Among the multiple components of propolis, flavonoids contribute greatly to the antioxidant activities of propolis. Flavonoids mainly exist in the form of sugar-conjugated derivatives. Quercetin ...glycosides represent the predominant flavonoid fraction in propolis. In this work, density functional theory (DFT) calculations were applied to analyze the antioxidative properties of quercetin and its glucosides in the gas and in the liquid phase (ethanol, water). Three main antioxidant mechanisms, hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET) were used to analyze the antioxidative capacity of the investigated compounds. Solvent effects dominantly affect SET-PT and SPLET. Thus, the thermodynamically preferred mechanism can be altered. HAT and SPLET are the thermodynamically dominant mechanisms in gas and solvent phases, respectively. Therefore, in the gas phase, the sequence of the antioxidative capacity is similar with the bond dissociation enthalpy values: quercetin > quercetin-5-O-glucoside > quercetin-7-O-glucoside > quercetin-3-O-glucoside > quercetin-3'-O-glucoside > quercetin-4'-O-glucoside. While, in the solvent phases, the sequence is similar with the proton affinity values: quercetin-4'-O-glucoside > quercetin-5-O-glucoside > quercetin > quercetin-3-O-glucoside > quercetin-7-O-glucoside > quercetin-3'-O-glucoside. OH groups in B-ring and C-ring contribute mainly to the antioxidative activities of quercetin and glucosides compared with A-ring.
Short single‐wall carbon nanotubes (SWCNTs)/epoxy composites were fabricated by a procedure of cutting and functionalizing SWCNTs, dispersion and curing, with a very low weight ratio of SWCNTs from ...0.03% to 0.5%. It was found that the tensile fracture strength of composites increases with increasing SWCNT content initially, then reaches a maximum at 0.05 wt% and finally decreases with further increasing the SWCNT content. The fracture strength of the composite with 0.05 wt% SWCNTs (78.46 MPa) is ~160% of that of pure epoxy sample (48.64 MPa). A model analysis based on the competition of local matrix fracture and the debonding of short SWCNT was proposed. It revealed that there is a transition from local matrix fracture to the debonding of SWCNT when the SWCNT content increases, this transition leads to the decrease in the tensile strength. Further analyses indicated that the content of carbon nanotubes (CNTs) at the transition depends on their radius and length. Considering the effect of surface cracks on the fracture strength of pure epoxy sample, the theoretical analyses are qualitatively in agreement with the experimental results. The present study partly explains why the fracture strength of CNT reinforced composites has various results for a similar CNT loading in previous works.
A schematic representation of the model cell of single‐wall carbon nanotubes (SWCNT) reinforced epoxy composites (A) and the forces suffered by SWCNT and matrix (B). A prediction dependence of fracture stress on the SWCNT content in the composites is given in (C).
Summary
Previous works indicated that aluminum hydroxide could be acted as catalyst to speed up the hydrolysis reaction of Al, and the catalytic activity of aluminum hydroxide is related to its ...microscopic properties. In this work, the effect of crystalline phases of aluminum hydroxide catalysts on Al hydrolysis is researched systematically. The results show that the hydrogen production performance of Al hydrolysis catalyzed by aluminum hydroxide with different crystalline phases is obviously different, and the catalytic activity of different aluminum hydroxides is in the order of boehmite > bayerite > gibbsite. A possible mechanism is given, which reveals that the surface hydroxyl groups of aluminum hydroxide act as anchoring sites for the adsorption of H2O molecules and decrease the dissociative adsorption energy of H2O molecules on aluminum hydroxides surface, promoting the hydrolysis reaction of Al. Consequently, the difference in catalytic activity of aluminum hydroxide with different crystalline phases is attributed to the difference in surface hydroxyl group density. This study provides a new idea for the synthesization of aluminum hydroxide with excellent catalytic activity.
The effect of crystalline phases of aluminum hydroxide on Al‐water reaction was investigated. The activity of different aluminum hydroxides is in the order of boehmite > bayerite > gibbsite. The activity of aluminum hydroxide is closely related to its surface hydroxyl group density.
The multiple free radical scavenging reactions of flavonoids have been studied considering the gas, benzene, and water phases by applying density functional theory (DFT). Intramolecular ...hydrogen-bonds are found in all the most stable geometries of flavonoids and can reduce the antioxidant activity of hydroxyl groups, acting as hydrogen-bond donors (5−OH, 3−OH and 3’−OH), while enhancing the antioxidant activity of hydroxyl groups, acting as hydrogen-bond acceptors (4’−OH). In the gas and benzene phases, all of the flavonoids first prefer performing continuous di-hydrogen atom transfer (HAT) reaction from the B ring OH groups to trap two free radicals with the formation of stable quinones for ampelopsin, taxifolin, dihydro orobol and eriodictyol and benzodioxole for hesperetin. They would trap the third free radical via the HAT in the gas phase. In the benzene phase, ampelopsin also favours to apply HAT to trap the third free radical, and the other flavonoids would use the sequential proton loss electron transfer (SPLET) mechanism. In the water phase, the investigated flavonoids would first perform consecutive proton loss (PL) reactions from all of the OH groups with the formation of polyanions. The multiple PL reactions begin from the 7−OH group. The second PL reaction prefers performing in the OH groups on the B benzene ring due to the better delocalization of the negative charge via conjugation over the entire skeleton. The polyanions of the investigated flavonoids scavenge three free radicals via three consecutive ET reactions.
•The antioxidative mechanisms of the five flavonoids are analysed.•Intramolecular hydrogen-bonds can affect the hydrogen atom loss sequence.•Flavonoids can perform multiple H+/e‒ processes.•The preferred mechanisms vary among the different phases.
The Arabidopsis thaliana protein UVR8 is a photoreceptor for ultraviolet-B. Upon ultraviolet-B irradiation, UVR8 undergoes an immediate switch from homodimer to monomer, which triggers a signalling ...pathway for ultraviolet protection. The mechanism by which UVR8 senses ultraviolet-B remains largely unknown. Here we report the crystal structure of UVR8 at 1.8 Å resolution, revealing a symmetric homodimer of seven-bladed β-propeller that is devoid of any external cofactor as the chromophore. Arginine residues that stabilize the homodimeric interface, principally Arg 286 and Arg 338, make elaborate intramolecular cation-π interactions with surrounding tryptophan amino acids. Two of these tryptophans, Trp 285 and Trp 233, collectively serve as the ultraviolet-B chromophore. Our structural and biochemical analyses identify the molecular mechanism for UVR8-mediated ultraviolet-B perception, in which ultraviolet-B radiation results in destabilization of the intramolecular cation-π interactions, causing disruption of the critical intermolecular hydrogen bonds mediated by Arg 286 and Arg 338 and subsequent dissociation of the UVR8 homodimer.
Single-wall carbon nanotube (SWCNT) reinforced copper (Cu) matrix composites were fabricated by a simple procedure of rotating mixing and then spark plasma sintering (SPS). It was found that addition ...of SWCNTs as low as 0.05 wt% can increase the ultimate tensile strength (223.5 MPa) and elongation rate (48.0%) of composites by 20.8% and 57.9%, respectively, relative to those (185 MPa and 30.4%) of pure Cu material, while the composite electrical conductivity retained a high value of 94.3% International Annealed Copper Standard (IACS). Mechanism analyses revealed that there are nanometer Ni particles tightly attached to SWCNTs, which were formed in situ during SWCNT growth by arc discharge method. These nanometer Ni particles promoted the bonding between SWCNTs and Cu matrix, leading to a fracture-mode change from intergranular in pure Cu material to ductile in the composites. The related factors to influence the microstructure of composites were discussed as well.
•Ni nanoparticles were formed in situ and tightly attached to SWCNT surfaces.•Addition of 0.05 wt% SWCNTs greatly increased both of strength and elongation of Cu.•A fracture-mode change occurs from intergranular in pure Cu to ductile in composites.•Load transfer is main mechanism responsible for enhancement in mechanical property.
Alongside the development of small-sized fuel cells, in situ hydrogen production technologies are essential to enable a compact, reliable and stable fuel supply. Among these technologies, Al-water ...reaction has tremendous potential and broad market prospect due to its low price, high gravimetric hydrogen storage density, environmentally benign byproducts and convenient storage and transportation. However, there is a passive oxide film on Al surface, hindering the direct reaction of Al with water, which has captured a surge of interest in enhancing the hydrogen production performance of Al-water reaction. In this review, the research progress in Al-water reaction is summarized. First, the physicochemical processes and related influencing factors of Al-water reaction are presented. Second, the strategies for enhancing Al-water reaction are critically discussed from five points of view: addition of alkalis, addition of catalysts, Al alloys, mechanical ball milling and surface modification. After that, the feasibility of hydrogen production from Al scraps is evaluated, and the potential applications of Al-water reaction are mapped. Finally, the challenges and opportunities of Al-hydrogen technology for fuel cell applications are outlined.
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•Mechanisms and related influencing factors of Al-water reaction were presented.•The strategies for enhancing Al-water reaction were critically discussed.•The feasibility of hydrogen production from Al scraps was evaluated.•The potential applications of Al-water reaction were mapped.•Challenges and opportunities of Al-hydrogen technology for fuel cells were outlined.
Magnetic anisotropy is the key element in the construction of single‐ion magnets, a kind of nanomagnets for high‐density information storage. This works describes an unusual large easy‐plane magnetic ...anisotropy (with a zero‐field splitting parameter D of +40.2 cm−1), mainly arising from the second‐order spin‐orbit coupling effect in a trigonal‐planar CoII complex Li(THF)4Co(NPh2)3, revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations. Meanwhile, the field‐induced slow magnetic relaxation in this complex was mainly attributed to the Raman process.
Single‐ion magnets, how do they work? The first three‐coordinate trigonal‐planar CoII complex that exhibits large easy‐plane magnetic anisotropy (D=+40.2 cm−1) mainly arising from second‐order spin‐orbit coupling effects was unambiguously revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations.
Bromate, a toxic by-product of bromide-containing drinking water after disinfecting with ozone, has attracted much attention in the past two decades. Traditional methods to activate zero-valent ...metals for reducing bromate are to eliminate their surface oxide layer by acid washing. In this work, for the first time, zero-valent Al (ZVAl) was surface treated by the following procedures including soaking, soaking and freeze-drying, soaking and heat-treating, and γ-Al2O3 covering Al particle surfaces (GCAP). It was found that all of above surface treated ZVAls have an obvious high efficiency for bromate reduction relative to pristine ZVAl. The bromate reduction rate is GCAP > soaking Al > freeze-drying Al > soaking and heat-treating Al > pristine Al, and using GCAP just 30 min is taken to completely reduce bromate to bromide in neutral solution. Mechanism analyses revealed that Al surface treating or covered by fine γ-Al2O3 phase can promote the hydration and breakage of Al surface passive oxide layer, resulting in a fast contact of inner Al with outside ions, leading to a high reduction rate of bromate in neutral solution. XPS analyses indicated that there are no bromate or bromide ions adsorbed on Al particle surfaces, implying that there is a high direct donating efficiency of electrons from inner Al to bromate ions in solution. Furthermore, GCAP has a good reusability and >90% bromate can be reduced even it was reused up to 4 cycles.
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•Zero-valent Al were treated by soaking or direct covering loose γ-Al2O3 grains.•Surface treated Al has a high efficiency for reducing bromate in neutral solution.•Surface treated Al has a good reusability for reducing bromate in solution.•Al treatment by soaking or γ-Al2O3 decreases induction time of reduction reaction.
2D magnetic materials with dimerized honeycomb lattices can be treated as mixed‐spin square lattices, in which a quantum phase transition may occur to realize the Bose–Einstein condensation of ...magnons under reachable experimental conditions. However, this has never been successfully realized with integer spin centers. Herein, a spin integer (S = 2) dimerized honeycomb lattice in an iron(II)‐azido compound Fe(4‐etpy)2(N3)2n (FEN, 4‐etpy = 4‐ethylpyridine) is realized. Morphology characterization by transmission electron microscopy, scanning electron microscopy, and atomic force microscopy spectroscopies show that the thinnest place of the sample is ≈13 nm, which is equal to ten layers of the compound. In contrast to the common magnetic properties of long‐range magnetic ordering, Mössbauer and polarized neutron scattering studies reveal that FEN exhibits a reentrant spin glass behavior owing to competing ferro‐ and antiferromagnetic exchange‐coupling interactions within the lattice. Two spin glass phases with disparate canting angles are characterized at 39 and 28 K, respectively. By using Curély's model, two exchange‐coupling constants (J1 = +35.8 cm−1 and J2 = −3.7 cm−1) can be simulated. Moreover, a very large coercive field of ≈1.9 Tesla is observed at 2 K, making FEN a “very hard” van der Waals 2D magnetic material.
A dimerized honeycomb lattice with integer spins is realized in an iron(II)‐azido compound. Morphology characterization shows that the thinnest place of the sample is ≈13 nm, which comprises ten layers of the compound. Magnetic, Mössbauer, and polarized neutron scattering studies reveal a reentrant spin glass behavior, which leads to a “very hard” magnetic hysteresis loop at lower temperatures.
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