The conformational properties of the delta opioid peptide [ D-Pen²,D-Pen⁵ ]enkephalin in aqueous solution determined by nmr and energy minimization calculations

The conformational properties of the highly potent delta opioid receptor selective cyclic peptide [D-Pen²,D- Pen⁵]enkephalin (DPDPE) have been investigated by use of one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy, molecular modeling based on the NMR results, and molecular mechanics energy minimization. A new method for elimination of H₂O (HOD) signals was used which, in conjunction with 2D methods, made possible a complete assignment of all hydrogen atoms in DPDPE. Additional computer simulations allowed an accurate determination of all ³J and ²J coupling constants. NOESY experiments gave direct evidence for transannular interactions of the Tyr¹ and Phe⁴ aromatic rings with the β,β-dimethyl groups of D-Pen². Utilizing NMR parameters in conjunction with model building, extensive energy minimization studies led to two pairs of very similar energy-minimized conformations for DPDPE. Each pair of conformations primarily differed by the sign of the disulfide helicity. One pair was of lower energy and satisfied all of the NMR criteria. Its conformation is distinguished by an amphiphilic conformation with a type IV β-turn and transannular interactions between the aromatic side chains of Tyr¹ and Phe⁴ with the β,β-dimethyl groups of D-Pen². The factors which stabilize this conformation are discussed, and the possible relationship of this conformation to the high delta opioid receptor selectivity is suggested.