五味子多糖提取、结构特征及生物活性研究进展

五味子是木兰科植物五味子(Schisandra chinensis(Turcz.)Baill),或华中五味子(Schisandra sphenanchera Rehd.etWils.)的干燥成熟果实,富含多糖、木脂素、挥发油、多酚等多种活性成分。多糖作为其主要的活性成分,提取工艺有双水相萃取、酶解法、超声法、微波法、超声微波协同、微波酶法协同、水提法等,其中双水相萃取效率最高,超声法和水提法应用最广泛。五味子多糖主要由鼠李糖、甘露糖、葡萄糖、半乳糖、阿拉伯糖和半乳糖醛酸组成,平均分子量大致范围在10^(3)~10^(5) Da。现代药理学研究表明,五味子多糖具有显著的抗氧化、免疫调节、抗肿瘤、保肝、降血糖、血脂等功效,在食品、医药领域高值化开发具有重要的意义。因此,本文对近年来国内外五味子多糖提取工艺、结构特征和生物活性及其相关机制进行系统总结,并对其研究方向与应用前景进行展望,以期为五味子深入研究和应用开发提供参考。

Enhanced Ferromagnetism of CrI3 Bilayer by Self-Intercalation

Two-dimensional(2D)ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications.CrI 3 is a rising star of intrinsic 2D ferromagnets,however,it suffers from weak exchange coupling.Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI 3.We show that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI 3 bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling.According to our first-principles calculations,the intercalated native atoms act as covalent bridge between two CrI 3 layers and lead to discrepant oxidation states for the Cr atoms.These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.

Physical properties and device applications of graphene oxide

Graphene oxide(GO),the functionalized graphene with oxygenated groups(mainly epoxy and hydroxyl),has attracted resurgent interests in the past decade owing to its large surface area,superior physical and chemical properties,and easy composition with other materials via surface functional groups.Usually,GO is used as an important raw material for mass production of graphene via reduction.However,under different conditions,the coverage,types,and arrangements of oxygen-containing groups in GO can be varied,which give rise to excellent and controllable physical properties,such as tunable electronic and mechanical properties depending closely on oxidation degree,suppressed thermal conductivity,optical transparency and fluorescence,and nonlinear optical properties.Based on these outstanding properties,many electronic,optical,optoelectronic,and thermoelectric devices with high performance can be achieved on the basis of GO.Here we present a comprehensive review on recent progress of GO,focusing on the atomic structures,fundamental physical properties,and related device applications,including transparent and flexible conductors,field-effect transistors,electrical and optical sensors,fluorescence quenchers,optical limiters and absorbers,surface enhanced Raman scattering detectors,solar cells,light-emitting diodes,and thermal rectifiers.

Superatomic Signature and Reactivity of Silver Clusters with Oxygen:Double Magic Ag_(17)^– with Geometric and Electronic Shell Closure

Understanding the stability and reactivity of silver clusters toward oxygen provides insights to design new materials of coinage metals with atomic precision.Herein,we report a systematic study on anionic silver clusters,Ag_(n)^(−)(n=10-34),by reacting them with O_(2) under multiple-collision conditions.Mass spectrometry observation presents the odd-even alternation effect on the reaction rates of these Agn−clusters.

Selective CO_(2) conversion tuned by periodicities in Au_(8n+4)(TBBT)_(4n+8) nanoclusters

The structure-dependent catalytic behavior is one of the most important issues in catalysis science.However,it has not been fully understood how different types of atom-packing structures of heterogeneous catalysts precisely impact the reaction sites and pathways.Here we investigate a periodic series of Au_(8n+4)(TBBT)_(4n+8 )nanoclusters with layer-by-layer structural pattern to catalyze CO_(2) hydrogenation(where n=3–6 is the number of(001)layers;TBBT=4-tert-butyl-benzenethiolate).An encouraging evolution of CO_(2) conversion can be identified:The product selectivity from methanol,formic acid to ethanol can be switched by the structure-dependent deformation from the flattened,perfect,to elongated cuboids in Au_(8n+4)(TBBT)_(4n+8).Through a combined study of experiment and theory,we demonstrate that the variation in structural patterns of catalysts can exclusively tune their adsorption strength with reaction intermediates and further control the CO_(2) conversion toward the different products.

Low-dimensional non-metal catalysts:principles for regulating p-orbital-dominated reactivity

Activation of p-block elements to replace the rare and precious transition metals for renewable energy applications is highly desirable.In this review,we go over recent experimental and theoretical progress on the low-dimensional non-metal materials for clean energy production,including carbon,silicon,oxide,boron,and phosphorus-based nanostructures,with the p-block elements serving as active sites.We aim to elucidate the mechanism for triggering activity in different kinds of non-metal systems,and extract general principles for controlling the p-orbital-mediated reactivity from a theoretical point of view.The perspectives and challenges for developing high-efficiency non-metal catalysts are provided in the end.

Pt overlayer for direct oxidation of CH_(4)to CH_(3)OH

Highly selective conversion of methane(CH_(4))to methanol(CH_(3)OH)is an emerging attractive but challenging process for future development of hydrogen economy,which requires efficient catalysts.Herein,we systematically explore the catalytic properties of Pt(111)overlayer on transition metal oxides(TMOs)for CH_(4) conversion by first principles calculations.The Pt(111)monolayer supported by Ce-terminated CeO_(2)(111)substrate exhibits high activity and selectivity for CH_(4) conversion to CH_(3)OH,with the kinetic barrier of rate-limiting step of 1.05 eV.Intriguingly,the surface activity of Pt overlayer is governed by its d-band center relative to the energy of bonding states of adsorbed molecules,which in turn depends on the number of charge transfer between Pt(111)monolayer and underlying TMOs substrates.These results provide useful insights in the design of metal overlayers as catalysts with high-ultra performance and atomic utilization.

The reactivity of O_(2) with copper cluster anions Cu_(n)^(-)(n = 7-20):Leveling effect of spin accommodation

The activation of molecular oxygen is an important step in metal-catalyzed oxidation reactions and a hot subject for the research of gas-phase metal clusters.It is known that the Ag and Au clusters readily re-act with O_(2)when they have open shell electronic structures.Distinct from this,here we observed Cu_(n)^(-)(n=7−20)clusters of both open and closed shells possess high reactivity with O_(2)with few exceptions.In a combination with ab initio calculations,we demonstrate that the activation of O_(2)on the even-and odd-sized Cu_(n)^(-)clusters follows the single and double electron transfer models,respectively.Such phe-nomenon of metal clusters with different basicity to activate oxygen is enabled by the leveling effect of spin accommodation.The activity of Cu_(n)^(-)clusters is correlated to the HOMO level,and for the close-shell clusters is also governed by the vertical spin excitation energy(VSE).In encountering the attack of dioxygen,the activity of the copper cluster anions not only depends on their basicity to donate electrons,but also closely associated with the cluster sizes.Small copper clusters Cu_(n)^(-)(n=7−13)can dissociate O_(2)spontaneously,while large clusters require extra energies and display close relationship between the reaction rates and electronic vertical detachment energies(VDE).Our work illuminates a novel reaction mechanism between Cu_(n)^(-)clusters and O_(2),which sheds light in manipulating the activity and stability of coinage clusters by controlling the spin and charge states.

An electrocatalyst with anti-oxidized capability for overall water splitting

An anti-oxidized NiS2 electrocatalyst with improved catalytic activity was developed using a Fe-induced conversion strateg3a X-ray photoelectron spectroscopy reveals that betatopic Ni species with high valence states are present within the Fe-NiS2 matrix and relatively less oxidized layers exist on the catalyst＇s surface, indicating its greatly enhanced anti-oxidized capability. Density functional theory calculations reveal that the Ni and Fe sites on the Fe-NiS2 catalyst surface possess strong adsorption capacity toward hydroxyl ions compared with the Ni sites on NiS2. Benefiting from its unique microstructure and modulated electronic structure due to the effects of iron species, the Fe-NiS2 catalyst prepared on carbon fiber delivers a remarkably enhanced catalytic activity and superior long-life durability for overall water splitting. The present results provide an efficient strategy for the design and configuration of anti-oxidation catalysts, especially for energy storage and catalysis.

Characterization of a novel regulatory pathway for mannitol metabolism and its coordination with biofilm formation in Mycobacterium smegmatis

Biofilm formation has been implicated to be tightly regulated in bacteria. Mycobacterial species possess a unique cell-wall structure; however, the underlying regulation mechanism for their biofilm formation remains largely unclear. In this study, we characterized a hypothetical mannitol metabolism and transportation gene cluster（Ms5571-Ms5576）, designated as mmt operon, whose expression significantly contributes to the biofilm formation in Mycobacterium smegmatis. We showed that in the operon the Ms5575 gene encodes a GntR-like transcriptional repressor and the Ms5576 gene encodes a mannitol2-dehydrogenase which can produce D-mannitol from D-mannose. Strikingly, the D-mannitol molecule can derepress the negative regulation of Ms5575 on the mmt operon to stimulate the operon＇s expression. Consistently, addition of D-mannitol into the medium can obviously induce mycobacterial biofilm formation. Furthermore, we found that Ms0179 positively regulates the mmt operon through its downstream regulator Ms0180. Ms0180 directly binds the mmt operon to positively regulate its expression. Both Ms0179 and Ms0180 significantly affect the mycobacterial biofilm formation. Taken together, we explored a regulatory pathway for the mannitol metabolism and its coordination with the biofilm formation in M. smegmatis. This finding provides novel insights into the unique mechanism of biofilm formation regulation in mycobacteria.

Currency Exposure in China under the New Exchange Rate Regime： National Level Evidence

The present paper studies China＇s national level currency exposure since 2005 when the country adopted a new exchange rate regime allowing the renminbi （RMB） to move towards greater flexibility. Using generalized autoregressive conditional heteroskedastic and constant conditional correlation-generalized autoregressive conditional heteroskedastic methods to estimate the augmented capital asset pricing models with orthogonalized stock returns, we find that China equity indexes are significantly exposed to exchange rate movements. In a static setting, there is strong sensitivity of stock returns to movements of China＇s trade- weighted exchange rate, and to the bilateral exchange rates except the RMB/dollar rate. However, in a dynamic framework, exposure to all the bilateral currency pairs under examination is significant. The results indicate that under the new exchange rate regime, China＇s gradualist approach to moving towards greater exchange rate flexibility has managed to keep exposure to a moderate level. However, we find evidence that in a dynamic setting, the exposure of the RMB to the dollar and other major currencies is significant. For China, the challenge of managing currency risk exposure is looming greater.