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2 ), and dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH 2 ). Analogous heptapeptides containing L-alanine instead of D-alanine are not biologically active. In a previous paper (J. Phys. Chem. B 2004, 108 (14), 4535-4545), we reported X-ray and NMR data for Tyr-D-Ala-Phe. In the current report, we present structural studies of Tyr-Ala-Phe, a "false message sequence" of opioid peptides. It has been found that Tyr-Ala-Phe crystallizes in two forms, as anhydrate (Form I) and dihydrate (Form II). Crystal and molecular structure of both forms was established by means of low-temperature X-ray measurements. Form I is orthorhombic with space group P2 1 2 1 2 1 , while II is hexagonal with space group P6 5 . Solid-state NMR was employed to study the structure and molecular dynamics of I and II. Analysis of cross-polarization buildup curves and 13 C chemical shift tensor (CST) parameters obtained by a two-dimensional PASS experiment have revealed a dramatic difference in the molecular motion of both modifications. 13 C T 1 relaxation times have provided further evidence confirming distinct molecular dynamics. The attempt to understand the role of the stereochemistry of Ala residue in opioid peptide sequences in relation to intramolecular interactions and preorganization mechanisms is presented.

L-selenomethionine 1 crystallizes in P2(1) space group with two molecules in the asymmetric unit. Solid-state NMR spectroscopy is used for searching of structure and dynamics of 1 in the crystal lattice. The distinct molecular motion of side chains for A and B molecules of 1 is apparent from measurements of relaxation parameters (1H 1rho, 13C T1) and analysis of CSA data (2D-PASS experiment). The 13C delta(ii) and 77Se delta(ii) parameters are correlated with theoretical shielding parameters obtained by means DFT GIAO calculations. Attempt to explain the mechanism of phase transition of crystals of 1 at 313K is presented.

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