961 research outputs found
Cyanogen Chloride with Organomagnesium Compounds
A Grignard reagent is commonly represented by the formula RMgX. It has been established, however, that the following equilibrium exists in such a reagent (1, 2, 3). 2RMgXR2Mg + MgX2. The assumption has been made that almost without exception the reactions of RMgX and R2Mg are the same and that therefore the representation of the reagent as RMgX is permissible. One of the first exceptions noted was in the reaction between alkyl sulfonates and the Grignard reagent (4, 5, 6). In this work it was shown that each component in the above equilibrium has a different effect in the reaction
Imaging cell surface glycosylation in vivo using "double click" chemistry.
Dynamic alterations in cell surface glycosylation occur in numerous biological processes that involve cell-cell communication and cell migration. We report here imaging of cell surface glycosylation in live mice using double click chemistry. Cell surface glycans were metabolically labeled using peracetylated azido-labeled N-acetylgalactosamine and then reacted, in the first click reaction, with either a cyclooctyne, in a Huisgen [3 + 2] cycloaddition, or with a Staudinger phosphine, via Staudinger ligation. The second click reaction was a [4 + 2] inverse electron demand Diels-Alder reaction between a trans-cyclooctene and a tetrazine, where the latter reagent had been fluorescently labeled with a far-red fluorophore. After administration of the fluorescent tetrazine, the bifunctional cyclooctyne-cyclooctene produced significant azido sugar-dependent fluorescence labeling of tumor, kidney, liver, spleen, and small intestine in vivo, where the kidney and tumor could be imaged noninvasively in the live mouse
The derivation of the formyl-group oxygen of chlorophyll b in higher plants from molecular oxygen.
The mechanism of formation of the formyl group of chlorophyll b has long been obscure but, in this paper, the origin of the 7-formyl-group oxygen of chlorophyll b in higher plants was determined by greening etiolated maize leaves, excised from dark-grown plants, by illumination under white light in the presence of either H218O or 18O2 and examining the newly synthesized chlorophylls by mass spectroscopy. To minimize the possible loss of 18O label from the 7-formyl substituent by reversible formation of chlorophyll b-71-gem-diol (hydrate) with unlabelled water in the cell, the formyl group was reduced to a hydroxymethyl group during extraction with methanol containing NaBH4: chlorophyll a remained unchanged during this rapid reductive extraction process.
Mass spectra of chlorophyll a and [7-hydroxymethyl]-chlorophyll b extracted from leaves greened in the presence of either H218O or 18O2 revealed that 18O was incorporated only from molecular oxygen but into both chlorophylls: the mass spectra were consistent with molecular oxygen providing an oxygen atom not only for incorporation into the 7-formyl group of chlorophyll b but also for the well-documented incorporation into the 131-oxo group of both chlorophylls a and b [see Walker, C. J., Mansfield, K. E., Smith, K. M. & Castelfranco, P. A. (1989) Biochem. J. 257, 599â602]. The incorporation of isotope led to as much as 77% enrichment of the 131-oxo group of chlorophyll a: assuming identical incorporation into the 131 oxygen of chlorophyll b, then enrichment of the 7-formyl oxygen was as much as 93%. Isotope dilution by re-incorporation of photosynthetically produced oxygen from unlabelled water was negligible as shown by a greening experiment in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea.
The high enrichment using 18O2, and the absence of labelling by H218O, unequivocally demonstrates that molecular oxygen is the sole precursor of the 7-formyl oxygen of chlorophyll b in higher plants and strongly suggests a single pathway for the formation of the chlorophyll b formyl group involving the participation of an oxygenase-type enzyme
Brane-bulk energy exchange : a model with the present universe as a global attractor
The role of brane-bulk energy exchange and of an induced gravity term on a
single braneworld of negative tension and vanishing effective cosmological
constant is studied. It is shown that for the physically interesting cases of
dust and radiation a unique global attractor which can realize our present
universe (accelerating and 0<Omega_{m0}<1) exists for a wide range of the
parameters of the model. For Omega_{m0}=0.3, independently of the other
parameters, the model predicts that the equation of state for the dark energy
today is w_{DE,0}=-1.4, while Omega_{m0}=0.03 leads to w_{DE,0}=-1.03. In
addition, during its evolution, w_{DE} crosses the w_{DE}=-1 line to smaller
values.Comment: 8 pages, 2 figures, RevTex; references added, to appear in JHE
Holography and Brane-bulk Energy Exchange
The five-dimensional description of generalized Randall-Sundrum cosmology is
mapped via holography to a generalization of the Starobinsky model. This
provides a holographic dual description of the cosmological brane-bulk energy
exchange processes studied previously. Some simple solutions are presented in
four dimensions.Comment: 41 pages, LaTe
Upgrade of the MIT Linear Electrostatic Ion Accelerator (LEIA) for nuclear diagnostics development for Omega, Z and the NIF
A dark radiation era prior to the inflationary phase
A cosmological model dominated at the beginning by a dark radiation followed
by a period of inflation is presented. This model is based on a Randall-Sundrum
II type brane-world. Current observational data are used to fix the parameters
associated to the dark radiation.Comment: 14 pages, 1 Figure, Accepted for publication in Phys. Lett.
Brane-World Gravity
The observable universe could be a 1+3-surface (the "brane") embedded in a
1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model
particles and fields trapped on the brane while gravity is free to access the
bulk. At least one of the \textit{d} extra spatial dimensions could be very
large relative to the Planck scale, which lowers the fundamental gravity scale,
possibly even down to the electroweak ( TeV) level. This revolutionary
picture arises in the framework of recent developments in M theory. The
1+10-dimensional M theory encompasses the known 1+9-dimensional superstring
theories, and is widely considered to be a promising potential route to quantum
gravity. At low energies, gravity is localized at the brane and general
relativity is recovered, but at high energies gravity "leaks" into the bulk,
behaving in a truly higher-dimensional way. This introduces significant changes
to gravitational dynamics and perturbations, with interesting and potentially
testable implications for high-energy astrophysics, black holes, and cosmology.
Brane-world models offer a phenomenological way to test some of the novel
predictions and corrections to general relativity that are implied by M theory.
This review analyzes the geometry, dynamics and perturbations of simple
brane-world models for cosmology and astrophysics, mainly focusing on warped
5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover
the simplest brane-world models in which 4-dimensional gravity on the brane is
modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati
models. Then we discuss co-dimension two branes in 6-dimensional models.Comment: A major update of Living Reviews in Relativity 7:7 (2004)
"Brane-World Gravity", 119 pages, 28 figures, the update contains new
material on RS perturbations, including full numerical solutions of
gravitational waves and scalar perturbations, on DGP models, and also on 6D
models. A published version in Living Reviews in Relativit
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