Visualizing chemical reactions as they occur requires atomic spatial and femtosecond temporal resolution. Here, we report imaging of the molecular structure of acetylene (C₂H₂) 9 femtoseconds after ...ionization. Using mid-infrared laser-induced electron diffraction (LIED), we obtained snapshots as a proton departs the C₂H₂²⁺ ion. By introducing an additional laser field, we also demonstrate control over the ultrafast dissociation process and resolve different bond dynamics for molecules oriented parallel versus perpendicular to the LIED field. These measurements are in excellent agreement with a quantum chemical description of field-dressed molecular dynamics.
•A unifying DC approximation, including all standard approximations, of the zero-norm is proposed.•The consistency between global/local minima of approximate and original problems are proved.•The ...equivalence between approximate and original problems are established for some approximations.•Four DCA schemes are developed that cover all standard nonconvex approximation algorithms.•A careful empirical experiment for feature selection in SVM are performed.
Sparse optimization refers to an optimization problem involving the zero-norm in objective or constraints. In this paper, nonconvex approximation approaches for sparse optimization have been studied with a unifying point of view in DC (Difference of Convex functions) programming framework. Considering a common DC approximation of the zero-norm including all standard sparse inducing penalty functions, we studied the consistency between global minimums (resp. local minimums) of approximate and original problems. We showed that, in several cases, some global minimizers (resp. local minimizers) of the approximate problem are also those of the original problem. Using exact penalty techniques in DC programming, we proved stronger results for some particular approximations, namely, the approximate problem, with suitable parameters, is equivalent to the original problem. The efficiency of several sparse inducing penalty functions have been fully analyzed. Four DCA (DC Algorithm) schemes were developed that cover all standard algorithms in nonconvex sparse approximation approaches as special versions. They can be viewed as, an ℓ1-perturbed algorithm/reweighted-ℓ1 algorithm / reweighted-ℓ2 algorithm. We offer a unifying nonconvex approximation approach, with solid theoretical tools as well as efficient algorithms based on DC programming and DCA, to tackle the zero-norm and sparse optimization. As an application, we implemented our methods for the feature selection in SVM (Support Vector Machine) problem and performed empirical comparative numerical experiments on the proposed algorithms with various approximation functions.
Supramolecular structures with strain‐stiffening properties are ubiquitous in nature but remain rare in the lab. Herein, we report on strain‐stiffening supramolecular hydrogels that are entirely ...produced through the self‐assembly of synthetic molecular gelators. The involved gelators self‐assemble into semi‐flexible fibers, which thereby crosslink into hydrogels. Interestingly, these hydrogels are capable of stiffening in response to applied stress, resembling biological intermediate filaments system. Furthermore, strain‐stiffening hydrogel networks embedded with liposomes are constructed through orthogonal self‐assembly of gelators and phospholipids, mimicking biological tissues in both architecture and mechanical properties. This work furthers the development of biomimetic soft materials with mechanical responsiveness and presents potentially enticing applications in diverse fields, such as tissue engineering, artificial life, and strain sensors.
Biomimetic strain‐stiffening hydrogels have been prepared through the self‐assembly of molecular gelators. On the basis of orthogonal self‐assembly of the molecular gelators and lipids, hydrogel networks embedded with liposomes are formed and show strain‐stiffening behavior as well, resembling both the structures and mechanical responsiveness of biological tissue.
Summary
Background
Conventional magnetic resonance imaging (MRI) techniques that measure hepatic steatosis are limited by T1 bias, T2* decay and multi‐frequency signal‐interference effects of protons ...in fat. Newer MR techniques such as the proton density‐fat fraction (PDFF) that correct for these factors have not been specifically compared to liver biopsy in adult patients with non‐alcoholic fatty liver disease (NAFLD).
Aim
To examine the association between MRI‐determined PDFF and histology‐determined steatosis grade, and their association with fibrosis.
Methods
A total of 51 adult patients with biopsy‐confirmed NAFLD underwent metabolic‐biochemical profiling, MRI‐determined PDFF measurement of hepatic steatosis and liver biopsy assessment according to NASH‐CRN histological scoring system.
Results
The average MRI‐determined PDFF increased significantly with increasing histology‐determined steatosis grade: 8.9% at grade‐1, 16.3% at grade‐2, and 25.0% at grade‐3 with P ≤ 0.0001 (correlation: r2 = 0.56, P < 0.0001). Patients with stage‐4 fibrosis, when compared with patients with stage 0–3 fibrosis, had significantly lower hepatic steatosis by both MRI‐determined PDFF (7.6% vs. 17.8%, P < 0.005) and histology‐determined steatosis grade (1.4 vs. 2.2, P < 0.05). NAFLD patients with grade 1 steatosis were more likely to have characteristics of advanced liver disease including higher average AST:ALT (0.87 vs. 0.60, P < 0.02), GGT (140 vs. 67, P < 0.01), and INR (1.06 vs. 0.99, P < 0.01), higher stage of fibrosis and hepatocellular ballooning.
Conclusions
MRI‐determined proton density‐fat fraction correlates with histology‐determined steatosis grade in adults with NAFLD. Steatosis is non‐linearly related to fibrosis progression. In patients with NAFLD, a low amount of hepatic steatosis on imaging does not necessarily indicate mild disease.
Dynamic foldamer chemistry Le Bailly, Bryden A. F; Clayden, Jonathan
Chemical communications,
04/2016, Letnik:
52, Številka:
27
Journal Article
Recenzirano
Odprti dostop
Foldamers can be made more than pieces of static, conformationally uniform molecular architecture by designing into their structure the conformational dynamism characteristic of functional molecular ...machines. We show that these dynamic foldamers display biomimetic properties reminiscent of allosteric proteins and receptor molecules. They can translate chemical signals into conformational changes, and hence into chemical outputs such as control of reactivity and selectivity. Future developments could see dynamic foldamers operating in the membrane phase providing artificial mechanisms for communication and control that integrate synthetic chemistry into synthetic biology.
Dynamic foldamers translate chemical signals into conformational changes, and hence into chemical outputs such as control of reactivity and selectivity.
Electron heating and mixing during asymmetric reconnection are studied with a 3‐D kinetic simulation that matches plasma parameters from Magnetospheric Multiscale (MMS) spacecraft observations of a ...magnetopause diffusion region. The mixing and heating are strongly enhanced across the magnetospheric separatrix compared to a 2‐D simulation. The transport of particles across the separatrix in 3‐D is attributed to lower hybrid drift turbulence excited at the steep density gradient near the magnetopause. In the 3‐D simulation (and not the 2‐D simulation), the electron temperature parallel to the magnetic field within the mixing layer is significantly higher than its upstream value in agreement with the MMS observations.
Key Points
A fully kinetic 3‐D simulation resolves electron mixing and heating at a magnetopause reconnection site matching MMS observations
Lower hybrid range drift turbulence mixes plasma across the magnetospheric separatrix
Strong parallel electron heating near the current layer, as observed by MMS, is reproduced only in 3‐D
Reconnection is the process by which stress in the field of a magnetized plasma is reduced by a topological rearrangement of its magnetic-field lines. The process is often accompanied by an explosive ...release of magnetic energy and is implicated in a range of astrophysical phenomena. In the Earth's magnetotail, reconnection energizes electrons up to hundreds of keV (ref. ) and solar-flare events can channel up to 50% of the magnetic energy into the electrons, resulting in superthermal populations in the MeV range. Electron energization is also fundamentally important to astrophysical applications yielding a window into the extreme environments. Here we show that during reconnection powerful energization of electrons by magnetic-field-aligned electric field (E|| ) can occur over spatial scales that hugely exceed previous theories and simulations. In our kinetic simulation E|| is supported by non-thermal and strongly anisotropic features in the electron distributions not permitted in standard fluid formulations, but routinely observed by spacecraft in the Earth's magnetosphere. This allows for electron energization in spatial regions that exceed the regular de -scale electron-diffusion region by at least three orders of magnitude.
François’ langur is an Endangered colobine inhabiting limestone habitats in southern China and northern Vietnam. Its global population has been estimated to be just more than 2000 mature individuals. ...Populations in Vietnam are highly fragmented with reportedly fewer than 200 adults in total and 50 in a single location. Although the François’ langur in Vietnam is highly imperiled as remnant populations persist in only three to four sites, little research has been carried out to provide a reliable estimate of its remaining population. In this study, we conducted field surveys in Lam Binh District, Tuyen Quang Province, northeastern Vietnam. In total, we recorded at least 16 groups of François’ langurs, with 156 individuals, raising the total number of individuals by approximately 10% compared to a previous study. The group structure, group size, activity budget, and density of the Lam Binh population resemble those reported in François’ langurs in China and other limestone langur species. The results show that the behavior ecology of limestone langurs significantly differs from that of forest langurs probably because they occupy separate habitats with distinctly different environmental variables. During our surveys, we detected a number of direct threats to this population, namely illegal logging, hunting, firewood collecting, hydropower development, grazing, and mining. It is recommended that the protection forest be elevated to the nature reserve status to better protect the most important population of the François’ langur in Vietnam.
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