Search results for "conformation"

Comparative morphology of the postpharyngeal gland in the Philanthinae (Hymenoptera, Crabronidae) and the evolution of an antimicrobial brood protect…

2015

Background Hymenoptera that mass-provision their offspring have evolved elaborate antimicrobial strategies to ward off fungal infestation of the highly nutritive larval food. Females of the Afro-European Philanthus triangulum and the South American Trachypus elongatus (Crabronidae, Philanthinae) embalm their prey, paralyzed bees, with a secretion from a complex postpharyngeal gland (PPG). This coating consists of mainly unsaturated hydrocarbons and reduces water accumulation on the prey’s surface, thus rendering it unfavorable for fungal growth. Here we (1) investigated whether a North American Philanthus species also employs prey embalming and (2) assessed the occurrence and morphology of …

Three-dimensional structure of hepatitis B virus core particles determined by electron cryomicroscopy

1994

Human hepatitis B virus core protein expressed in E. coli assembles into two sizes of particle. We have determined their three-dimensional structures by electron cryomicroscopy and image processing. The large and small particles correspond to triangulation number T = 4 and T = 3 dimer clustered packings, containing 240 and 180 protein subunits, respectively. The local packing of subunits is very similar in the two sizes of particle and shows holes or channels through the shell. The native viral core particle packages RNA and is active in reverse transcription to DNA. The holes we observe may provide access for the necessary small molecules. Shells assembled from the intact core protein cont…

Head-to-tail connected double calix[4]arenes

1997

New macrotricyclic compounds consisting of two calix[4]arene substructures connected by aliphatic chains of various length (three to five carbon atoms) between two oppositep-positions and two distal phenolic oxygens have been synthesized. Starting withp-tert-butyl-calix[4]arene, two O-protected phenolic units are attachedvia ether links in 1,3-position by reaction with the corresponding tosylates. After deprotection, the new calix[4]arene is formed by fragment condensation with 2,6-bisbromomethylated 4-alkylphenols. The structure of one example (8c) has been confirmed by single crystal X-ray analysis. Both calixarene parts assume the cone conformation, a molecule of acetonitrile being inclu…

Reversible Dimerization of Tetraureas Derived from Calix[4]Arenes

1999

Calix[4]arene derivatives, substituted at the wider rim by four urea functions are easily available in great variety. In apolar solvents like benzene or chloroform they dimerize via NH…O=C hydrogen bonds encapsulating a single solvent molecule as guest. The exclusive formation of dimers has been proved in solution by NMR, while their shape has been characterized by single crystal X-ray analysis. The kinetic stability and the rate of the guest exchange have been studied by NOESY experiments using a derivative with lower symmetry.

Ab Initio Studies of Disilazanes:  Structures and Vibrational Properties of Hexachloro-, Hexamethyl-, and Tetrachlorodisilazane

1999

The structures of hexachlorodisilazane, NH(SiCl3)2, and hexamethyldisilazane, NH(SiMe3)2, have been calculated at HF/6-31G* and MP2/6-31G* levels. Both contain planar HNSi2 skeletons and show the expected staggering of the substituents as seen along the Si‚‚‚Si direction, as previously found in the electron diffraction structure of the hexamethyl compound. Unlike the latter, however, the ab initio structures both belong to the C2 point group. Force fields for these two molecules and for tetrachlorodisilazane, NH(SiHCl 2)2, were calculated at the HF level and scaled to produce vibration frequencies for comparison with previously obtained spectra. For NH(SiCl3)2, all but five of the fundament…

Zinn - Chalkogen - Heterocyclen. I. 2.4.6-Hexamethyl-1.3.5-triselena-2.4.6-tristannacyclohexan, Massen- und Schwingungsspektren

2010

The mass spectrum of the title compound exhibits very similar fragmentation patterns compared with the sulfur analogon. The high stability of triply coordinated organotin cations is remarkable. There is no evidence for any stabilisation effects by pπ-dπ bonding concerning the ring bonds. The assignment of the vibration spectra leads to the conclusion that the molecule of triselenastanninane possesses a boat conformation.

Conformational equilibrium and intramolecular hydrogen bond of 4′X and 4X substituted 2′(OH)chalcones

1999

Abstract By using the AM1 method on 4X and 4′X substituted 2′(OH)chalcones (X=H, OH, F, Cl, CH 3 , CH 3 O, N(CH 3 ) 2 , and NO 2 ), certain currently unclear structural characteristics, conformational equilibria, intramolecular hydrogen bonds and UV spectroscopic properties had been clarified. The compounds studied have non-planar structures. The 2′(OH),4′Xchalcones only present the trans - s - cis conformation. trans - s - cis and trans - s - trans conformers of 2′(OH),4X-chalcones have a comparable thermodynamic stability. For these chalcones, a conformational equilibrium at 298 K exists, with 91% of the trans - s - cis form. The proposal of analyzing the intramolecular hydrogen bond of 2…

ChemInform Abstract: Asymmetrically Substituted Calix(4)arenes. A Two-Dimensional 1H NMR Study of a Tetraester Derivative in the cone-Conformation.

2010

Bioactive Constituents of Juniperus turbinata Guss. from La Maddalena Archipelago.

2018

A comprehensive phytochemical study of Juniperus turbinata (Cupressaceae) collected from La Maddalena Archipelago (Sardinia, Italy) is reported. Both the essential oil and the ethanolic extract obtained from the aerial parts were analyzed. The essential oil appears to belong to a new chemotype compared to other Mediterranean juniper accessions, as it was favored by geographic isolation of the isles. It showed a low content of monoterpene hydrocarbons and -terpineol, ent-manoyl oxide, 1,10-di-epi-cubenol as the major constituents. The ethanolic fraction contained mainly diterpenoids. Among these, 15-formyloxyimbricatolic acid (7) is a new natural product since it has hitherto been obtained o…

The structural plasticity of the C terminus of p21Cip1 is a determinant for target protein recognition.

2003

The cyclin-dependent kinase inhibitory protein p21(Cip1) might play multiple roles in cell-cycle regulation through interaction of its C-terminal domain with a defined set of cellular proteins such as proliferating cell nuclear antigen (PCNA), calmodulin (CaM), and the oncoprotein SET. p21(Cip1) could be described as an intrinsically unstructured protein in solution although the C-terminal domain adopts a well-defined extended conformation when bound to PCNA. However, the molecular mechanism of the interaction with CaM and the oncoprotein SET is not well understood, partly because of the lack of structural information. In this work, a peptide derived from the C-terminal domain of p21(Cip1) …