Abstract
The α/β^3^‐mixed tripeptides R‐CO‐β^3^‐HMet‐Leu‐Phe‐OMe (1a,b), R‐CO‐Met‐β^3^‐HLeu‐Phe‐OMe (2a,b) and R‐CO‐Met‐Leu‐β^3^‐HPhe‐OMe (3a,b) (a, R = tert‐butyloxy‐; b, R = H−), analogues of the potent chemoattractant For‐Met‐Leu‐Phe‐OMe, have been synthesized by classical solution methods and fully characterized. The activities of the new analogues as chemoattractants, superoxide anion producers and lysozyme releasers have been determined on human neutrophils. Whereas all of the three N‐formyl derivatives are significantly less active than the parent tripeptide as chemoattractants, compound 1b has been found to be highly active as a superoxide anion producer and 3b as a lysozyme releaser. The results show that the replacement of the native Leu residue at the central position is, in each of the examined cases, the least favourable modification. The three N‐Boc derivatives are, as expected, devoid of activity as agonists, but they are all good inhibitors of chemotaxis. Information on the solution conformation has been obtained by examining the involvement of the NH groups in intramolecular H‐bonds using ^1^H NMR. The conformation of the N‐Boc analogue 1a has also been determined in the crystal state by x‐ray diffraction analysis. The molecule is extended at the β^3^‐HMet residue (φ~1~ = −87°;θ~1~ = 172°;ψ~1~ = 126° ) and no intramolecular H‐bond is present. Copyright © 2004 European Peptide Society and John Wiley & Sons, Ltd.