Anomalous Nitrogen Isotope Ratio in Comets Arpigny, Claude; Jehin, Emmanuël; Manfroid, Jean ...
Science (American Association for the Advancement of Science),
09/2003, Letnik:
301, Številka:
5639
Journal Article, Web Resource
Recenzirano
High-resolution spectra of the$CN B^2 \Sigma^+-X^2 \Sigma^+$(0,0) band at 390 nanometers yield isotopic ratios for comets C/1995 O1 (Hale-Bopp) and C/2000 WM1 (LINEAR) as follows:$165 \pm 40$and$115 ...\pm 20$for12C/13C,$140 \pm 35$and$140 \pm 30$for14N/15N. Our N isotopic measurements are lower than the terrestrial$^{14}N/^{15}N = 272$and the ratio for Hale-Bopp from measurements of HCN, the presumed parent species of CN. This isotopic anomaly suggests the existence of other parent(s) of CN, with an even lower N isotopic ratio. Organic compounds like those found in interplanetary dust particles are good candidates.
The CN isotopic ratios in comets Manfroid, J.; Jehin, E.; Hutsemékers, D. ...
Astronomy and astrophysics (Berlin),
08/2009, Letnik:
503, Številka:
2
Journal Article, Web Resource
Recenzirano
Odprti dostop
Our aim is to determine the isotopic ratios 12C/13C and 14N/15N in a variety of comets and link these measurements to the formation and evolution of the solar system. The 12C/13C and 14N/15N isotopic ...ratios are measured for the CN radical by means of high-resolution optical spectra of the R branch of the B-X (0, 0) violet band. 23 comets from different dynamical classes have been observed, sometimes at various heliocentric and nucleocentric distances, in order to estimate possible variations of the isotopic ratios in parent molecules. The 12C/13C and 14N/15N isotopic ratios in CN are remarkably constant (average values of, respectively, 91.0 ± 3.6 and 147.8 ± 5.7) within our measurement errors, for all comets whatever their origin or heliocentric distance. While the carbon isotopic ratio does agree with the terrestrial value (89), the nitrogen ratio is a factor of two lower than the terrestrial value (272), indicating a fractionation in the early solar system, or in the protosolar nebula, common to all the comets of our sample. This points towards a common origin of the comets independently of their birthplaces, and a relationship between HCN and CN.
Architecture of the symmetric core of the nuclear pore Lin, Daniel H.; Stuwe, Tobias; Schilbach, Sandra ...
Science (American Association for the Advancement of Science),
04/2016, Letnik:
352, Številka:
6283
Journal Article
Recenzirano
Odprti dostop
The nuclear pore complex (NPC) controls the transport of macromolecules between the nucleus and cytoplasm, but its molecular architecture has thus far remained poorly defined. We biochemically ...reconstituted NPC core protomers and elucidated the underlying protein-protein interaction network. Flexible linker sequences, rather than interactions between the structured core scaffold nucleoporins, mediate the assembly of the inner ring complex and its attachment to the NPC coat. X-ray crystallographic analysis of these scaffold nucleoporins revealed the molecular details of their interactions with the flexible linker sequences and enabled construction of full-length atomic structures. By docking these structures into the cryoelectron tomographic reconstruction of the intact human NPC and validating their placement with our nucleoporin interactome, we built a composite structure of the NPC symmetric core that contains ~320,000 residues and accounts for ~56 megadaltons of the NPC's structured mass. Our approach provides a paradigm for the structure determination of similarly complex macromolecular assemblies.
Context. Comets are believed to hold a relatively pristine record of the physical and chemical processes that occurred during the formation and evolution of the solar system. Thorough investigations ...of these small bodies, such as the one that will be performed by the ESA/Rosetta cornerstone mission, are thus supposed to bring strong and unique constraints on the origins of the solar system. Aims. Because comet 67P/Churyumov-Gerasimenko was only recently selected as the target for the ESA/Rosetta mission, there has been little opportunity to study its pre-perihelion activity. This phase is, however, very important for the mission, since the global mapping of the nucleus and the choice of landing site for Philae will be performed during this pre-perihelion phase. Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008. Methods. The gas and dust activity of comet 67P/Churyumov-Gerasimenko are studied through visible spectroscopy and broadband imaging, respectively, covering a range of pre-perihelion heliocentric distances between 2.99 and 2.22 AU. Results. The data we have gathered on the dust activity are consistent with trends observed by other authors and show a strong asymmetry between the pre- and post-perihelion phases of the orbit. The spectra do not show any lines due to the emission of volatiles, and upper limits on their production rates are typically one order of magnitude lower than at the equivalent post-perihelion heliocentric distances. The asymmetry in the pre- and post-perihelion phases of the activity may be due to a dusty crust quenching the activity at the surface of 67P. We estimate that this crust could be about 12 cm thick, although not uniform across the surface. Even if no gas is individually detected, the coma surface brightness profiles might indicate a possible contamination from gaseous species emitted before the comet actually reaches perihelion.
Architecture of the fungal nuclear pore inner ring complex Stuwe, Tobias; Bley, Christopher J.; Thierbach, Karsten ...
Science (American Association for the Advancement of Science),
10/2015, Letnik:
350, Številka:
6256
Journal Article
Recenzirano
Odprti dostop
The nuclear pore complex (NPC) constitutes the sole gateway for bidirectional nucleocytoplasmic transport. We present the reconstitution and interdisciplinary analyses of the ∼425-kilodalton inner ...ring complex (IRC), which forms the central transport channel and diffusion barrier of the NPC, revealing its interaction network and equimolar stoichiometry. The Nsp1•Nup49•Nup57 channel nucleoporin heterotrimer (CNT) attaches to the IRC solely through the adaptor nucleoporin Nic96. The CNT•Nic96 structure reveals that Nic96 functions as an assembly sensor that recognizes the three-dimensional architecture of the CNT, thereby mediating the incorporation of a defined CNT state into the NPC. We propose that the IRC adopts a relatively rigid scaffold that recruits the CNT to primarily form the diffusion barrier of the NPC, rather than enabling channel dilation.
The nucleocapsid protein (N) of SARS-CoV-2 is essential for virus replication, genome packaging, evading host immunity, and virus maturation. N is a multidomain protein composed of an independently ...folded monomeric N-terminal domain that is the primary site for RNA binding and a dimeric C-terminal domain that is essential for efficient phase separation and condensate formation with RNA. The domains are separated by a disordered Ser/Arg-rich region preceding a self-associating Leu-rich helix. Phosphorylation in the Ser/Arg region in infected cells decreases the viscosity of N:RNA condensates promoting viral replication and host immune evasion. The molecular level effect of phosphorylation, however, is missing from our current understanding. Using NMR spectroscopy and analytical ultracentrifugation, we show that phosphorylation destabilizes the self-associating Leu-rich helix 30 amino-acids distant from the phosphorylation site. NMR and gel shift assays demonstrate that RNA binding by the linker is dampened by phosphorylation, whereas RNA binding to the full-length protein is not significantly affected presumably due to retained strong interactions with the primary RNA-binding domain. Introducing a switchable self-associating domain to replace the Leu-rich helix confirms the importance of linker self-association to droplet formation and suggests that phosphorylation not only increases solubility of the positively charged elongated Ser/Arg region as observed in other RNA-binding proteins but can also inhibit self-association of the Leu-rich helix. These data highlight the effect of phosphorylation both at local sites and at a distant self-associating hydrophobic helix in regulating liquid–liquid phase separation of the entire protein.
The evolution of the activity and composition of the coma of ROSETTA target comet 67P/Churyumov-Gerasimenko was studied along its postperihelion orbit from 2.29 AU to 3.22 AU. The comet had a major ...drop of activity between 2.5 AU and 2.9 AU, which manifested most obviously in its light curve (the brightness decreased by 3 mag), but is also confirmed by the production rates derived from spectrophotometric measurements. The strong decrease of activity indicates a change in the outgassing behaviour of the nucleus and might be due to water sublimation becoming less efficient. The spectrophotometric measurements at 2.9 AU show that the coma is at least slightly depleted in C sub(2) with respect to CN.
Context.Icy grains in the inner coma of a comet may play an important role in the energy balance and in the production of certain gas coma species. Their existence has therefore been assumed ...repeatedly to explain a variety of observed phenomena. However, owing to their extremely short life time no evidence for the presence of icy grains had been found in any active comet close to the Sun.Aims.We observed Comet 9P/Tempel 1 during the Deep Impact mission to look for phenomena induced by the impact.Methods.The comet was observed with the XMM-Newton Observatory. We used the EPIC camera for X-ray imaging and the Optical Monitor for monitoring in the ultraviolet and visible spectral range.Results.An outburst of the comet nucleus was observed as a result of the impact and the evolution of the coma was monitored in gas and dust. Our observations led to the first detection of icy grains in a comet at 1.5 AU from the Sun.Conclusions.We showed for the first time that the material ejected from the nucleus of a comet contains icy grains, even at small heliocentric distance.
Over the past 10 years the isotopic ratios of carbon (
12C/
13C) and nitrogen (
14N/
15N) have been determined for a dozen comets, bright enough to allow obtaining the required measurements from the ...ground. The ratios were derived from high-resolution spectra of the CN coma measured in the B
2∑
+−X
2∑
+ (0, 0) emission band around 387
nm. The observed comets belong to different dynamical classes, including dynamically new as well as long- and short-period comets from the Halley- and Jupiter-family. In some cases the comets could be observed at various heliocentric distances. All values determined for the carbon and nitrogen isotopic ratios were consistent within the error margin irrespective of the type of comet or the heliocentric distance at which it was observed. Our investigations resulted in average ratios of
12C/
13C=91±21 and nitrogen
14N/
15N=141±29. Whilst the value for the carbon isotopic ratio is in good agreement with the solar and terrestrial value of 89, the nitrogen isotopic ratio is very different from the telluric value of 272.
The 12C14N/12C15N and 12C14N/13C14N isotopic ratios are determined for the first time in a Jupiter-family comet, 88P/1981 Q1 Howell, and in the chemically peculiar Oort Cloud comet C/1999 S4 ...(LINEAR). By comparing these measurements to previous ones derived for six other Oort Cloud comets (including one of Halley-type), we find that both the carbon and nitrogen isotopic ratios are constant within the uncertainties. The mean values are 12C/13C 90 and 14N/15N for the eight comets. These results strengthen the view that CN radicals originate from refractory organics formed in the protosolar molecular cloud and subsequently incorporated in comets.
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