538 research outputs found
The Auroral Planetary Imaging and Spectroscopy (APIS) service
The Auroral Planetary Imaging and Spectroscopy (APIS) service, accessible
online, provides an open and interactive access to processed auroral
observations of the outer planets and their satellites. Such observations are
of interest for a wide community at the interface between planetology and
magnetospheric and heliospheric physics. APIS consists of (i) a high level
database, built from planetary auroral observations acquired by the Hubble
Space Telescope (HST) since 1997 with its mostly used Far-UltraViolet
spectro-imagers, (ii) a dedicated search interface aimed at browsing
efficiently this database through relevant conditional search criteria and
(iii) the ability to interactively work with the data online through plotting
tools developed by the Virtual Observatory (VO) community, such as Aladin and
Specview. This service is VO compliant and can therefore also been queried by
external search tools of the VO community. The diversity of available data and
the capability to sort them out by relevant physical criteria shall in
particular facilitate statistical studies, on long-term scales and/or
multi-instrumental multi-spectral combined analysis
Structure (X-Ray) analysis of manicoline B, a mixture of two diastereoisomers of a new alkaloid from Dulacia guianensis (Olacaceae)
[3+2] Cycloadditions. Part XXXIII. Selective cycloadditions of C-(1-naphthyl)-N-methyl nitrone and C-phenyl-N-benzyl nitrone to α,β-unsaturated carbonyl compounds1,2
488-501[3+2] Cycloadditions [32CA] involving nitrones as 1,3-dipolar species to alkenes yield isoxazolidines, which on further transformations can be converted to naturally occurring bioactive compounds. Thus the 32CA route provides conversion of simple natural products to more complex natural occurring bioactive nitrogen heterocycles, and close analogues. The present work deals with 32CA between the nitrones C-(1-naphthyl)-N-methyl nitrone and C-phenyl-N-benzyl nitrone to α,β-unsaturated carbonyl compounds as dipolarophiles, viz. arylidene malonate esters, methyl cinnamate and benzylidene acetophenone (chalcone). Methyl cinnamate is a naturally occurring compound, while the chalcone scaffold is present in several natural products. Structure elucidation of the generated cycloadducts have been achieved by means of detailed spectroscopic and XRD studies. All the cycloadditions investigated occurr regioselectively to yield tetra/penta-substituted isoxazolidines, where the carbonyl group(s) are situated at the 4-position of the isoxazolidine ring. DFT computations including optimised geometries, FMO energies, electronic chemical potentials, chemical hardness, chemical softness and reactivity indices of a number of the reactants have been calculated at DFT/B3LYP/6-31++G(d,p) level of theory. The calculated reactivity indices have been used to analyse the 32CAs studied and to predict the regioselectivities; the predictions are in excellent accord with the experimental results
Isolation and structure (X-ray analysis) of karinolide, a new quassinoid from Simaba multiflora
Manicol : a sesquiterpenoid hydroxytropolone from Dulacia guianensis; a revised structure (X-ray analysis)
Structural analysis of urate oxidase in complex with its natural substrate inhibited by cyanide: Mechanistic implications
<p>Abstract</p> <p>Background</p> <p>Urate oxidase (EC 1.7.3.3 or UOX) catalyzes the conversion of uric acid and gaseous molecular oxygen to 5-hydroxyisourate and hydrogen peroxide, in the absence of cofactor or particular metal cation. The functional enzyme is a homo-tetramer with four active sites located at dimeric interfaces.</p> <p>Results</p> <p>The catalytic mechanism was investigated through a ternary complex formed between the enzyme, uric acid, and cyanide that stabilizes an intermediate state of the reaction. When uric acid is replaced by a competitive inhibitor, no complex with cyanide is formed.</p> <p>Conclusion</p> <p>The X-ray structure of this compulsory ternary complex led to a number of mechanistic evidences that support a sequential mechanism in which the two reagents, dioxygen and a water molecule, process through a common site located 3.3 Å above the mean plane of the ligand. This site is built by the side chains of Asn 254, and Thr 57, two conserved residues belonging to two different subunits of the homo-tetramer. The absence of a ternary complex between the enzyme, a competitive inhibitor, and cyanide suggests that cyanide inhibits the hydroxylation step of the reaction, after the initial formation of a hydroperoxyde type intermediate.</p
A new paradigm for macromolecular crystallography beamlines derived from high-pressure methodology and results
Macromolecular crystallography at high pressure (HPMX) is a mature technique. Shorter X-ray wavelengths increase data collection efficiency on cryocooled crystals. Extending applications and exploiting spin-off of HPMX will require dedicated synchrotron radiation beamlines based on a new paradigm
Adaptation of the base-paired double-helix molecular architecture to extreme pressure
The behaviour of the d(GGTATACC) oligonucleotide has been investigated by X-ray crystallography at 295 K in the range from ambient pressure to 2 GPa (∼20 000 atm). Four 3D-structures of the A-DNA form (at ambient pressure, 0.55, 1.09 and 1.39 GPa) were refined at 1.60 or 1.65 Å resolution. In addition to the diffraction pattern of the A-form, the broad meridional streaks previously explained by occluded B-DNA octamers within the channels of the crystalline A-form matrix were observed up to at least 2 GPa. This work highlights an important property of nucleic acids, their capability to withstand very high pressures, while keeping in such conditions a nearly invariant geometry of base pairs that store and carry genetic information. The double-helix base-paired architecture behaves as a molecular spring, which makes it especially adapted to very harsh conditions. These features may have contributed to the emergence of a RNA World at prebiotic stage
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