n the present study, we report synthesis and biological evaluation of the N-Boc-protected tripeptides 4a-l and N-For protected tripeptides 5a-l as new For-Met-Leu-Phe-OMe (fMLF-OMe) analogues. All the new ligands are characterized by the C-terminal Phe residue variously substituted at position 4 of the aromatic ring. The agonism of 5a-l and the antagonism of 4a-l (chemotaxis, superoxide anion production, lysozyme release as well as receptor binding affinity) have been examined on human neutrophils. No synthesized compounds has higher activity than the standard fMLF-OMe tripeptide to stimulate chemotaxis, although compounds 5a and 5c with -CH 3 and -C(CH 3) 3, respectively, in position 4 on the aromatic ring, are better than the standard tripeptide to stimulate the production of superoxide anion, in higher concentration. Compounds 4f and 4i, containing -F and -I in position 4, respectively, on the aromatic ring of phenylalanine, exhibit significant chemotactic antagonism. The influence of the different substitution at the position 4 on the aromatic ring of phenylalanine is discussed. © 2012 European Peptide Society and John Wiley & Sons, Ltd.
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