X-ray and nuclear magnetic resonance (NMR) studies of signalizing the tripeptide sequence (Tyr-D-Ala-Phe) of dermorphin and deltorphins I and II. Comparative analysis in the liquid and solid phases

The crystal and molecular structure of compound 1, the "message" Tyr-D-Ala-Phe sequence of dermorphin and deltorphins I, II opioid peptides, was established using X-ray diffraction methods at a temperature of 100 K. Crystals of 1 are monoclinic, with the space group C2. The peptide chain has beta-conformation. All three side groups are located on the same side of the peptide chain, because of the D-conformation of the central alanine (Ala). The H atoms of the methyl group create a C-H(...)pi interaction with the phenyl ring of tyrosine (Tyr). The distances between the methyl group of D-Ala and the carbons of the phenyl ring of Tyr are in the range of 4.007-4.089 Angstrom. NMR spectroscopy measurements were performed in the liquid and solid states, to conclude a higher-order structure of 1 in both phases and correlate with X-ray data. The PASS-2D experiment was used to assign principal elements of the chemical shift tensor C-13 delta(ii). The differences between the experimental values of C-13 delta(ii) and the theoretical shielding parameters of C-13 sigma(ii) that are obtained using DFT GIAO calculation are explained in terms of distinction of the local motion of phenyl rings of Tyr and phenylalanine (Phe) at ambient temperature. C-13 T-1 measurements, analysis of the cross-polarization (CP) kinetics, and data obtained from dipolar dephasing experiment clearly proved the unique, dynamic features of tripeptide 1.