2,929 research outputs found
Living with Oxygen
Work on the electronic structures of metalâoxo complexes began in Copenhagen over 50 years ago. This work led to the prediction that tetragonal multiply bonded transition metalâoxos would not be stable beyond the ironârutheniumâosmium oxo wall in the periodic table and that triply bonded metalâoxos could not be protonated, even in the strongest BrĂžnsted acids. In this theory, only double bonded metalâoxos could attract protons, with basicities being a function of the electron donating ability of ancillary ligands. Such correlations of electronic structure with reactivity have gained importance in recent years, most notably owing to the widespread recognition that high-valent ironâoxos are intermediates in biological reactions critical to life on Earth.
In this Account, we focus attention on the oxygenations of inert organic substrates by cytochromes P450, as these reactions involve multiply bonded ironâoxos. We emphasize that P450 ironâoxos are strong oxidants, so strong that they would destroy nearby amino acids if substrates are not oxygenated rapidly; it is our view that these high-valent ironâoxos are such dangerous reactive oxygen species that Nature surely found ways to disable them. Looking more deeply into this matter, mainly by examining many thousands of structures in the Protein Data Bank, we have found that P450s and other enzymes that require oxygen for function have chains of tyrosines and tryptophans that extend from active-site regions to protein surfaces. Tyrosines are near the heme active sites in bacterial P450s, whereas tryptophan is closest in most human enzymes. High-valent ironâoxo survival times taken from hole hopping maps range from a few nanoseconds to milliseconds, depending on the distance of the closest Trp or Tyr residue to the heme. In our proposed mechanism, multistep hole tunneling (hopping) through Tyr/Trp chains guides the damaging oxidizing hole to the protein surface, where it can be quenched by soluble protein or small molecule reductants. As the Earthâs oxygenic atmosphere is believed to have developed about 2.5 billion years ago, the increase in occurrence frequency of tyrosine and tryptophan since the last universal evolutionary ancestor may be in part a consequence of enzyme protective functions that developed to cope with the environmental toxin, O_2
Pressure Dependence of the Magnetic Anisotropy in the "Single-Molecule Magnet" [Mn4O3Br(OAc)3(dbm)3]
The anisotropy splitting in the ground state of the single-molecule magnet
[Mn4O3Br(OAc)3(dbm)3] is studied by inelastic neutron scattering as a function
of hydrostatic pressure. This allows a tuning of the anisotropy and thus the
energy barrier for slow magnetisation relaxation at low temperatures. The value
of the negative axial anisotropy parameter changes from
-0.0627(1) meV at ambient to -0.0603(3) meV at 12 kbar pressure, and in the
same pressure range the height of the energy barrier between up and down spins
is reduced from 1.260(5) meV to 1.213(9) meV. Since the bond is
significantly softer and thus more compressible than the bonds,
pressure induces a tilt of the single ion Mn anisotropy axes, resulting
in the net reduction of the axial cluster anisotropy.Comment: 4 pages, 3 figure
The effectiveness of mouthwashes in alleviating radiation-induced oral mucositis in head and neck cancer patients: A systematic review
Objective The aim of the study was to perform a systematic literature search and meta-analysis to reveal the most effective mouthwash for head and neck cancer patients who are experiencing radiation therapy-induced mucositis. Methods Using two electronic databases, a literature search and data interpretation were systematically performed as follows: (i) problem specification, (ii) devising of a literature search plan, (iii) literature search and retrieval of publications, and (iv) meta-analysis and data interpretation. The main problem was specified as follows: what mouthwash is effective in alleviating oral mucositis for head and neck cancer patients who are undergoing radiotherapy? Results The literature search yielded 354 titles and abstracts. After reviewing the extracted literature, 25 publications met the inclusion criteria for this study and 17 of 25 were eventually evaluated in the meta-analysis. Conclusion The results of the meta-analysis indicated that the use of a mouthwash that includes anti-inflammatory properties contributes the most to alleviating oral mucositis in patients who are undergoing radiotherapy to treat head and neck cancer
ClaimChain: Improving the Security and Privacy of In-band Key Distribution for Messaging
The social demand for email end-to-end encryption is barely supported by
mainstream service providers. Autocrypt is a new community-driven open
specification for e-mail encryption that attempts to respond to this demand. In
Autocrypt the encryption keys are attached directly to messages, and thus the
encryption can be implemented by email clients without any collaboration of the
providers. The decentralized nature of this in-band key distribution, however,
makes it prone to man-in-the-middle attacks and can leak the social graph of
users. To address this problem we introduce ClaimChain, a cryptographic
construction for privacy-preserving authentication of public keys. Users store
claims about their identities and keys, as well as their beliefs about others,
in ClaimChains. These chains form authenticated decentralized repositories that
enable users to prove the authenticity of both their keys and the keys of their
contacts. ClaimChains are encrypted, and therefore protect the stored
information, such as keys and contact identities, from prying eyes. At the same
time, ClaimChain implements mechanisms to provide strong non-equivocation
properties, discouraging malicious actors from distributing conflicting or
inauthentic claims. We implemented ClaimChain and we show that it offers
reasonable performance, low overhead, and authenticity guarantees.Comment: Appears in 2018 Workshop on Privacy in the Electronic Society
(WPES'18
Irreducible triangulations of surfaces with boundary
A triangulation of a surface is irreducible if no edge can be contracted to
produce a triangulation of the same surface. In this paper, we investigate
irreducible triangulations of surfaces with boundary. We prove that the number
of vertices of an irreducible triangulation of a (possibly non-orientable)
surface of genus g>=0 with b>=0 boundaries is O(g+b). So far, the result was
known only for surfaces without boundary (b=0). While our technique yields a
worse constant in the O(.) notation, the present proof is elementary, and
simpler than the previous ones in the case of surfaces without boundary
Dispersion of Resonant Raman Peaks of CO and OH in SnO2, Mo1-x FexO2 Thin Films and SiO2 bulk glass
Resonance Raman (RR) peaks of and stretching modes and their higher harmonics
have been observed superimposed on photoluminescence (PL) spectrum of thin
films. Commercial fluorine doped thin films deposited by sputtering on glass
and thin films deposited on Si by laser ablation have been studied. The
dispersions of CO and OH stretching RR modes are ~ 600 cm-1/eV and 800 cm-1
respectively. The dispersion of the third harmonic of CO stretching mode is ~
2000 cm-1/eV. Similar dispersion of RR peak of stretching modes and higher
harmonics superimposed on PL spectra has been observed in Mo1-xFexO2 thin films
and SiO2 bulk glass. Large dispersion of RR peaks seems to be a common property
of oxides with impurities of and .Comment: 13 pages including three figure
A Cost-Effective Design for a Neutrino Factory
There have been active efforts in the U.S., Europe, and Japan on the design
of a Neutrino Factory. This type of facility produces intense beams of
neutrinos from the decay of muons in a high energy storage ring. In the U.S., a
second detailed Feasibility Study (FS2) for a Neutrino Factory was completed in
2001. Since that report was published, new ideas in bunching, cooling and
acceleration of muon beams have been developed. We have incorporated these
ideas into a new facility design, which we designate as Study 2B (ST2B), that
should lead to significant cost savings over the FS2 design.Comment: 46 pages, 38 figures; to be submitted to Physical Review Special
Topics: Accelerators and Beam
Morphological, Structural, and Spectral Characteristics of Amorphous Iron Sulfates
Current or past brine hydrologic activity on Mars may provide suitable conditions for the formation of amorphous ferric sulfates. Once formed, these phases would likely be stable under current Martian conditions, particularly at low- to mid-latitudes. Therefore, we consider amorphous iron sulfates (AIS) as possible components of Martian surface materials. Laboratory AIS were created through multiple synthesis routes and characterized with total X-ray scattering, thermogravimetric analysis, scanning electron microscopy, visible/near-infrared (VNIR), thermal infrared (TIR), and Mössbauer techniques. We synthesized amorphous ferric sulfates (Fe(III)2(SO4)3â·â~â6â8H2O) from sulfate-saturated fluids via vacuum dehydration or exposure to low relative humidity
A Realistic Description of Nucleon-Nucleon and Hyperon-Nucleon Interactions in the SU_6 Quark Model
We upgrade a SU_6 quark-model description for the nucleon-nucleon and
hyperon-nucleon interactions by improving the effective meson-exchange
potentials acting between quarks. For the scalar- and vector-meson exchanges,
the momentum-dependent higher-order term is incorporated to reduce the
attractive effect of the central interaction at higher energies. The
single-particle potentials of the nucleon and Lambda, predicted by the G-matrix
calculation, now have proper repulsive behavior in the momentum region q_1=5 -
20 fm^-1. A moderate contribution of the spin-orbit interaction from the
scalar-meson exchange is also included. As to the vector mesons, a dominant
contribution is the quadratic spin-orbit force generated from the rho-meson
exchange. The nucleon-nucleon phase shifts at the non-relativistic energies up
to T_lab=350 MeV are greatly improved especially for the 3E states. The
low-energy observables of the nucleon-nucleon and the hyperon-nucleon
interactions are also reexamined. The isospin symmetry breaking and the Coulomb
effect are properly incorporated in the particle basis. The essential feature
of the Lambda N - Sigma N coupling is qualitatively similar to that obtained
from the previous models. The nuclear saturation properties and the
single-particle potentials of the nucleon, Lambda and Sigma are reexamined
through the G-matrix calculation. The single-particle potential of the Sigma
hyperon is weakly repulsive in symmetric nuclear matter. The single-particle
spin-orbit strength for the Lambda particle is very small, in comparison with
that of the nucleons, due to the strong antisymmetric spin-orbit force
generated from the Fermi-Breit interaction.Comment: Revtex v2.09, 69 pages with 25 figure
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Atmospheric response to solar radiation absorbed by phytoplankton
Phytoplankton alter the absorption of solar radiation, affecting upper ocean temperature and circulation. These changes, in turn, influence the atmosphere through modification of the sea surface temperature (SST). To investigate the effects of the present-day phytoplankton concentration on the atmosphere, an atmospheric general circulation model was forced by SST changes due to phytoplankton. The modified SST was obtained from ocean general circulation model runs with space- and time-varying phytoplankton abundances from Coastal Zone Color Scanner data. The atmospheric simulations indicate that phytoplankton amplify the seasonal cycle of the lowest atmospheric layer temperature. This amplification has an average magnitude of 0.3°K but may reach over 1°K locally. The surface warming in the summer is marginally larger than the cooling in the winter, so that on average annually and globally, phytoplankton warm the lowest layer by about 0.05°K. Over the ocean the surface air temperature changes closely follow the SST changes. Significant, often amplified, temperature changes also occur over land. The climatic effect of phytoplankton extends throughout the troposphere, especially in middle latitudes where increased subsidence during summer traps heat. The amplification of the seasonal cycle of air temperature strengthens tropical convection in the summer hemisphere. In the eastern tropical Pacific Ocean a decreased SST strengthens the Walker circulation and weakens the Hadley circulation. These significant atmospheric changes indicate that the radiative effects of phytoplankton should not be overlooked in studies of climate change.Keywords: Atmospheric general circulation model (AGCM), seasonal cycle, Phytoplankton, sea surface temperature (SST), Absorption of solar radiatio
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