Synthesis and properties of chemotactic peptide analogs. I. Crystal structure and molecular conformation of HCO-Met-leu-Ain-OMe

HCO-Met-Leu-Ain-OMe (2), an analog of the chemotactic peptide HCO-Met-Leu-Phe-OH, containing the conformationally blocked residue of the 2-aminoindane-2-carboxylic acid (Ain) has been synthesized and its crystal and molecular conformation has been determined. Crystals of 2 are monoclinic, space group P2(1), with a = 15.059(7), b = 18.548(7), c = 9.600(4) A; beta = 85.04(3) degrees. The structure has been solved by direct methods and refined to R = 0.069 for 2813 independent reflections with I greater than 2.5 sigma (I). Two independent molecules A and B have been found in the asymmetric unit of the crystal of 2. Their conformation can be described as extended at the Met and Leu residues, but folded at the C-terminal Ain residue. The helical folding is left- and right-handed in the A and B molecule, respectively. The crystal packing is characterized by ribbons of intermolecular hydrogen bonded molecules extended along the c direction. The constrained analog 2 is highly active in the superoxide production, thus indicating that a stabilization of a helical folding at the C-terminal region of chemotactic tripeptides maintains the activity. The orientation of the aromatic ring, with respect to its adjacent backbone atoms, does not seem critical for the activity.