1,756 research outputs found
Ethyl and isopropyl 4-ferrocenylbenzoate.
The title compounds, [Fe(C5H5)(C14H13O2)] and [Fe(C5H5)-
(C15H15O2)], respectively, contain the ferrocenyl 5(C5H4) and
phenylene ±C6H4± rings in a nearly coplanar arrangement,
with interplanar angles of 6.88 (12) and 10.5 (2), respectively.
Molecules of the ethyl ester form dimers through 5(C5H5)CÐ
H O C hydrogen bonds, with graph set R22
(20), and,
together with Csp3ÐH (C5H5) interactions, generate a
one-dimensional column (irregular ladder). Molecules of the
isopropyl ester aggregate through 5(C5H5)CÐH (C6H4)
interactions
Spatial and Temporal Organization of Chromosome Duplication and Segregation in the Cyanobacterium Synechococcus elongatus PCC 7942
The spatial and temporal control of chromosome duplication and segregation is crucial for proper cell division. While this process is well studied in eukaryotic and some prokaryotic organisms, relatively little is known about it in prokaryotic polyploids such as Synechococcus elongatus PCC 7942, which is known to possess one to eight copies of its single chromosome. Using a fluorescent repressor-operator system, S. elongatus chromosomes and chromosome replication forks were tagged and visualized. We found that chromosomal duplication is asynchronous and that the total number of chromosomes is correlated with cell length. Thus, replication is independent of cell cycle and coupled to cell growth. Replication events occur in a spatially random fashion. However, once assembled, replisomes move in a constrained manner. On the other hand, we found that segregation displays a striking spatial organization in some cells. Chromosomes transiently align along the major axis of the cell and timing of alignment was correlated to cell division. This mechanism likely contributes to the non-random segregation of chromosome copies to daughter cells
Renormalons in Effective Field Theories
We investigate the high-order behavior of perturbative matching conditions in
effective field theories. These series are typically badly divergent, and are
not Borel summable due to infrared and ultraviolet renormalons which introduce
ambiguities in defining the sum of the series. We argue that, when treated
consistently, there is no physical significance to these ambiguities. Although
nonperturbative matrix elements and matching conditions are in general
ambiguous, the ambiguity in any physical observable is always higher order in
than the theory has been defined. We discuss the implications for the
recently noticed infrared renormalon in the pole mass of a heavy quark. We show
that a ratio of form factors in exclusive decays (which is related
to the pole mass) is free from renormalon ambiguities regardless of the mass
used as the expansion parameter of HQET. The renormalon ambiguities also cancel
in inclusive heavy hadron decays. Finally, we demonstrate the cancellation of
renormalons in a four-Fermi effective theory obtained by integrating out a
heavy colored scalar.Comment: Minor changes mad
Architecture and Selectivity in Aquaporins: 2.5 Å X-Ray Structure of Aquaporin Z
Aquaporins are a family of water and small molecule channels found in organisms ranging from bacteria to animals. One of these channels, the E. coli protein aquaporin Z (AqpZ), has been shown to selectively conduct only water at high rates. We have expressed, purified, crystallized, and solved the X-ray structure of AqpZ. The 2.5 Å resolution structure of AqpZ suggests aquaporin selectivity results both from a steric mechanism due to pore size and from specific amino acid substitutions that regulate the preference for a hydrophobic or hydrophilic substrate. This structure provides direct evidence on the molecular mechanisms of specificity between water and glycerol in this family of channels from a single species. It is to our knowledge the first atomic resolution structure of a recombinant aquaporin and so provides a platform for combined genetic, mutational, functional, and structural determinations of the mechanisms of aquaporins and, more generally, the assembly of multimeric membrane proteins
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The Bacterial Carbon-Fixing Organelle Is Formed by Shell Envelopment of Preassembled Cargo
Background: Cyanobacteria play a significant role in the global carbon cycle. In Synechococcuselongatus, the carbon-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is concentrated into polyhedral, proteinaceous compartments called carboxysomes. Methodology/Principal Findings Using live cell fluorescence microscopy, we show that carboxysomes are first detected as small seeds of RuBisCO that colocalize with existing carboxysomes. These seeds contain little or no shell protein, but increase in RuBisCO content over several hours, during which time they are exposed to the solvent. The maturing seed is then enclosed by shell proteins, a rapid process that seals RuBisCO from the cytosol to establish a distinct, solvent-protected microenvironment that is oxidizing relative to the cytosol. These closure events can be spatially and temporally coincident with the appearance of a nascent daughter RuBisCO seed. Conclusions/Significance: Carboxysomes assemble in a stepwise fashion, inside-to-outside, revealing that cargo is the principle organizer of this compartment’s biogenesis. Our observations of the spatial relationship of seeds to previously formed carboxysomes lead us to propose a model for carboxysome replication via sequential fission, polymerization, and encapsulation of their internal cargo
Rubisco function, evolution, and engineering
Carbon fixation is the process by which CO2 is converted from a gas into
biomass. The Calvin Benson Bassham (CBB) cycle is the dominant carbon fixation
pathway on earth, driving >99.5% of the ~120 billion tons of carbon that are
"fixed" as sugar, by plants, algae and cyanobacteria. The carboxylase enzyme in
the CBB, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fixes one
CO2 molecule per turn of the cycle. Despite being critical to the assimilation
of carbon, rubisco's kinetic rate is not very fast and it is a bottleneck in
flux through the pathway. This presents a paradox - why hasn't rubisco evolved
to be a better catalyst? Many hypothesize that the catalytic mechanism of
rubisco is subject to one or more trade-offs, and that rubisco variants have
been optimized for their native physiological environment. Here we review the
evolution and biochemistry of rubisco through the lens of structure and
mechanism in order to understand what trade-offs limit its improvement. We also
review the many attempts to improve rubisco itself and, thereby, promote plant
growth
SU(3) Predictions for Weak Decays of Doubly Heavy Baryons -- including SU(3) breaking terms
We find expressions for the weak decay amplitudes of baryons containing two b
quarks (or one b and one c quark -- many relationship are the same) in terms of
unknown reduced matrix elements. This project was originally motivated by the
request of the FNAL Run II b Physics Workshop organizers for a guide to
experimentalists in their search for as yet unobserved hadrons. We include an
analysis of linear SU(3) breaking terms in addition to relationships generated
by unbroken SU(3) symmetry, and relate these to expressions in terms of the
complete set of possible reduced matrix elements.Comment: 49 page
Long-lived space observatories for astronomy and astrophysics
NASA's plan to build and launch a fleet of long-lived space observatories that include the Hubble Space Telescope (HST), the Gamma Ray Observatory (GRO), the Advanced X Ray Astrophysics Observatory (AXAF), and the Space Infrared Telescope Facility (SIRTF) are discussed. These facilities are expected to have a profound impact on the sciences of astronomy and astrophysics. The long-lived observatories will provide new insights about astronomical and astrophysical problems that range from the presence of planets orbiting nearby stars to the large-scale distribution and evolution of matter in the universe. An important concern to NASA and the scientific community is the operation and maintenance cost of the four observatories described above. The HST cost about 160 million (1986 dollars) a year to operate and maintain. If HST is operated for 20 years, the accumulated costs will be considerably more than those required for its construction. Therefore, it is essential to plan carefully for observatory operations and maintenance before a long-lived facility is constructed. The primary goal of this report is to help NASA develop guidelines for the operations and management of these future observatories so as to achieve the best possible scientific results for the resources available. Eight recommendations are given
Experimenting with ecosystem interaction networks in search of threshold potentials in real-world marine ecosystems
Thresholds profoundly affect our understanding and management of ecosystem dynamics, but we have yet to develop practical techniques to assess the risk that thresholds will be crossed. Combining ecological knowledge of critical system interdependencies with a large-scale experiment, we tested for breaks in the ecosystem interaction network to identify threshold potential in real-world ecosystem dynamics. Our experiment with the bivalves Macomona liliana and Austrovenus stutchburyi on marine sandflats in New Zealand demonstrated that reductions in incident sunlight changed the interaction network between sediment biogeochemical fluxes, productivity, and macrofauna. By demonstrating loss of positive feedbacks and changes in the architecture of the network, we provide mechanistic evidence that stressors lead to break points in dynamics, which theory predicts predispose a system to a critical transition
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