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The molecular and crystal structures of the terminally blocked homo-tri- and homo-pentapeptides from 1-amino-1-cyclopentane-carboxylic acid, Z-(Ac5c)3-OtBu (1) and Z-(Ac5c)5-OtBu (2), were determined by X-ray diffraction. The two compounds exhibit the following parameters: (1) orthorhombic, P212121, a = 11.539(7) Å, b = 16.185(2) Å, c = 30.925(4) Å and Z = 8; (2) triclinic, P[unk], a = 10.835(5) Å, b = 11.631(9) Å, c = 18.212(5) Å, α = 92.20(7)°, β = 94.79(10)°, γ = 107.6(3)° and Z = 2. The crystal structures were solved by direct methods. The least-squares refinements led to R values of 0.080 and 0.060 for 4061 and 4018 reflections with I ≥ 2σ(I) (according to the SHELXL-93 computer program weighting scheme) for (1) and I ≥ 3σ(I) (Structure Determination Programs, SDP crystallographic package weighting scheme) for (2), respectively. The two independent molecules, A and B, in the asymmetric unit of the tripeptide (1) are folded into a type-I/III and a type-I′/β-bend conformation, respectively, each stabilized by an intramolecular C=O…H–N H-bond. The pentapeptide molecules adopt a 310-helical structure, stabilized by three consecutive, intramolecularly H-bonded type-III (III′) β-bend conformations.

Using a combined chemical/chiral chromatographic approach we synthesized an N-protected derivative of (R)-c(3)Val, a severely conformationally restricted C-alpha-tetrasubstituted alpha-amino acid characterized by a C-beta,C-beta-dimethylated cyclopropane system. A set of terminally protected derivatives and model peptides (to the heptamer level), containing one or two (R)-c(3)Val residues in combination with either Aib or Gly residues, was prepared by solution methods. A detailed solution and crystal-state conformational investigation, based on Fourier transform infrared (FTIR) absorption, H-1-NMR, and x-ray diffraction techniques, performed in comparison with a similar study on related derivatives and peptides rich in (alphaMe)Val, the prototype of C-alpha-tetrasubstituted alpha-amino acids of this subfamily, allowed us to conclude the following: (a) c(3)Val is a good beta-bend and helix former, although less efficient than (alphaMe)Val. (b) The relationship between alpha-carbon chirality and screw sense of the folded structure formed is the same as that of (alphaMe) Val, i.e., the (R)-enantiomer has a strong left-handed bias. (c) c(3)Val seems more prone than (alphaMe)Val to fold into a gamma-bend conformation. The conformational propensities of C-beta,C-beta-disubstituted Ac(3)c residues are also discussed in comparison with those of the parent cyclopropane residue.

A series of Na-protected, monodispersed homo-oligopeptide esters to the octamer level from L-Ca-Me, Ca-propylglycine [or Ca-methylnorvaline, (aMe)Nva] has been synthesized by soln. methods and fully characterized. The preferred conformation of these homo-oligomers in soln. has been assessed by FT-IR absorption and 1H NMR techniques. Moreover, the mol. structures of the homotrimer and homotetramer have been detd. in the crystal state by X-ray diffraction. The obtained results strongly support the view that right-handed, single or multiple, and consecutive b bends are preferentially adopted by the conformationally restricted L-(aMe)Nva homo-oligomers. In particular, 310 helixes are formed by the longest homo-oligomers. It is our contention that the [(aMe)Nva]n peptides represent the best available choice among Ca-tetrasubstituted a-amino acid-based homo-oligomers for the construction of relatively easy to make, rigid foldamers with a well-defined screw-sense bias.

The achiral, nitroxyl-contg. a-amino acid TOAC (TOAC = 2,2,6,6- tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral a-amino acid Ca-Me valine [(aMe)Val], was used to prep. short peptides (from di- to hexa-) that induced the enantioselective oxidn. of racemic 1-phenylethanol to acetophenone. The best catalyst was an Na-acylated dipeptide alkylamide with the -TOAC-(aMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded b-turn conformation with large, lipophilic (hydrophobic) N- and C-terminal blocking groups. We rationalized our findings by proposing models for the diastereomeric intermediates between (R)-[and (S)]-1-phenylethanol and the catalyst Fmoc- TOAC-L-(aMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.

Three copies of sequences of peptides of the peptaibol family, known to affect the permeability of the lipid bilayer of membranes, were connected to tris(2-aminoethyl)amine (TREN), a tripodal metal ion ligand, in order to prepare functional peptides capable of modifying the permeability of liposomal membranes. Some of the resulting tripodal polypeptide derivatives are very effective in promoting carboxyfluorescein (CF) leakage from CF-loaded unilamellar vesicles composed of a 70/30 phosphatidylcholine-cholesterol blend. The activity of these novel compounds was shown to be tunable upon metal ion coordination of the TREN subunit, the tripodal apopeptide being far more effective than its ZnII complex. Leakage experiments showed that a minimum number of five aminoacid per peptide chain is required to form active systems. A mechanism is proposed in which the ZnII ion affects the conformation of the template from an extended to a globular one thus acting as an allosteric regulator of the activity of the systems. Molecular modeling studies indicate that when the three peptide chains are connected to the template in the extended conformation, the resulting tripodal polypeptide is able to span the membrane across, thus allowing the formation of permeable channels made of a cluster of molecules. The same change of conformation induces, to some extent, fusion of the membranes of different liposomes.

Esculentins-1 are a family of frog skin antimicrobial peptides (AMPs), characterized by a 46 amino acids primary structure and a broad range of antimicrobial activity. Studies on the mode of action of amphibian AMPs evidenced that bacterial membranes are their major target. Esculentins possess the basic features common to most linear AMPs, that is an overall positive charge at neutral pH and a considerable proportion of hydrophobic residues. These properties allow the peptides to bind to the negatively-charged microbial membranes by folding into an amphiphilic structure, thus perturbing membrane permeability. Esculentin 1-21 [Esc(1-21)], consists of the first 20 aminoacids of the naturally occurring Esculentin 1a, with a glycinamide residue at the C-terminus: H-Gly-Ile-Phe-Ser-Lys-Leu-Ala-Gly-Lys-Gly-Leu-Lys-Asn-Leu-Leu-Ile-Ser-Gly-Leu-Lys-Gly-NH2. It was previously demonstrated that Esc(1-21) is highly potent against the most common mastitis-causing microbes in dairy cattle (e.g. Streptococcus agalactiae) both in vitro and in vivo. Furthermore, recent studies have indicated that this peptide can rapidly kill both planktonic and biofilm forms of the opportunist pathogen Pseudomonas aeruginosa and improve survival in a murine model of Pseudomonas lung infection upon its topical administration. These properties make Esc(1-21) an attractive starting point for the development of novel antimicrobial agents able to cure mastitis in dairy cows and to treat Pseudomonas-related infections. Current therapies in clinical and veterinary medicine suffer from drawbacks that include the increased emergence of multidrug-resistant pathogens, and the production of milk contaminated with antibiotics has become a serious threat in the livestock. In this connection, we thought significant to explore the effects of the incorporation of ?-aminoisobutyric acid (Aib) residues into the sequence of Esc(1-21). When inserted into peptides, this strongly helicogenic, non-coded, C?-tetrasubstituted ?-amino acid might increase the population and stability of helical structures and also potentially confer resistance against enzymatic degradation. In this work we report on the synthesis, the characterization, a preliminary conformational study in different environments by CD and 2D-NMR techniques, and the biological activity of an analog of Esc(1-21) bearing three residues of ?-aminoisobutyric acid (Aib) at sequence position 1, 10 and 18.