Structural versatility of peptides containing C alpha, alpha-dialkylated glycines. An X-ray diffraction study of six 1-aminocyclopropane-1-carboxylic acid rich peptides

The molecular and crystal structures of six fully blocked, Ac3c-rich peptides to the tetramer level were determined by X-ray diffraction. The peptides are Fmoc-(Ac3c)2-OMe-CH3OH, Ac-(Ac3c)2-OMe, t-Boc-Ac3c-L-Phe-OMe, pBrBz-(Ac3c)3-OMe.H2O, Z-Gly-Ac3c-Gly-OTmb.(CH3)2CO, and t-Boc-(Ac3c)4-OMe.2H2O. Type-I (I') beta-bends and distorted 3(10)-helices were found to be typical of the tri- and tetrapeptides, respectively. In the dipeptides, too short to form beta-bend conformations, other less common structural features may be observed. The average geometry of the cyclopropyl moiety of the Ac3c residue is asymmetric and the N-C alpha-C' bond angle is significantly expanded from the regular tetrahedral value. A comparison with the structural preferences of other extensively investigated C alpha, alpha-dialylated alpha-amino acids is made and the implications for the use of the Ac3c residue in conformational design are examined.     

Nonrandom location of IS1 elements in the genomes of natural isolates of Escherichia coli

We have studied the spatial distribution of IS1 elements in the genomes of natural isolates comprising the ECOR reference collection of Escherichia coli. We find evidence for nonrandomness at three levels. Many pairs of IS1 elements are in much closer proximity (< 10 kb) than can be accounted for by chance. IS1 elements in close proximity were identified by long-range PCR amplification of the genomic sequence between them. Each amplified region was sequenced and its map location determined by database screening of DNA hybridization. Among the ECOR strains with at least two IS1 elements, 54% had one or more pairs of elements separated by < 10 kb. We propose that this type of clustering is a result of "local hopping," in which we assume that a significant proportion of tranposition events leads to the insertion of a daughter IS element in the vicinity of the parental element. A second level of nonrandomness is found in strains with a modest number of IS1 elements that are mapped through the use of inverse PCR to amplify flanking genomic sequences: in these strains, the insertion sites tend to be clustered over a smaller region of chromosome than would be expected by chance. A third level of nonrandomness is observed in the composite distribution of IS elements across strains: among 20 mapped IS1 elements, none were found in the region of 48-77 minutes, a significant gap. One region of the E. coli chromosome, at 98 min, had a cluster of IS1 elements in seven ECOR strains of diverse phylogenetic origin. We deduce from sequence analysis that this pattern of distribution is a result of initial insertion in the most recent common ancestor of these strains and therefore not a hot spot of insertion. Analysis using long- range PCR with primers for IS2 and IS3 also yielded pairs of elements in close proximity, suggesting that these elements may also occasionally transpose by local hopping.      

A novel peptide conformation: First unequivocal observation of the oxy-analog of a β-bend

The x-ray diffraction analysis of the N-benzyloxycarbonyl homo-tripeptide from α-amino-isobutyric acid has shown the occurrence of an incipient 3₁₀-helix characterized by one type-III (or type-III′) β-bend followed by one oxy-analog of the same type of β-bend. This represents the first unequivocal observation of the latter conformation, where the O—H group of the COOH moiety present at the C-terminus of the peptide main chain plays the role of the hydrogen-bonding donor. These results have been compared with those of the same peptide in its monohydrate form and of its methyl ester derivative, the x-ray diffraction structures of which are also described here.     

8S-lipoxygenase products activate peroxisome proliferator-activated receptor alpha and induce differentiation in murine keratinocytes.

To determine the function and mechanism of action of the 8S-lipoxygenase (8-LOX) product of arachidonic acid, 8S-hydroxyeicosatetraenoic acid (8S-HETE), which is normally synthesized only after irritation of the epidermis, transgenic mice with 8-LOX targeted to keratinocytes through the use of a loricrin promoter were generated. Histological analyses showed that the skin, tongue, and stomach of transgenic mice are highly differentiated, and immunoblotting and immunohistochemistries of skin showed higher levels of keratin-1 expression compared with wild-type mice. The labeling index, however, of the transgenic epidermis was twice that of the wild-type epidermis. Furthermore, 8S-HETE treatment of wild-type primary keratinocytes induced keratin-1 expression. Peroxisome proliferator activated receptor alpha (PPARalpha) was identified as a crucial component of keratin-1 induction through transient transfection with expression vectors for PPARalpha, PPARgamma, and a dominant-negative PPAR, as well as through the use of known PPAR agonists. From these studies, it is concluded that 8S-HETE plays an important role in keratinocyte differentiation and that at least some of its effects are mediated by PPARalpha.     

Conformational rigidity versus flexibility in a novel peptidic neurokinin A receptor antagonist

Neurokinin A receptor antagonists have been proposed as a new class of drugs for several applications in humans (asthama, intestinal motility, etc.). The rational design, synthesis, structural characterization and biological activity evaluation of a new potent, highly selective, long-lasting, peptide-based receptor antagonist are reported. The structure–activity relationship indicates that the conformational rigidity determines potency, specificity and especially the long life of the molecule in the living body. MEN 10627 is the prototype of a new class of cyclic, peptide-based, neurokinin A receptor antagonists and it is a suitable candidate for clinical testing in humans.     

Defect peptide chemistry: Perturbations in the structure of a homopentapeptide induced by a guest residue interrupting side-chain regularity

The fully blocked pentapeptide Tfa-(Deg)₂-L -Abu-(Deg)₂-OtBu (Tfa:triflouroacetyl; Deg: C^α,α-diethylglycine; OtBu: tert-butoxy) adopts in the crystal state a regular, right-handed 3₁₀-helical structure stabilized by three N ? H …? O ? C intramolecular 1 ← 4 (or C₁₀) H bonds, as determined by an x-ray diffraction analysis. However, a Fourier transform ir absorption and ¹H-nmr study strongly supports the view that in deuterochloroform solution the four Deg residues at both termini of the peptide main chain are involved in successive, fully extended C₅ forms. A comparison with the stable, fully developed, multiple C₅ conformation of Tfa-(Deg)₅-OtBu indicates that incorporation of an Abu guest residue, interrupting the side-chain uniformity of the host (Deg)₅ homopeptide, while altering only marginally the conformation in a solvent of low polarity, is responsible for a dramatic perturbation of the crystal-state structure. © 1994 John Wiley & Sons, Inc.     

Long, chiral polypeptide 3(10)-helices at atomic resolution

The crystal-state preferred conformation of the terminally blocked hepta- and octapeptides with the general formula -(Aib)n L-Leu-(Aib)2- (n = 4 and 5, respectively), determined by X-ray diffraction, was found to be a right-handed 3(10)-helix stabilized by five and six consecutive intramolecular NH...O = C H-bonds of the C(10)-III type, respectively. The octapeptide structure represents the first observation at atomic resolution of a regular, chiral 3(10)-helix larger than two complete turns. In both cases the right handed screw sense of the helix is dictated by the presence of the single, internal L-residue. This study confirms the propensity of short peptides rich in Aib, the prototype of the amino acid residues dialkylated at the alpha carbon, to adopt a 3(10)-helical structure and is expected to help our understanding of the conformational preferences of the membrane-active, channel-forming, ion-transporting peptaibol antibiotics.     

Jasione sphaerocephala sp.nov. (Campanulaceae) from Italy

Jasione sphaerocephala Brullo, Marcend et Pavone sp.nov. is described from Calabria, S Italy. It grows in rocky places and has some resemblance to J. laevis and J. montana. The chromosome number is 2n = 14.