Synthesis and biological activity of some partially modified retro-inverso analogues of cholecystokinin

Effects of the phi[NHCO] retromodification on dehydroalanine dipeptide

The potential energy surface of the phi [NHCO] delta Ala dipeptide has been computed using ab initio quantum mechanical calculations at different theoretical levels. Three degenerate minima were found and characterized. The most favoured conformation is stabilized by an intramolecular hydrogen bonding interaction. The other two minima correspond to helical and malonamide-like conformations, being 4.6 kcal/mol and 6.9 kcal/mol less stable than the lowest energy conformation, respectively. Influence of solvent effects on the relative stabilities of the different conformations have been accounted using SCRF calculations. Two implicit solvent models have been considered: water and CCl4. The results indicate that in the both cases the conformation with an intramolecular hydrogen bonding interaction retains its lowest energy.

Crystal structures of peptides and modified peptides

The X-ray diffraction experiments on peptides and related molecules which have been carried out in Western Europe, except Italy, in the last eight years are reviewed. The crystal structures of some bioactive peptides such as Leu-enkephalin (a neurotransmitter), cyclosporin A (an immunomodulator in both the free and protein-bound state), balhimycin (an antibiotic) and octreotide (a somatostatin analogue) are briefly presented. Crystallized N- and C-protected model peptides have given an insight into the folding tendency and folding modes depending on the peptide sequences. The crystal structures of various pseudopeptide molecules reveal how the three-dimensional structure of peptide analogues can be modulated by substituting non-peptide groups for the peptide bond. A few examples of structural mimetics of the beta- and gamma-turns, and of templates for alpha-helix induction are also presented.

Conformational studies of retro-inverso peptides: the crystal and molecular structure of the hydantoin from H-Ala-g-Ala-mGly-OBzl

The crystal structure of the hydantoin 1-[(S)-1'-aminoethylmalonyl benzyl ester]-(S)-4-methylimidazolidin-2,5-dione (1) derived from the peptide H-Ala-gAla-mGly-OBzl, having the retro-inverso modification of the Ala-Gly bond, has been determined by x-ray diffraction analysis. The crystals are orthorhombic, space group P2(1)2(1)2(1) with a = 6.539, b = 14.721, c = 17.101 A, z = 4. The structure was solved by direct methods and refined with anisotropic thermal factors to a final R value of 0.067 for the 947 observed reflections. Reversal of the Ala-Gly amide bond perturbs the folding tendency of the backbone shown by the parent peptide t-BuCO-Ala-Gly-NHiPr. The gem-diamino residue, gAla, and the malonyl moieties are found in the helical and the extended conformations, respectively. Intramolecular hydrogen bonding is not observed. The molecules in the crystal are held together by the formation of two intermolecular hydrogen bonds of the N-H ... O=C type with N ... O distances of 2.86 and 3.17 A, respectively.

H-Gly-His psi (NHCO)Lys-OH, partially modified retro-inverso analogue of the growth factor glycyl-L-histidyl-L-lysine with enhanced enzymatic stability

A partially retro-inverso analogue of the natural growth factor GHK was synthesized, in which the -CONH- bond between histidine and lysine was modified as -NHCO-. This modification is not expected to perturb the spatial distribution of the side-chains, and therefore the binding processes, compared to the native peptide. In the synthesis of the analogue two possible systems for deblocking of N pi-Bom group of histidine have been applied and compared. An alternative method is also described for the incorporation of malonyllysine into the peptide chain. When evaluated with respect to resistance toward degradation by human plasma in vitro, the new peptide analogue showed approximately a ten-fold increase in stability versus the parent peptide.

Synthesis of cyclic analogues of cholecystokinin highly selective for central receptors

Cyclic CCK analogues in which positions 28 and 31 have been replaced by lysine residues and whose side chains are bridged by a succinic moiety, were synthesized. They were tested for their ability to inhibit the binding of 125I-BH-CCK-8 to isolated rat pancreatic acini and to guinea pig brain membranes. These cyclic CCK-analogues were compared to the potent CCK analogue Boc-[Nle28,31]-CCK-7 and to Boc-Trp-Leu-Asp-Phe-NH2, analogue of CCK-4. These cyclic compounds appeared to be highly selective for central CCK receptors.

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.