpt(5)) (DPDMPT), were designed employing energy calculations . Geometrical comparison showed that some low-energy 3D structures of DPMPT and DPDMPT are compatible with the model for the delta-receptor- bound conformation of the well-known delta-selective DPDPE peptide Tyr (1)-cyclo(D-Pen(2)-Gly(3)-Phe(4)-D-Pen(5), which was proposed by us ea rlier. DPMPT and DPDMPT were tested for their binding to delta-, mu- a nd kappa-opioid receptors. The corresponding K-i values were 3.5, 68, and >5000 nM for DPMPT, and 103.7, >5000, and >5000 nM for DPDMPT, res pectively. Independent studies by homo- and heteronuclear NMR spectros copy and energy calculations showed that DPMPT exists in DMSO solution in conformational equilibrium among several backbone conformations wi th the same type of 3D structure for the cyclic moiety, but with somew hat different conformers of the acyclic part of the molecule and two t ypes of rotamers for the D-Pen side chain, namely, t and g(-). For DPD MPT, energy calculations combined with the NMR data suggest that any o ne out of four low-energy conformers belonging to the same type of the backbone of the cyclic moiety may be a possible candidate for the DPD MPT conformer in DMSO. The DPDMPT structure revealed by X-ray crystall ography showed remarkable similarity to DPDMPT solution conformations. The determined solution conformations of both compounds were compared to their suggested delta-receptor-bound conformers. Results of compar ison showed that all four of the possible solution conformations of DP DMPT are nonsimilar to the DPDMPT delta-receptor-bound conformation, w hereas two of the possible solution conformations of DPMPT are compati ble with the suggested delta-receptor-bound conformation of DPMPT. Thi s finding can explain the difference in binding of DPMPT and DPDMPT to delta-opioid receptors by a suggestion that the delta-receptor-bound conformation of DPMPT already preexists in solution, whereas solution conformations of DPDMPT should be more significantly distorted to matc h the delta-receptor-bound conformation of DPDMPT.