220 research outputs found
A variational problem on Stiefel manifolds
In their paper on discrete analogues of some classical systems such as the
rigid body and the geodesic flow on an ellipsoid, Moser and Veselov introduced
their analysis in the general context of flows on Stiefel manifolds. We
consider here a general class of continuous time, quadratic cost, optimal
control problems on Stiefel manifolds, which in the extreme dimensions again
yield these classical physical geodesic flows. We have already shown that this
optimal control setting gives a new symmetric representation of the rigid body
flow and in this paper we extend this representation to the geodesic flow on
the ellipsoid and the more general Stiefel manifold case. The metric we choose
on the Stiefel manifolds is the same as that used in the symmetric
representation of the rigid body flow and that used by Moser and Veselov. In
the extreme cases of the ellipsoid and the rigid body, the geodesic flows are
known to be integrable. We obtain the extremal flows using both variational and
optimal control approaches and elucidate the structure of the flows on general
Stiefel manifolds.Comment: 30 page
Lewis-base silane activation: from reductive cleavage of aryl ethers to selective ortho-silylation
We report a transition-metal-free protocol for the efficient reductive cleavage of diaryl and aryl alkyl ethers. The combination of triethylsilane with common bases forms an unusually powerful reductive couple that regioselectively ruptures lignin- and coal-related C–O bonds in aromatic ethers. Interestingly, with certain bases and temperature regimes ortho-directed C–H silylation efficiently competes with the latter process. However, careful tuning of the reactions conditions allows for the selective reductive cleavage of lignin model compounds to their corresponding phenolic and aromatic constituents
An essential bifunctional enzyme in Mycobacterium tuberculosis for itaconate dissimilation and leucine catabolism
Mycobacterium tuberculosis (Mtb) is the etiological agent of tuberculosis. One-fourth of the global population is estimated to be infected with Mtb, accounting for ∼1.3 million deaths in 2017. As part of the immune response to Mtb infection, macrophages produce metabolites with the purpose of inhibiting or killing the bacterial cell. Itaconate is an abundant host metabolite thought to be both an antimicrobial agent and a modulator of the host inflammatory response. However, the exact mode of action of itaconate remains unclear. Here, we show that Mtb has an itaconate dissimilation pathway and that the last enzyme in this pathway, Rv2498c, also participates in L-leucine catabolism. Our results from phylogenetic analysis, in vitro enzymatic assays, X-ray crystallography, and in vivo Mtb experiments, identified Mtb Rv2498c as a bifunctional β-hydroxyacyl-CoA lyase and that deletion of the rv2498c gene from the Mtb genome resulted in attenuation in a mouse infection model. Altogether, this report describes an itaconate resistance mechanism in Mtb and an L-leucine catabolic pathway that proceeds via an unprecedented (R)-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) stereospecific route in nature
Plague from Eating Raw Camel Liver
We investigated a cluster of 5 plague cases; the patients included 4 with severe pharyngitis and submandibular lymphadenitis. These 4 case-patients had eaten raw camel liver. Yersinia pestis was isolated from bone marrow of the camel and from jirds (Meriones libycus) and fleas (Xenopsylla cheopis) captured at the camel corral
Versatile Wideband Balanced Detector for Quantum Optical Homodyne Tomography
We present a comprehensive theory and an easy to follow method for the design
and construction of a wideband homodyne detector for time-domain quantum
measurements. We show how one can evaluate the performance of a detector in a
specific time-domain experiment based on electronic spectral characteristic of
that detector. We then present and characterize a high-performance detector
constructed using inexpensive, commercially available components such as
low-noise high-speed operational amplifiers and high-bandwidth photodiodes. Our
detector shows linear behavior up to a level of over 13 dB clearance between
shot noise and electronic noise, in the range from DC to 100 MHz. The detector
can be used for measuring quantum optical field quadratures both in the
continuous-wave and pulsed regimes with pulse repetition rates up to about 250
MHz.Comment: 11 pages, 8 figures, 1 tabl
Conductivity of underdoped YBa2Cu3O7-d : evidence for incoherent pair correlations in the pseudogap regime
Conductivity due to superconducting fluctuations studied in optimally doped
YBa2Cu3O7-d films displays a stronger decay law in temperature than explainable
by theory. A formula is proposed, which fits the data very well with two
superconductive parameters, Tc and the coherence length ksi_c0, and an energy
scale Delta*. This is also valid in underdoped materials and enables to
describe the conductivity up to 300 K with a single-particle excitations
channel in parallel with a channel whose contribution is controlled by ksi_c0,
Tc and Delta*.
This allows to address the nature of the pseudogap in favour of incoherent
pairing.Comment: 14 pages, 4 figures, 1 tabl
Catalysis by hen egg-white lysozyme proceeds via a covalent intermediate
Hen egg-white lysozyme (HEWL) was the first enzyme to have its three-dimensional structure determined by X-ray diffraction techniques(1). A catalytic mechanism, featuring a long-lived oxo-carbenium-ion intermediate, was proposed on the basis of model-building studies(2). The `Phillips' mechanism is widely held as the paradigm for the catalytic mechanism of beta -glycosidases that cleave glycosidic linkages with net retention of configuration of the anomeric centre. Studies with other retaining beta -glycosidases, however, provide strong evidence pointing to a common mechanism for these enzymes that involves a covalent glycosyl-enzyme intermediate, as previously postulated(3). Here we show, in three different cases using electrospray ionization mass spectrometry, a catalytically competent covalent glycosyl-enzyme intermediate during the catalytic cycle of HEWL. We also show the three-dimensional structure of this intermediate as determined by Xray diffraction. We formulate a general catalytic mechanism for all retaining beta -glycosidases that includes substrate distortion, formation of a covalent intermediate, and the electrophilic migration of C1 along the reaction coordinate
A Model Study of Discrete Scale Invariance and Long-Range Interactions
We investigate the modification of discrete scale invariance in the bound
state spectrum by long-range interactions. This problem is relevant for
effective field theory descriptions of nuclear cluster states and
manifestations of the Efimov effect in nuclei. As a model system, we choose a
one dimensional inverse square potential supplemented with a long-range Coulomb
interaction. We study the renormalization and bound-state spectrum of the
system as a function of the Coulomb interaction strength. Our results indicate,
that the counterterm required to renormalize the inverse square potential alone
is sufficient to renormalize the full problem. However, the breaking of the
discrete scale invariance through the Coulomb interaction leads to a modified
bound state spectrum. The shallow bound states are strongly influenced by the
Coulomb interaction while the deep bound states are dominated by the inverse
square potential.Comment: 8 pages, 6 figures, EPJ style, published versio
- …