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

Conformational investigations on analogs of inflammation response inducing chemotactic tripeptide fMLP

Conformations of three analogs of for-L-Met-L-Leu-L-Phe-OH (fMLP), which initiates inflammatory response by interaction with the formyl peptide receptor (FPR), have been investigated by the application of the X-ray crystallographic technique. The investigated analogs of fMLP peptides are as follows: for-L-Met-1-amino-1-cyclooctane-carbonyl(Ac8c)-L-Phe-OMe; for-L-Met-L-Leu-L-p-iodo-Phe-OH; and for-L-Met-di-n-propylglycyl(Dpg)-L-Phe-OMe. The peptide backbone in and is constrained at position of fMLP by the introduction of Calpha,alpha-disubstituted glycines. In peptide, Phe-OMe is substituted by p-iodo-Phe-OH. Crystal structures reveal an overall folded conformation adopted by and. The former is folded in the type II beta-turn, which is stabilized by an intramolecular 1<--4 (formyl) C==O...H--N (Phe) hydrogen bond, whereas the latter is folded in an open turn without any intramolecular hydrogen bond. On the other hand, peptide has an extended conformation, and two different molecules in a crystallographic asymmetric unit form an antiparallel beta-sheet-like structure. In and, residues Ac8c and Dpg adopt left-handed helical and fully extended (C5) conformations, respectively. The cyclooctane ring in Ac8c acquires a boat-chair conformation. Crystal packing of is characterized by the association of aliphatic-aromatic rings via a C--H...pi interaction. In the crystal of, contrary to the usual observations, peptides are interlinked via networks of head-to-tail hydrogen bond and pi...pi interactions, which are generally observed to be mutually exclusive. The structure-function mechanism of the ligand-receptor interaction is discussed.

Control of peptide conformation by the Thorpe-Ingold effect (C alpha-tetrasubstitution)

The preferred conformations of peptides heavily based on the currently extensively exploited achiral and chiral alpha-amino acids with a quaternary alpha-carbon atom, as determined by conformational energy computations, crystal-state (x-ray diffraction) analyses, and solution ((1)H-NMR and spectroscopic) investigations, are reviewed. It is concluded that 3(10)/alpha-helical structures and the fully extended (C(5)) conformation are preferentially adopted by peptide sequences characterized by this family of amino acids, depending upon overall bulkiness and nature (e.g., whether acyclic or C(alpha) (i) <--> C(alpha) (i) cyclized) of their side chains. The intriguing relationship between alpha-carbon chirality and bend/helix handedness is also illustrated. gamma-Bends and semiextended conformations are rarely observed. Formation of beta-sheet structures is prevented.

Modified chemotactic peptides: synthesis and activity of an azaTic-containing fMLP-OMe analogue

The synthesis and the biological activity of a pseudopeptide analogue of the chemotactic N-formyltripeptide fMLP-OMe, containing the azaTic (3,4-dihydro-2(1H)-phthalazinecarboxylic acid) residue replacing the native phenylalanine, is described. Whereas pseudopeptides containing linear alpha-azaamino acids are currently studied, data on the new group of analogues containing cyclic alpha-aza residues capable of limiting the rotameric distribution of the side chains (topological control) are just emerging in the literature. At our best knowledge, the here described [azaTic3]fMLP-OMe represents the first example of the introduction of this new type of alpha-aza residue into a natural bioactive peptide.

Conformational investigation of alpha,beta-dehydropeptides. VIII. N-acetyl-alpha,beta-dehydroamino acid N'-methylamides: conformation and electron density perturbation from infrared and theoretical studies

The Fourier transform infrared spectra are analyzed in the regions of Vs(N-H), amide I, amide II and Vs(C alpha = C beta) bands for a series of Ac-delta Xaa-NHMe, where delta Xaa = delta Ala, (Z)-delta Abu, (Z)-delta Leu, (Z)-delta Phe and delta Val, to determine the predominant solution conformation of these alpha,beta-dehydropeptide-related molecules and the electron distribution perturbation in their amide bonds. The measurements were performed in dichloromethane (DCM). To confirm and rationalize the assignments, the spectra of the respective series of saturated Ac-Xaa-NHMe, recorded in DCM, and the spectra of these two series of unsaturated and saturated compounds, recorded in acetonitrile, were examined. To help interpret the spectroscopic results, the equilibrium geometrical parameters for some selected amides were used. These were optimized with ab initio methods in the 6-31G** basis set. Each of the dehydroamides studied adopted a C5 structure, which in Ac-delta Ala-NHMe is fully extended and accompanied by the strong C5 hydrogen bond. Interaction with the C alpha = C beta bond lessened the amidic resonance within each of the flanking amide groups. The N-terminal C = O bond was noticeably shorter, both amide bonds were longer than the corresponding bonds in the saturated entities and the N-terminal amide system was distorted. Ac-delta Ala-NHMe constituted an exception. Its C-terminal amide bond was shorter than the standard one and both amide systems were prototypically planar.

Synthesis, conformation, and biological activity of two fMLP-OMe analogues containing the new 2-[2'-(methylthio)ethyl]methionine residue

The new C alpha-tetrasubstituted alpha-amino acid residue 2-[2'-(methylthio)ethyl]methionine (Dmt) has been introduced into the reference chemotactic tripeptide HCO-Met-Leu-Phe-OMe (fMLP-OMe) in place of the leucine or methionine, respectively. The biological activity of the new analogues [Dmt2]fMLP-OMe (2) and [Dmt1]fMLP-OMe (3) has been determined; whereas 2 is active toward human neutrophils, stimulating directed migration, superoxide anion generation, and lysozyme release, 3 results practically inactive in all tested assays. A conformational analysis on 2 and 3 has been performed in solution by using ir absorption and 1H-nmr. The conformation of 2 was also examined in the crystal by x-ray diffraction methods. Both 2 and 3 adopt fully extended conformation in correspondence with the Dmt residue. Biological and conformational results are discussed and compared with related previously studied models.

Modified chemotactic peptides: synthesis, conformation, and activity of HCO-Thp-Ac6c-Phe-OMe

HCO-Thp-Ac6c-Phe-OMe (3) has been synthesized as a new analogue of the prototypical chemotactic agent HCO-Met-Leu-Phe-OMe (fMLP-OMe). Compound 3 contains 4-aminotetra-hydrothiopyran-4-carboxylic acid (Thp) and 1-aminocyclohexane-1-carboxylic acid (Ac6c) as achiral, conformationally restricted mimics of Met and Leu, respectively. In the crystal, the formyltripeptide adopts an helical conformation at the Thp and Ac6c residues, of the type alpha R and alpha L, respectively, whereas the C-terminal phenylalanine is quasi-extended. A system of two consecutive gamma-turns, centered at the first two residues, better explains the nmr data as compared with an alternative beta-turn structure. The conformation of the new analogue 3 is compared with those of two related peptides containing Thp as N-terminal residue. The biological activity of 3 has been determined on human neutrophils and compared to that of the previously studied model [Ac6c2] fMLP-OMe. While the above analogue is highly active in the superoxide anion production, the new tripeptide 3 is practically unable to elicit any of the tested biological activities.

The role of (Z)-2,3-didehydrophenylalanine and phenylalanine C-terminal residues in determining the chemotactic activity of formylpeptides

Several formylpeptides, analogs of the chemotactic agent HCO-Met-Leu-Phe-OMe, having the HCO-Xaa-Leu-delta ZPhe-OMe and HCO-Xaa-Leu-delta ZPhe-Phe-OMe structures (delta ZPhe = (Z)-2,3-didehydrophenylalanine), have been synthesized. The biological activity of these ligands has been determined on human neutrophils and compared to that of the corresponding HCO-Xaa-Leu-Phe-OMe derivatives not containing the unsaturated residue. The replacement of the C-terminal Phe with delta ZPhe causes, in all the examined tripeptides, the loss of any biological activity. On the other hand, the introduction into the delta ZPhe containing models of an additional C-terminal Phe residue leads to the formyltetrapeptides HCO-Xaa-Leu-delta ZPhe-Phe-OMe which show a biological activity very similar to that exhibited by the corresponding HCO-Xaa-Leu-Phe-OMe analogues.

Chemotactic peptide analogues. Synthesis and chemotactic activity of N-formyl-Met-Leu-Phe analogues containing (S)-phenylalaninol derivatives

The synthesis and the biological activity towards human neutrophils of some N-formyl-Met-Leu-Phe-OMe analogues containing (S)-phenylalaninol (Pheol) or its derivatives in place of the native phenylalanine are reported. While the analogue containing Pheol (4) was found to be devoid of significant biological activity, its esters 3 and 5, although inactive as chemoattractants, are able to strongly stimulate superoxide production and are active with a lower efficacy in the lysozyme release.

X-ray crystallography of peptides: the contributions of the Italian laboratories

The review article summarizes the most relevant solid state structural and conformational results obtained in the laboratories involved in Italy in the studies of synthetic and natural peptides by x-ray diffraction analyses. Some of the topics will include research studies carried out in other European countries, whereas in other cases studies carried out in Italy will be included in other review articles included in this volume. The review deals with peptides containing symmetrically achiral and unsymmetrically chiral C alpha,alpha-dialkylated glycine residues, peptides containing beta-alanine residues, alpha,beta-dehydroamino acid residues, and aminosuccinyl residues, peptides containing the thioamide surrogate, heterochiral peptides and several bioactive peptides systems with the proposed relationships between function and structure.