Chemotactic tripeptides incorporating at position 2 alpha-aminoacid residues with unsaturated side chains

New N-For-Met-Leu-Phe-OMe (fMLF-OMe) analogues incorporating three different g-d-didehydro-a-aminoacid residues (namely: Alg¼(S)-Allylglycine; Dag¼Diallylglycine; Cpg¼1-Aminocyclopent- 3-ene-1-carboxylic acid) replacing the native (S)-Leucine have been synthesized and their activity towards human neutrophils has been evaluated in comparison with that shown by the reference tripeptide fMLF-OMe. Chemotaxis, lysozyme release and superoxide anion production have been measured. 1H NMR titration experiments and NOESY spectrum of the Cpg containing model 10 have been discussed in order to ascertain the preferred solution conformations. A fully extended (C5) conformation at position 2 and a folded conformation with two consecutive g-turns (C7 structure) have been proposed for the Dag and Cpg containing tripeptides, respectively.