Modified chemotactic peptides: synthesis, conformation, and biological activity of For-Thp-Leu-delta ZPhe-OMe.

For-Thp-Leu-delta ZPhe-OMe (2), an analogue of the chemotactic tripeptide For-Met-Leu-Phe-OMe, containing 4-aminotetrahydrothiopyran-4-carboxylic acid (Thp) and (Z)-2,3-didehydrophenylalanine (delta ZPhe) as achiral, conformationally restricted mimics of Met and Phe, respectively, has been synthesized. In the crystal the new formyltripeptide adopts a type I beta-turn conformation stabilized by a weak H bond between the formylic oxygen and the delta ZPhe NH. 1H-nmr analysis based on NH solvent accessibility and nuclear Overhauser effect experiments suggests that the beta-turn is not preferred in CDCl3 solution where a gamma-turn, centered at the Thp residue, prevails. The biological activity of 2 has been determined on human neutrophils and compared to that of previously studied analogues. The tripeptide 2 is practically unable to elicit superoxide anion production and lysozyme release, while slight, but not statistically significant activity was induced in chemotaxis. The role of the orientation of the aromatic ring with respect to the backbone adjacent atoms is discussed.