Breaking Lorentz reciprocity to overcome the time-bandwidth limit in physics and engineering

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

A century-old tenet in physics and engineering asserts that any type of system, having bandwidth , can interact with a wave over only a constrained time period t inversely proportional to the bandwidth (t· ~ 2). This law severely limits the generic capabilities of all types of resonant and wave-guiding systems in photonics, cavity quantum electrodynamics and optomechanics, acoustics, continuum mechanics, and atomic and optical physics but is thought to be completely fundamental, arising from basic Fourier reciprocity. We propose that this "fundamental" limit can be overcome in systems where Lorentz reciprocity is broken. As a system becomes more asymmetric in its transport properties, the degree to which the limit can be surpassed becomes greater. By way of example, we theoretically demonstrate how, in an astutely designed magnetized semiconductor heterostructure, the above limit can be exceeded by orders of magnitude by using realistic material parameters. Our findings revise prevailing paradigms for linear, time-invariant resonant systems, challenging the doctrine that high-quality resonances must invariably be narrowband and providing the possibility of developing devices with unprecedentedly high time-bandwidth performance.

Quantum correlations from a room-temperature optomechanical cavity

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

The act of position measurement alters the motion of an object being measured. This quantum measurement backaction is typically much smaller than the thermal motion of a room-temperature object and thus difficult to observe. By shining laser light through a nanomechanical beam, we measure the beam’s thermally driven vibrations and perturb its motion with optical force fluctuations at a level dictated by the Heisenberg measurement-disturbance uncertainty relation. We demonstrate a cross-correlation technique to distinguish optically driven motion from thermally driven motion, observing this quantum backaction signature up to room temperature. We use the scale of the quantum correlations, which is determined by fundamental constants, to gauge the size of thermal motion, demonstrating a path toward absolute thermometry with quantum mechanically calibrated ticks.

On the generation of solar spicules and Alfvenic waves

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfvénic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfvénic waves.

A catalytic fluoride-rebound mechanism for C(sp3)-CF3 bond formation

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

The biological properties of trifluoromethyl compounds have led to their ubiquity in pharmaceuticals, yet their chemical properties have made their preparation a substantial challenge, necessitating innovative chemical solutions. We report the serendipitous discovery of a borane-catalyzed formal C(sp3)-CF3 reductive elimination from Au(III) that accesses these compounds by a distinct mechanism proceeding via fluoride abstraction, migratory insertion, and C-F reductive elimination to achieve a net C-C bond construction. The parent bis(trifluoromethyl)Au(III) complexes tolerate a surprising breadth of synthetic protocols, enabling the synthesis of complex organic derivatives without cleavage of the Au-C bond. This feature, combined with the "fluoride-rebound" mechanism, was translated into a protocol for the synthesis of 18F-radiolabeled aliphatic CF3-containing compounds, enabling the preparation of potential tracers for use in positron emission tomography.

Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe–S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe–S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.

Human-in-the-loop optimization of exoskeleton assistance during walking

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

Exoskeletons and active prostheses promise to enhance human mobility, but few have succeeded. Optimizing device characteristics on the basis of measured human performance could lead to improved designs. We have developed a method for identifying the exoskeleton assistance that minimizes human energy cost during walking. Optimized torque patterns from an exoskeleton worn on one ankle reduced metabolic energy consumption by 24.2 ± 7.4% compared to no torque. The approach was effective with exoskeletons worn on one or both ankles, during a variety of walking conditions, during running, and when optimizing muscle activity. Finding a good generic assistance pattern, customizing it to individual needs, and helping users learn to take advantage of the device all contributed to improved economy. Optimization methods with these features can substantially improve performance.

Mitotic chromosome assembly despite nucleosome depletion in Xenopus egg extracts

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

The nucleosome is the fundamental structural unit of eukaryotic chromatin. During mitosis, duplicated nucleosome fibers are organized into a pair of rod-shaped structures (chromatids) within a mitotic chromosome. However, it remains unclear whether nucleosome assembly is indeed an essential prerequisite for mitotic chromosome assembly. We combined mouse sperm nuclei and Xenopus cell-free egg extracts depleted of the histone chaperone Asf1 and found that chromatid-like structures could be assembled even in the near absence of nucleosomes. The resultant "nucleosome-depleted" chromatids contained discrete central axes positive for condensins, although they were more fragile than normal nucleosome-containing chromatids. Combinatorial depletion experiments underscored the central importance of condensins in mitotic chromosome assembly, which sheds light on their functional cross-talk with nucleosomes in this process.

Local protein kinase A action proceeds through intact holoenzymes

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

Hormones can transmit signals through adenosine 3',5'-monophosphate (cAMP) to precise intracellular locations. The fidelity of these responses relies on the activation of localized protein kinase A (PKA) holoenzymes. Association of PKA regulatory type II (RII) subunits with A-kinase–anchoring proteins (AKAPs) confers location, and catalytic (C) subunits phosphorylate substrates. Single-particle electron microscopy demonstrated that AKAP79 constrains RII-C subassemblies within 150 to 250 angstroms of its targets. Native mass spectrometry established that these macromolecular assemblies incorporated stoichiometric amounts of cAMP. Chemical-biology– and live cell–imaging techniques revealed that catalytically active PKA holoenzymes remained intact within the cytoplasm. These findings indicate that the parameters of anchored PKA holoenzyme action are much more restricted than originally anticipated.

Assembly principles and structure of a 6.5-MDa bacterial microcompartment shell

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

Many bacteria contain primitive organelles composed entirely of protein. These bacterial microcompartments share a common architecture of an enzymatic core encapsulated in a selectively permeable protein shell; prominent examples include the carboxysome for CO2 fixation and catabolic microcompartments found in many pathogenic microbes. The shell sequesters enzymatic reactions from the cytosol, analogous to the lipid-based membrane of eukaryotic organelles. Despite available structural information for single building blocks, the principles of shell assembly have remained elusive. We present the crystal structure of an intact shell from Haliangium ochraceum, revealing the basic principles of bacterial microcompartment shell construction. Given the conservation among shell proteins of all bacterial microcompartments, these principles apply to functionally diverse organelles and can inform the design and engineering of shells with new functionalities.

New Products

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

Chemical transformation of xenobiotics by the human gut microbiota

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

The human gut microbiota makes key contributions to the metabolism of ingested compounds (xenobiotics), transforming hundreds of dietary components, industrial chemicals, and pharmaceuticals into metabolites with altered activities, toxicities, and lifetimes within the body. The chemistry of gut microbial xenobiotic metabolism is often distinct from that of host enzymes. Despite their important consequences for human biology, the gut microbes, genes, and enzymes involved in xenobiotic metabolism are poorly understood. Linking these microbial transformations to enzymes and elucidating their biological effects is undoubtedly challenging. However, recent studies demonstrate that integrating traditional and emerging technologies can enable progress toward this goal. Ultimately, a molecular understanding of gut microbial xenobiotic metabolism will guide personalized medicine and nutrition, inform toxicology risk assessment, and improve drug discovery and development.

An environment-dependent transcriptional network specifies human microglia identity

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

Microglia play essential roles in central nervous system (CNS) homeostasis and influence diverse aspects of neuronal function. However, the transcriptional mechanisms that specify human microglia phenotypes are largely unknown. We examined the transcriptomes and epigenetic landscapes of human microglia isolated from surgically resected brain tissue ex vivo and after transition to an in vitro environment. Transfer to a tissue culture environment resulted in rapid and extensive down-regulation of microglia-specific genes that were induced in primitive mouse macrophages after migration into the fetal brain. Substantial subsets of these genes exhibited altered expression in neurodegenerative and behavioral diseases and were associated with noncoding risk variants. These findings reveal an environment-dependent transcriptional network specifying microglia-specific programs of gene expression and facilitate efforts to understand the roles of microglia in human brain diseases.

A global brain state underlies C. elegans sleep behavior

ScienceNOW Daily News Feed - Jue, 06/22/2017 - 11:20

How the brain effectively switches between and maintains global states, such as sleep and wakefulness, is not yet understood. We used brainwide functional imaging at single-cell resolution to show that during the developmental stage of lethargus, the Caenorhabditis elegans brain is predisposed to global quiescence, characterized by systemic down-regulation of neuronal activity. Only a few specific neurons are exempt from this effect. In the absence of external arousing cues, this quiescent brain state arises by the convergence of neuronal activities toward a fixed-point attractor embedded in an otherwise dynamic neural state space. We observed efficient spontaneous and sensory-evoked exits from quiescence. Our data support the hypothesis that during global states such as sleep, neuronal networks are drawn to a baseline mode and can be effectively reactivated by signaling from arousing circuits.

Air-guns used in offshore oil exploration can kill tiny marine life

Nature News - Jue, 06/22/2017 - 10:23

Lethal effects from pulses of sound used to probe the sea floor can travel over a kilometer.

Nature News doi: 10.1038/nature.2017.22167

'Momentum microscope' measures quantum correlations - Jue, 06/22/2017 - 09:44
Many-body problem solved for colliding condensates

Flash Physics: Dead galaxy mystifies astronomers, a photon dam bursts, Space Industry Bill for the UK - Jue, 06/22/2017 - 07:17
Today's selection of need-to-know updates from the world of physics

Discovery of a new mechanism involved in the migration of cancer cells

EurekAlert! - Mié, 06/21/2017 - 22:00
A team of young French researchers has discovered a new mechanism which facilitates cell migration. On the surface of its membrane, the cell develops multiple small hooks which help it to attach to fibers outside the cell and move along them. This action helps us to understand better how a cell escapes from the tumor mass and moves around the body to form a new focus.

Correct connections are crucial

EurekAlert! - Mié, 06/21/2017 - 22:00
Working with colleagues from Harvard Medical School and Würzburg, researchers from Charité - Universitätsmedizin Berlin have been examining the use of deep brain stimulation in the treatment of Parkison's disease in an attempt to optimize treatment effectiveness. The results, describing an effective network profile of deep brain stimulation has been reported in the journal Annals of Neurology*.


Subscribe to Facultad de Ciencias agregador