93 research outputs found
Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization
Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.
Origins of archaeal tetraether lipids in sediments : insights from radiocarbon analysis
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 72 (2008): 4577-4594, doi:10.1016/j.gca.2008.06.021.Understanding the supply and preservation of glycerol dibiphytanyl glycerol tetraethers
(GDGTs) in marine sediments helps inform their use in paleoceanography. Compound-specific
radiocarbon measurements of sedimentary alkenones from multiple environments have been
used to gain insight into processes that affect paleo-temperature reconstructions. Similar
analyses are warranted to investigate how analogous processes affecting GDGTs impact TEX86
paleotemperatures. Here we present radiocarbon measurements on individual GDGTs from
Bermuda Rise and Santa Monica Basin sediments and discuss the results in the context of
previous studies of co-depositional alkenones and foraminifera. The 149 C contents of GDGTs and
planktonic foraminifera in Bermuda Rise are very similar, suggesting a local source; and TEX86-
derived temperatures agree more closely with foraminiferal temperatures than do
temperatures. In contrast, GDGTs in Santa Monica Basin are depleted in 1412 C relative to both
alkenones and foraminifera, and TEX86 temperatures agree poorly with known surface water
values. We propose three possible factors that could explain these results: (i) GDGTs may be
labile relative to alkenones during advective transport through oxic waters; (ii) archaeal
production deep in the water column may contribute 1416 C-depleted GDGTs to sediments; and (iii)
some GDGTs also may derive from sedimentary archaeal communities. Each of these three
processes is likely to occur with varying relative importance depending on geographic location.
The latter two may help to explain why TEX86 temperature reconstructions from Santa Monica
Basin do not appear to reflect actual sea surface temperatures. Terrigenous GDGTs are unlikely
to be major contributors to Bermuda Rise or Santa Monica Basin sediments, based on values of
the BIT index. The results also indicate that the crenarchaeol regioisomer is governed by
processes different from other GDGTs. Individual measurements of the crenarchaeol regioisomer
are significantly depleted in 1424 C relative to co-occurring GDGTs, indicating an alternative origin
for this compound that presently remains unknown. Re-examination of the contribution of
crenarchaeol regioisomer to the TEX86 index shows that it is a significant influence on the
sensitivity of temperature reconstructions.This work was supported by the David & Lucille Packard Foundation and by NSF-OCE-0241363 and EAR-0311937 (to A.P.)
Congenital Diaphragmatic hernia – a review
Congenital Diaphragmatic hernia (CDH) is a condition characterized by a defect in the diaphragm leading to protrusion of abdominal contents into the thoracic cavity interfering with normal development of the lungs. The defect may range from a small aperture in the posterior muscle rim to complete absence of diaphragm. The pathophysiology of CDH is a combination of lung hypoplasia and immaturity associated with persistent pulmonary hypertension of newborn (PPHN) and cardiac dysfunction. Prenatal assessment of lung to head ratio (LHR) and position of the liver by ultrasound are used to diagnose and predict outcomes. Delivery of infants with CDH is recommended close to term gestation. Immediate management at birth includes bowel decompression, avoidance of mask ventilation and endotracheal tube placement if required. The main focus of management includes gentle ventilation, hemodynamic monitoring and treatment of pulmonary hypertension followed by surgery. Although inhaled nitric oxide is not approved by FDA for the treatment of PPHN induced by CDH, it is commonly used. Extracorporeal membrane oxygenation (ECMO) is typically considered after failure of conventional medical management for infants ≥ 34 weeks’ gestation or with weight >2 kg with CDH and no associated major lethal anomalies. Multiple factors such as prematurity, associated abnormalities, severity of PPHN, type of repair and need for ECMO can affect the survival of an infant with CDH. With advances in the management of CDH, the overall survival has improved and has been reported to be 70-90% in non-ECMO infants and up to 50% in infants who undergo ECMO
Small shelly fossils and carbon isotopes from the early Cambrian (Stage 3-4) Mural Formation of western Laurentia
The extraordinary window of phosphatised and phosphatic Small Shelly Fossils (SSFs) during the early and middle Cambrian is an important testament to the radiation of biomineralising metazoans. While SSF are well known from most Cambrian palaeocontinents during this time interval, western Laurentia has relatively few SSF faunas. Here we describe a diverse SSF fauna from the early Cambrian (Stage 3-4) Mural Formation at three localities in Alberta and British Columbia, Canada, complemented by carbon isotope measurements to aid in a potential future bio-chemostratigraphic framework. The fauna expands the recorded SSF assemblage diversity in western Laurentia and includes several brachiopods, four bradoriids, three chancelloriids, two hyoliths, a tommotiid and a helcionellid mollusc as well as echinoderm ossicles and specimens of Microdictyon, Volborthella and Hyolithellus. New taxa include the tommotiid genus Canadiella gen. nov., the new bradoriid species Hipponicharion perforata sp. nov. and Pseudobeyrichona taurata sp. nov. Compared to contemporaneous faunas from western Laurentia, the fauna is relatively diverse, particularly in taxa with originally phosphatic shells, which appear to be associated with archaeocyathid buildups. This suggests that the generally low faunal diversity in western Laurentia may be at least partly a consequence of poor sampling of suitable archaeocyathan reef environments. In addition, the tommotiid Canadiella filigrana appears to be of biostratigraphic significance in Cambrian Stage 3 strata of western Laurentia and the unexpected high diversity of bradoriid arthropods in the fauna also suggests that this group may prove useful for biostratigraphic resolution in the region
The fine structure of neurons and other elements in the nervous system of the giant African land snail Archachatina marginata
Microstructure of Crystallizable α‑Olefin Molecular Bottlebrushes: Isotactic and Atactic Poly(1-octadecene)
Isotactic
and atactic poly(1-octadecene) (iPOD and aPOD) have been
synthesized by organometallic coordinative insertion polymerization
of 1-octadecene. Analyzing X-ray and neutron scattering data of POD
melts identifies their bottlebrush structures as flexible rods where
the rod length is the extended backbone length and rod radius is the
side chain coil dimension. Upon cooling, both iPOD and aPOD melts
crystallize by fully extending their coiled side chains to form orthorhombic
alkane crystals in iPOD and nematically ordered rotator alkane crystals
in aPOD, as determined by X-ray scattering and Raman spectroscopy.
Molecular dynamics simulations of isotactic and atactic 48-mers of
1-octadecene were applied to define and verify melt and crystalline
structures and scattering peak assignments, respectively. Modeling
suggests that side chains of both crystallized isotactic and atactic
PODs align at 70° and 160° to the 4/1 spiral backbone of
equal probability, at an average of 115°, and POD chains pack
in an antiparallel pattern. Large wheat-sheaf structural assembly
of fibril bundles can be observed in aPOD, which render high opacity
to these samples. Each of those fibrils is made of several bottlebrush
molecules packed into a hexagonal lattice. Faster crystallization
observed in iPODs hinders the formation of large crystallites, which
results in translucent samples
Neurosecretory processes projecting from the preoptic nucleus into the third ventricle of Zoarces viviparus L.
Self-Assembly of the 2-His-1-carboxylate Facial Triad in Mononuclear Iron(II) and Zinc(II) Models of Metalloenzyme Active Sites
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