150 research outputs found
Clustering of MgII absorption line systems around massive galaxies: an important constraint on feedback processes in galaxy formation
We use the latest version of the metal line absorption catalogue of Zhu &
M\'enard (2013) to study the clustering of MgII absorbers around massive
galaxies (~10^11.5 M_sun), quasars and radio-loud AGN with redshifts between
0.4 and 0.75. Clustering is evaluated in two dimensions, by binning absorbers
both in projected radius and in velocity separation. Excess MgII is detected
around massive galaxies out to R_p=20 Mpc. At projected radii less than 800
kpc, the excess extends out to velocity separations of 10,000 km/s. The extent
of the high velocity tail within this radius is independent of the mean stellar
age of the galaxy and whether or not it harbours an active galactic nucleus. We
interpret our results using the publicly available Illustris and Millennium
simulations. Models where the MgII absorbers trace the dark matter particle or
subhalo distributions do not fit the data. They overpredict the clustering on
small scales and do not reproduce the excess high velocity separation MgII
absorbers seen within the virial radius of the halo. The Illustris simulations
which include thermal, but not mechanical feedback from AGN, also do not
provide an adequate fit to the properties of the cool halo gas within the
virial radius. We propose that the large velocity separation MgII absorbers
trace gas that has been pushed out of the dark matter halos, possibly by
multiple episodes of AGN-driven mechanical feedback acting over long
timescales.Comment: 10 pages, 11 figures, accepted in MNRA
Stabilization of an α-helix by short adjacent accessory foldamers
AbstractTemplate-based stabilization of α-peptide helices with short accessory non-peptide helical foldamers fused either at the N- or C-terminus or at both ends of the peptide segment has been investigated by NMR spectroscopy in polar solvents and by X-ray diffraction. In this work, we focused on aliphatic N,NâČ-linked oligoureas that form predictable and well-defined helical structures akin to α-helices. Our results indicate that urea oligomers have the ability to enforce a peptide segment to adopt a well-defined α-helical structure and may suggest a general approach to stabilize short helical peptide epitopes for the development of modulators of proteinâprotein interactions
Hybrid Sequences that Express both Aromatic Amide and 뱉Peptidic Folding Features
Foldamers combining aliphatic and aromatic mainâchain units often produce atypical structures that cannot easily be accessed from purely aromatic or aliphatic sequences. We report solidâstate evidence that sequences comprising αâamino acids and quinolineâbased monomers adopt conformations that combine the folding propensities of both components. Foldamers 2 and 3 having an XQQ repeat motif (X=αâamino acid, Q=quinoline) were synthesized. Crystals of 2 (X=Phe, Q with an anionic side chain) obtained from water revealed an aromatic helix where amide groups belonging to the αâamino acids created a hydrogenâbond array typical of peptidic helices. Crystals of 3 (X=Ser, Q with a lipophilic side chain) obtained from organic solvents revealed a helixâturnâhelix structure in which αâamino acid side chains interfere with mainâchain hydrogen bonding. High sequenceâdependency of the conformation is typical of peptides but is shown here to include aromatic folding features
Racemic crystal structures of A-DNA duplexes
The ease with which racemic mixtures crystallize compared with the equivalent chiral systems is routinely taken advantage of to produce crystals of small molecules. However, biological macromolecules such as DNA and proteins are naturally chiral, and thus the limited range of chiral space groups available hampers the crystallization of such molecules. Inspiring work over the past 15 years has shown that racemic mixtures of proteins, which were made possible by impressive advances in protein chemical synthesis, can indeed improve the success rate of protein crystallization experiments. More recently, the racemic crystallization approach was extended to include nucleic acids as a possible aid in the determination of enantiopure DNA crystal structures. Here, findings are reported that suggest that the benefits may extend beyond this. Two racemic crystal structures of the DNA sequence d(CCCGGG) are described which were found to fold into A-form DNA. This form differs from the Z-form DNA conformation adopted by the chiral equivalent in the solid state, suggesting that the use of racemates may also favour the emergence of new conformations. Importantly, the racemic mixture forms interactions in the solid state that differ from the chiral equivalent (including the formation of racemic pseudo-helices), suggesting that the use of racemic DNA mixtures could provide new possibilities for the design of precise self-assembled nanomaterials and nanostructures
Mechanism of reconstitution/activation of the soluble PQQ-dependent glucose dehydrogenase from Acinetobacter calcoaceticus: a comprehensive study
International audienc
Allosteric Recognition of Homomeric and Heteromeric Pairs of Monosaccharides by a Foldamer Capsule
The recognition of either homomeric or heteromeric pairs of pentoses in an aromatic oligoamide double helical foldamer capsule was evidenced by circular dichroism (CD), NMR spectroscopy, and Xâray crystallography. The cavity of the host was predicted to be large enough to accommodate simultaneously two xylose molecules and to form a 1:2 complex (one container, two saccharides). Solution and solidâstate data revealed the selective recognition of the αâ4C1âdâxylopyranose tautomer, which is bound at two identical sites in the foldamer cavity. A step further was achieved by sequestering a heteromeric pair of pentoses, that is, one molecule of αâ4C1âdâxylopyranose and one molecule of ÎČâ1C4âdâarabinopyranose despite the symmetrical nature of the host and despite the similarity of the guests. Subtle inducedâfit and allosteric effects are responsible for the outstanding selectivities observed
Aromatic oligoamide foldamers as versatile scaffolds for induced circularly polarized luminescence at adjustable wavelengths
International audienceQuinoline oligoamide foldamers appended with non-chiral fluorophores and derivatized with a camphanyl chiral inducer display strong chiroptical properties at tunable wavelengths as proved by CD and CPL spectroscopies. Induced CPL activity with high luminescence dissymmetry factors was observed in the visible range at wavelengths specific to the fluorophores
A post-elongation strategy for the introduction of guanidinium units in the main chain of helical oligourea foldamers
International audienc
Photolariats: synthesis, metal ion complexation and photochromism
International audiencePhotolariat development, as an extension to the family of synthetic photochromic crown ether receptors, or photocrowns, is reported. Incorporated additional chelating groups, namely two anisoles or thioanisoles, contribute in completing the metal ion coordination sphere with different affinities and selectivities for a range of ions. Single-crystal X-ray diffraction analysis suggests that the thermally stable trans-form of the receptor is unsuitable for ion binding, consistent with spectrophotometric and NMR titration results, which is largely improved in the cis-form as the basis for the photocontrolled ion coordination/ejection. In terms of the azobenzene-containing receptor photochemistry, a photostationary state highly enriched in the cis-form (94:6, cis-/trans-) is reached, with slow thermal return (1.1-1.4 Ă 10â 5 sâ 1) in the dark, which can undergo multiple cycles without discernable photodegradation
Chem. Sci.
The isobutyl side chain is a highly prevalent hydrophobic group in drugs, and it notably constitutes the side chain of leucine. Its replacement by a hexafluorinated version containing two CF3 groups may endow the target compound with new and advantageous properties, yet this modification remains overlooked due to the absence of a general and practical synthetic methodology. Herein, we report the first general method to introduce the hexafluoroisobutyl group into ketoesters, malonates, 1,3-diketones, Schiff base esters and malononitrile. We demonstrated that the reaction occurs through an elimination/allylic shift/hydrofluorination cascade process which efficiently overcomes the usual fluoride ÎČ-elimination observed with α-CF3-vinyl groups. We showed that with alkali metal bases, a pentafluorinated alkene is obtained predominantly, whereas the use of tetrabutylammonium fluoride (TBAF) allows hydrofluorination to occur. This tandem process represents a conceptually new pathway to synthesize bis-trifluoromethylated compounds. This methodology was applied to the multigram-scale synthesis of enantiopure (S)-5,5,5,5âČ,5âČ,5âČ-hexafluoroleucine.Interactions supramolĂ©culaires sĂ©lectives et directionnelles Ă base de synthons fluorĂ©s hautement polaire
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