A turn conformation stabilized by the intramolecular salt bridge between the charged termini was confirmed for the heterochiral tetrapeptide by single-crystal X-ray diffraction (XRD) (Figure ). Surprisingly, the NH of Leu and Phe3 were engaged in H-bonding, albeit intermolecularly with the CO of the same residues of an adjacent tetrapeptide molecule, as a distinctive feature of the solid phase, as opposed to the intramolecular H-bond of the peptide in solution. The dihedral angles of the crystal structures are compatible with one of the visited conformations observed by MD, albeit not the most representative one (Fig. ).