Proton‐Coupled Electron Transfer of Unsaturated Fatty Acids and Mechanistic Insight into Lipoxygenase

A proton‐coupled electron transfer (PCET) process plays an important role in the initial step of lipoxygenases to produce lipid radicals which can be oxygenated by reaction with O₂ to yield the hydroperoxides stereoselectively. The EPR spectroscopic detection of free lipid radicals and the oxygenated radicals (peroxyl radicals) together with the analysis of the EPR spectra has revealed the origin of the stereo‐ and regiochemistry of the reaction between O₂ and linoleyl (= (2Z)‐10‐carboxy‐1‐[(1Z)‐hept‐1‐enyl]dec‐2‐enyl) radical in lipoxygenases. The direct determination of the absolute rates of H‐atom‐transfer reactions from a series of unsaturated fatty acids to the cumylperoxyl (= (1‐methyl‐1‐phenylethyl)dioxy) radical by use of time‐resolved EPR at low temperatures together with detailed kinetic investigations on both photoinduced and thermal electron‐transfer oxidation of unsaturated fatty acids provides the solid energetic basis for the postulated PCET process in lipoxygenases. A strong interaction between linoleic acid (= (9Z,12Z)‐octadeca‐9,12‐dienoic acid) and the reactive center of the lipoxygenases (Fe^III? OH) is suggested to be involved to make a PCET process to occur efficiently, when an inner‐sphere electron transfer from linoleic acid to the Fe^III state is strongly coupled with the proton transfer to the OH group.     

Mechanistic Insight into the Facilitation of β‐Lactam Fragmentation through Metal Assistance

The mechanism of OsH₆(PiPr₃)₂‐mediated fragmentation of a 4‐(2 pyridyl)‐2‐azetidinone has been investigated by DFT calculations. The addition of the C4?H bond of the substrate to OsH₂(PiPr₃)₂ allows the active participation of an osmium lone pair in the B‐type β‐lactam fragmentation process. This new mechanism makes the N1?C4/C2?C3 fragmentation of the lactamic core thermally accessible through a stepwise process.     

Mechanistic Insight into the Copper‐Catalyzed Regiodivergent Silacarboxylation of Allenes with CO2

DFT calculations were performed to investigate the detailed reaction mechanisms in the copper‐catalyzed regiodivergent silacarboxylation of allenes. According to our calculations, the catalysis would bifurcate at the allene silylcupration step, followed by CO₂ insertion, eventually leading to the carboxylated vinylsilane or allylsilane products. The gaps between the two silylcupration barriers were predicted to be ?2.3, ?0.4, and 2.2 kcal mol^?1 when using (rac)‐Me‐DuPhos, dcpe, and PCy₃ (+H₂O) as the ligands, which nicely accounted for the experimental vinylsilane/allylsilane ratios of 93:7, 50:50, and 15:85, respectively. By means of transition‐state‐energy decomposition, we found that the energy penalty of catalyst deformation into its transition‐state geometry was the key factor in determining the direction of the reaction. The switchable regioselectivity by using different P ligands could be ascribed to structural changes of the Cu?Si and Cu?P bonds during the silylcupration process.     

Mechanistic Insight into Binding of Huperzine A with Human Serum Albumin: Computational and Spectroscopic Approaches

Human serum albumin (HSA) is the most abundant protein in plasma synthesized by the liver and the main modulator of fluid distribution between body compartments. It has an amazing capacity to bind with multiple ligands, offering a store and transporter for various endogenous and exogenous compounds. Huperzine A (HpzA) is a natural sesquiterpene alkaloid found in Huperzia serrata and used in various neurological conditions, including Alzheimer’s disease (AD). This study elucidated the binding of HpzA with HSA using advanced computational approaches such as molecular docking and molecular dynamic (MD) simulation followed by fluorescence-based binding assays. The molecular docking result showed plausible interaction between HpzA and HSA. The MD simulation and principal component analysis (PCA) results supported the stable interactions of the protein–ligand complex. The fluorescence assay further validated the in silico study, revealing significant binding affinity between HpzA and HSA. This study advocated that HpzA acts as a latent HSA binding partner, which may be investigated further in AD therapy in experimental settings.     

Mechanistic Insight into the Reaction of Organic Acids with SO3 at the Air–Water Interface

The gas‐phase reaction of organic acids with SO₃ has been recognized as essential in promoting aerosol‐particle formation. However, at the air–water interface, this reaction is much less understood. We performed systematic Born–Oppenheimer molecular dynamics (BOMD) simulations to study the reaction of various organic acids with SO₃ on a water droplet. The results show that with the involvement of interfacial water molecules, organic acids can react with SO₃ and form the ion pair of sulfuric‐carboxylic anhydride and hydronium. This mechanism is in contrast to the gas‐phase reaction mechanisms in which the organic acid either serves as a catalyst for the reaction between SO₃ and H₂O or reacts with SO₃ directly. The distinct reaction at the water surface has important atmospheric implications, for example, promoting water condensation, uptaking atmospheric condesation species, and incorporating “SO₄^2?” into organic species in aerosol particles. Therefore, this reaction, typically occurring within a few picoseconds, provides another pathway towards aerosol formation.     

Mechanistic Insights into Palladium‐Catalyzed Silylation of Aryl Iodides with Hydrosilanes through a DFT Study

The catalytic cycles of palladium‐catalyzed silylation of aryl iodides, which are initiated by oxidative addition of hydrosilane or aryl iodide through three different mechanisms characterized by intermediates R₃Si−Pd^II−H (Cycle A), Ar−Pd^II−I (Cycle B), and Pd^IV (Cycle C), have been explored in detail by hybrid DFT. Calculations suggest that the chemical selectivity and reactivity of the reaction depend on the ligation state of the catalyst and specific reaction conditions, including feeding order of substrates and the presence of base. For less bulky biligated catalyst, Cycle C is energetically favored over Cycle A, through which the silylation process is slightly favored over the reduction process. Interestingly, for bulky monoligated catalyst, Cycle B is energetically more favored over generally accepted Cycle A, in which the silylation channel is slightly disfavored in comparison to that of the reduction channel. Moreover, the inclusion of base in this channel allows the silylated product become dominant. These findings offer a good explanation for the complex experimental observations. Designing a reaction process that allows the oxidative addition of palladium(0) complex to aryl iodide to occur prior to that with hydrosilane is thus suggested to improve the reactivity and chemoselectivity for the silylated product by encouraging the catalytic cycle to proceed through Cycles B (monoligated Pd⁰ catalyst) or C (biligated Pd⁰ catalyst), instead of Cycle A.     

[CpRu]‐Catalyzed Carbene Insertions into Epoxides: 1,4‐Dioxene Synthesis through SN1‐Like Chemistry with Retention of Configuration

Rather than lead to the usual deoxygenation pathway, metal carbenes derived from α‐diazo‐β‐ketoesters undergo three‐atom insertions into epoxides using a combination of 1,10‐phenanthroline and [CpRu(CH₃CN)₃][BAr_F] as the catalyst. Original 1,4‐dioxene motifs are obtained as single regio‐ and stereoisomers. A perfect syn stereochemistry (retention, e.r. up to 97:3) is observed for the ring opening, which behaves as an S_N1‐like transformation.     

Deeper Insight into the Volatile Profile of Rosa willmottiae with Headspace Solid-Phase Microextraction and GC–MS Analysis

As the distribution center of Rosa in the world, China has abundant wild germplasm resources, which can contribute to the breeding of modern roses. To explore the potential value of wild roses distributed in the Sichuan–Tibet region, solid phase microextraction (SPME) and gas chromatography–mass spectrometry (GC–MS) were used to determine the volatile organic compounds (VOCs) in Rosa willmottiae flowers at three flowering stages (bud stage, initial flowering stage, full flowering stage). Meanwhile, we compared the VOCs of R. willmottiae with different phenotypes (double flowers and single flowers). A total of 74 volatile compounds were identified. The results show that the essential substances belong to alcohols and terpenoids. The main volatile organic compounds are 2-phenyl ethanol (20.49%), benzyl alcohol (10.69%), β-maaliene (8.66%), geranyl acetate (8.47%), and (+)-α-long pinene (6.127%). Different flowering stages had great influence on the volatile profile, from the bud stage to full flowering stage; the content of terpenoids released decreased by 6.17%, whereas alcohols and esters increased by 8.58% and 11.56%, respectively. The chemical diversity and the content of the main components with a different phenotype were not significantly different. Our result will provide a theoretical basis for the development and utilization of Rosa willmottiae in Sichuan and Tibet.     

Insight into Seasonal Change of Phytochemicals,Antioxidant, and Anti-Aging Activities of Root Bark of Paeonia suffruticosa (Cortex Moutan) Combined with Multivariate Statistical Analysis

Chemical compositions, antioxidants, and anti-aging activities of Cortex Moutan (CM), from different collection periods and different producing areas, were measured and compared in order to obtain excellent CM extracts. The bioactivities of CM extracts were examined by an in vitro antioxidant method and a UVB irradiated human dermal fibroblast (HDF) model. Phytochemical properties were obtained from ultra-fast liquid chromatography quadrupole time-of-flight mass spectrometry (UFLC-Q-TOF-MS) prior to the multivariate statistical analysis. As for the results, the extracts of Heze CM (HZCM) and Luoyang CM (LYCM) collected in June had better in vitro antioxidant activities, significantly increased the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and reduced the content of malondialdehyde (MDA), compared to other CM extracts. HZCM and LYCM extracts could upregulate the relative expression of SOD and GSH-Px mRNA. The extract of HZCM collected in June could significantly repress the production of matrix metalloproteinase 1 (MMP-1) and improve the production of procollagen type I (PCOL)-I in UVB irradiated HDF. In total, 50 compounds, including 17 monoterpenoids, 19 flavonoids, 13 phenols, and 1 amino acid were identified or tentatively identified in the CM extracts. Gallic acid, p-hydroxybenzoic acid, oxypaeoniflorin, paeoniflorin, 1,2,3,4,6-O-pentagalloyl glucose, and paeonol were predominant compounds in the CM extracts. Taken together, CM collected from April to September had better antioxidant and anti-aging effects for external usage.