Magic angle spinning NMR study of interaction of N-terminal sequence of dermorphin (Tyr-d-Ala-Phe-Gly) with phospholipids.

Two modifications of the Tyr-d-Ala-Phe-Gly tetrapeptide with different C-terminal groups (Tyr-d-Ala-Phe-Gly-OH 1 and Tyr-d-Ala-Phe-Gly-NH(2)2) were investigated by various nuclear magnetic resonance sequences under magic angle spinning. The structural constraints obtained from the magic angle spinning nuclear magnetic resonance measurements suggest that both peptides are aligned on the surface of the membrane and that the sandwich-like π-CH(3)-π arrangement of the pharmacophore is preserved. The influence of the chemical modification of the C-terminal residue of 1 and 2 on their interaction with phosphate group of the phospholipid in the subgel phase L(c) and the conformation of the peptides in the liquid crystalline phase L(α) are discussed. The correlation between the X-ray structure of 1 in the solid state and 1 embedded into a membrane in the L(c) phase is presented on the basis of the comparative analysis of the two-dimensional (13)C-(13)C dipolar-assisted rotational resonance cross-peaks and the (13)C isotropic chemical shifts.

Study of the mechanism of thermal chemical processes in the crystals of YAF tripeptides by means of mass spectrometry and solid state NMR.

Thermal reactions in two Tyr-Ala-Phe (YAF) tripeptide crystals with different molecular packing (monoclinic and hexagonal), distinct stereochemistry of central amino acid (D or L alanine) and specific arrangement of molecules in the crystal lattice (head-to-tail) were investigated. Samples were heated up to 180 °C, while the melting point for YAF crystals is above the 220 °C. Below the melting temperature, in both cases the chemical reactions leading to formation of cyclic dipeptides (YA diketopiperazine) and leaving of phenylalanine were observed. Two possible mechanisms of chemical reaction in the crystal lattice assuming intra- and/or intermolecular pathways were considered. (13)C and (15)N enriched YAF samples were employed to study of mechanism of solid state reactivity using mass spectrometry and advanced solid state NMR techniques (2D DARR (Dipolar Assisted Rotational Resonance) and 2D Double CP (Cross-Polarization) correlations).

Computed and experimental chemical shift parameters for rigid and flexible YAF peptides in the solid state.

DFT methods were employed to compute the (13)C NMR chemical shift tensor (CST) parameters for crystals of YAF peptides (Tyr-Ala-Phe) with different stereochemistry for the Ala residue. Tyr-D-Ala-Phe 1 crystallizes in the C2 space group while Tyr-L-Ala-Phe crystallizes in either the P2(1)2(1)2 space group (2a) or the P6(5) space group (2b). PISEMA MAS measurements for samples with a natural abundance of (1)H and (13)C nuclei and (2)H QUADECHO experiments for samples with deuterium labeled aromatic rings were used to analyze the geometry and time scale of the molecular motion. At ambient temperature, the tyrosine ring of sample 1 is rigid and the phenylalanine ring undergoes a π-jump, both rings in sample 2a are static, and both rings in sample 2b undergo a fast regime exchange. The theoretical values of the CST were obtained for isolated molecules (IM) and clusters employing the ONIOM approach. The experimental (13)C δ(ii) parameters for all of the samples were measured via a 2D PASS sequence. Significant scatter of the computed versus the experimental (13)C CST parameters was observed for 1 and 2b, while the observed correlation was very good for 2a. In this report, we show that the quality of the (13)C σ(ii)/(13)C δ(ii) correlations, when properly interpreted, can be a source of important information about local molecular motions.

The influence of the stereochemistry of alanine residue on the solid state conformation and crystal packing of opioid peptides containing D-Ala or L-Ala in message domain--XRD and NMR study.

In this work, an X-ray diffraction (XRD) and solid state NMR study of two tetrapeptides with different stereochemistry of alanine residue is presented using Tyr-(D-Ala)-Phe-Gly (1), an N-terminal sequence of opioid peptide dermorphin, and its biologically inactive analog Tyr-(L-Ala)-Phe-Gly (2). Single-crystal XRD proved that 1 crystallized under different conditions from exclusively one structure: a monoclinic crystal with P2(1) space group. In contrast, 2 very easily formed at least three crystallographic modifications, 2a (monoclinic P2(1)), 2b (orthorhombic P2(1)2(1)2) and 2c (tetragonal P4(1)2(1)2). Solid-state NMR spectroscopy was employed to investigate the structure and molecular dynamics of 1, 2a, and 2b. By employing different NMR experiments (dipolar dephasing and PILGRIM) and an analysis of the (13)C principal elements of the chemical shift tensor (CST), it was proven that the main skeleton of tetrapeptides is rigid, whereas significant differences in the molecular motion of the aromatic residues were observed. Comparing current data with those of previous studies (J. Phys. Chem. B2004, 108, 4535-4545 and Cryst. Growth Des. 2009, 9, 4050-4059), it can be assumed that an important preorganization mechanism anticipating the formation of peptide crystals containing D-Ala in sequence is the intramolecular CH-π interaction, which occurs for the amino acid with D stereochemistry. This effect may be responsible for the formation of only one crystallographic form of D-Ala peptides.