Helical screw-sense preferences of peptides based on chiral, Cα-tetrasubstituted α-amino acids

The preferred helical screw senses of chiral α-amino acids with a C(α)-tetrasubstituted α-carbon atom, as determined in the crystal state by X-ray diffraction analyses on derivatives and peptides, are reviewed. This survey covers C(α)-methylated and C(α)-ethylated α-amino acids, as well as α-amino acids cyclized on the α-carbon, including those characterized by the combination of lack of chirality at the α-carbon with either side-chain or axial chirality. Although, in general, chiral C(α)-tetrasubstituted α-amino acids show a less pronounced bias toward a single helical screw sense than their proteinogenic (C(α)-trisubstituted) counterparts, our analysis highlights significant differences in terms of magnitude and direction of such a bias among the various sub-families of residues, and between individual amino acids within each sub-family as well. The experimental findings can be rationalized, at least in part, on the basis of steric considerations.

C(alpha)-tetrasubstituted amino acid based peptides in asymmetric catalysis

C(alpha)-tetrasubstituted alpha-amino acids constitute a powerful tool for controlling the conformation of short peptide sequences. Chiral peptides may be used in stereoselective reactions both for asymmetric induction and in kinetic resolution. By reviewing recent data from our own laboratories and by presenting new results on the stereoselective oxidation of chalcone to the corresponding oxide, this contribution shows that the control of peptide conformation is a critical issue in order to achieve successful stereoselective catalysis.

Conformational analysis by HRMAS NMR spectroscopy of resin-bound homo-peptides from C(alpha)-methyl-leucine

A series of [L-(alphaMe)Leu]n (n = 1-5) homo-peptides have been covalently linked to Tentagel and POEPOP resins and submitted to a conformational study using HRMAS NMR spectroscopy. Whereas the mono- and dipeptide are mainly fully-extended, stable 3(10)-helical structures are formed beginning from the trimer.

Crystal-state 3D-structural characterization of novel 310-helical peptides

The crystal-state conformations of two octapeptides, pBrBz-(D-Iva)8-OtBu (8I) and Ac-[L-(alphaMe)Val]8-OH (8II), the heptapeptide Z-[L-(alphaMe)Val]7-OH (7), the hexapeptide Z-[L-(alphaMe)Leu]6-OtBu (6) and the tetrapeptide alkylamide Z-(Aib)2-L-Glu(OMe)-L-Ala-L-Lol (5) were assessed by x-ray diffraction analyses. Two independent molecules are observed in the asymmetric unit of each L-(alphaMe)Val homo-peptide. All four homo-peptides are folded in a regular 3(10)-helical structure (only the C-terminal H-bonded conformation of the D-Iva octapeptide is distorted to a type-I beta-turn). The hydroxyl groups of the C-terminal carboxyl moieties of the two L-(alphaMe)Val homo-peptides participate in an oxy-analogue of the type-III beta-turn conformation. While the two L-(alphaMe)Val 3(10)-helices are right-handed, the D-Iva and L-(alphaMe)Leu helices are left-handed. The tetrapeptide alkylamide is 3(10)-helical at the N-terminus, but it is mixed 3(10)/alpha-helical at the C-terminus.

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.

Design and synthesis of glutathione analogues

This review reports recent structural modifications (since 1989) performed on the glutathione molecular both in the oxidized and reduced form. Relevant chemical aspects, biochemical consequences and therapeutical implications are illustrated. Natural thiols related to glutathione are also considered.

Preparation of dipeptoid mimetics for the tetrapeptide cholecystokinin, CCK(30-33)

The diastereoselective synthesis of 2,3-methanophenylalanine methyl esters (5) has been achieved in 58% yield. The preparation of the dehydropeptides (1, R = Me; 2, R = H) and the cyclopropylpeptides (3, R = Me; 4, R = H) possessing good binding affinities for the CCK-A and CCK-B receptors is described. Conformational studies of the dipeptide esters 1 and 3 indicated the presence of a beta-turn within the peptide backbone, although there was no preference in type. The Phe and Trp moieties, however, did prefer to be situated on the same side of the peptide turn which is favourable for receptor binding.

Inversion of 3(10)-helix screw sense in a (D-alpha Me)Leu homo-tetrapeptide induced by a guest D-(alpha Me)Val residue

The terminally blocked tetrapeptide pBrBz-[D-(alpha Me)Leu]2-D-(alpha Me)Val-D-(alpha Me)Leu-OfBu is folded in the crystal state in a left-handed 3(10)-helical structure stabilized by two consecutive 1<--4 C = O...H-N intramolecular H-bonds, as determined by X-ray diffraction analysis. A CD study strongly supports the view that this conformation is also that largely prevailing in MeOH solution. A comparison with the published conformation of pBrBz-[D-(alpha Me)Leu]4-OfBu indicates that incorporation of a single internal beta-branched (alpha Me)Val guest residue into the host homo-tetrapeptide from the gamma-branched (alpha Me)Leu residue is responsible for a dramatic structural perturbation, i.e. an inversion of the 3(10) screw sense from right to left-handed.

The preferred solid-state conformation of (alpha Me)Trp peptides

The two Z-L-Ala-DL-(alpha Me)Trp-NH2 diastereomeric dipeptides were synthesized from (Z-L-Ala)2(O) and H-DL-(alpha Me)Trp-NH2. The latter racemate, prepared by phase-transfer catalyzed alkylation of the N alpha-benzylidene derivative of alanine amide followed by acidic hydrolysis of the resulting Schiff base, was characterized by X-ray diffraction. The molecular and crystal structure of Z-L-Ala-L-(alpha Me)Trp-NH2, separated from its diastereomer by silica-gel column chromatography, was determined by X-ray diffraction analysis. Both independent molecules in the asymmetric unit of the dipeptide adopt a type-II beta-bend conformation. However, only the more regularly folded conformation of molecule B is stabilized by a 1<--4 C = O...H--N intramolecular H bond. The present results indicate that: (i) the C alpha-methylated (alpha Me)Trp residue is a strong beta-bend and helix former, and (ii) the relationship between (alpha Me)Trp chirality and helix screw sense tends to be opposite to that of protein amino acids. The implications for the use of the (alpha Me)Trp residue in designing conformationally restricted analogs of bioactive peptides are briefly discussed.

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.