Terminal Oxygen Research Articles

Reversible Multi‐Complexation of CO2 to Alkaline Earth Metal Ion‐Pair at 400 ppm and 298 K

AbstractThe efficient capture of low‐pressure CO2 remains a significant challenge due to the lack of established multi‐complexation of CO2 to active sites in microporous materials. In this study, we introduce a novel concept of reversible multi‐complexation of CO2 to alkaline earth metal (AEM) ion pairs, utilizing a host site in ferrierite‐type zeolite (FER). This unique site constrains two AEM ions in proximity, thereby enhancing and isotopically spreading their electrostatic potentials within the zeolite cavity. This electrostatic potential‐engineered micropore can trap up to four CO2 molecules, forming M2+−(CO2)n−M2+ (n=0–4, M=Ca, Sr, Ba) complexes, where each CO2 molecule is stabilized by interactions between terminal oxygen (Ot) in CO2 and the AEM ions. Notably, the Ba2+ pair site exhibits higher thermodynamic stability for multiple adsorptions due to the optimal binding mode of Ba2+−Ot−Ba2+. Through high‐accuracy energy calculations, we have established the relationship among structure, CO2 uptake, and operating temperature/pressure, demonstrating that the Ba2+ pair site can capture four CO2 molecules even at concentrations as low as 400 ppm and at 298 K. Three of the four molecules of CO2 trapped were removable at room temperature and under vacuum. The findings in the present study provide a new direction for developing efficient CO2 adsorbents.

Synthesis, crystallographic, spectroscopic, magnetic, and DFT characterization of a new strandberg-type polyoxometalate: (H2AEP)4[CoP2Mo5]2 ̶ first principles calculations

This work reports the synthesis of a novel diphosphopentamolybdate compound with the formula (C6H17N3)4[Co(H2O)4P2Mo5O23]2‧8·74H2O abbreviated as (H2AEP)4[CoP2Mo5]2. Characterization of this compound was carried out using various techniques, including X-ray diffraction, thermal analysis (TGA), and spectroscopies such as FT-IR, Raman, UV–visible, and fluorescence. The structural study reveals that the studied material crystallizes in the orthorhombic system Pca21 with lattice parameters of a = 18.6348(16) Å, b = 11.8613(10) Å, c = 35.074(3) Å, and volume V = 7752.5(11) Å3, and a number of formula units per primitive cell Z = 4. The crystal structure study reveals that the Strandberg anions (P2Mo5O23)6− are connected to Co(II) octahedra through the terminal oxygen atoms of the PO4 tetrahedra, creating 1-D anionic chains in the crystallographic direction [101]. The 1-(2-Aminoethyl)piperazinium (AEP) cations are located within the anionic framework to neutralize its negative charge. The magnetic characteristics of the compound were investigated by examining the temperature-dependent molecular susceptibility over the range of 2–300 K. The analysis revealed the presence of an antiferromagnetic exchange interaction. Moreover, the material's refractive index exhibited significant dispersive behavior within the visible spectrum, indicating its potential for use in visible optical communication devices. A first-principles density functional theory (DFT)-based computational study confirms the semiconducting behavior of (H2AEP)4[CoP2Mo5]2, revealing that in the ground state, high-spin octahedrally coordinated Co2+ cations exhibit a very weak antiferromagnetic coupling along the [Co(H2O)4P2Mo5O23]∞4−chains running in the crystal a direction.

Fundamental Insights into the Direct Electron Transfer Mechanism on Ag Atomic Cluster.

The nonradical oxidation pathway for pollutant degradation in Fenton-like catalysis is favorable for water treatment due to the high reaction rate and superior environmental robustness. However, precise regulation of such reactions is still restricted by our poor knowledge of underlying mechanisms, especially the correlation between metal site conformation of metal atom clusters and pollutant degradation behaviors. Herein, we investigated the electron transfer and pollutant oxidation mechanisms of atomic-level exposed Ag atom clusters (AgAC) loaded on specifically crafted nitrogen-doped porous carbon (NPC). The AgAC triggered a direct electron transfer (DET) between the terminal oxygen (Oα) of surface-activated peroxodisulfate and the electron-donating substituents-containing contaminants (EDTO-DET), rendering it 11-38 times higher degradation rate than the reported carbon-supported metal catalysts system with various single-atom active centers. Heterocyclic substituents and electron-donating groups were more conducive to degradation via the EDTO-DET system, while contaminants with high electron-absorbing capacity preferred the radical pathway. Notably, the system achieved 79.5% chemical oxygen demand (COD) removal for the treatment of actual pharmaceutical wastewater containing 1053 mg/L COD within 30 min. Our study provides valuable new insights into the Fenton-like reactions of metal atom cluster catalysts and lays an important basis for revolutionizing advanced oxidation water purification technologies.

Actual reliance of catalytic activities tuned oxygen vacancies on VOx/Ce1-xZrxO2 catalyst for selective oxydehydrogenation of methanol to dimethoxymethane

Metal oxide catalysts like ceria-supported vanadia (VOx/Ce1-xZrxO2) with different Zr contents were prepared by coupling ceria-zirconia solid solution and vanadia. Characterization revealed that the insertion of zirconia into ceria significantly improved catalytic activity due to the generation of more oxygen vacancies and distortion of the composite structure. The oxygen vacancy concentration initially increased with the increase of Zr (x= wt%) and peaked at x=0.7 and then decreased. The reliance of oxygen vacancy concentration in VOx/Ce1-xZrxO2 catalyst on their catalytic activity in methanol oxydehydrogenation was further investigated. Results indicated that oxygen vacancies not only activated oxygen species but also cleaved C–H bonds alongside V5+ active site contained terminal oxygen atoms for cleaving O–H and C–H bonds within the adsorbed methanol. The VOx/Ce0.3Zr0.7O2 exhibited the best catalytic activity with the methanol conversion of 76 % and the dimethoxymethane selectivity of 96 % at 200 ℃ and atmospheric pressure in a fixed-bed reactor.

Reversible Multi-Complexation of CO2 to Alkaline Earth Metal Ion-Pair at 400 ppm and 298 K.

The efficient capture of low-pressure CO2 remains a significant challenge due to the lack of established multi-complexation of CO2 to active sites in microporous materials. In this study, we introduce a novel concept of reversible multi-complexation of CO2 to alkaline earth metal (AEM) ion pairs, utilizing a host site in ferrierite-type zeolite (FER). This unique site constrains two AEM ions in proximity, thereby enhancing and isotopically spreading their electrostatic potentials within the zeolite cavity. This electrostatic potential-engineered micropore can trap up to four CO2 molecules, forming M2+-(CO2)n-M2+ (n = 0-4, M = Ca, Sr, Ba) complexes, where each CO2 molecule is stabilized by interactions between terminal oxygen (Ot) in CO2 and the AEM ions. Notably, the Ba2+ pair site exhibits higher thermodynamic stability for multiple adsorptions due to the optimal binding mode of Ba2+-Ot-Ba2+. Through high-accuracy energy calculations, we established the relationship among structure, CO2 uptake, and operating temperature/pressure, demonstrating that the Ba2+ pair site can reversibly capture four CO2 molecules even at concentrations as low as 400 ppm and at 298 K. The findings in the present study provide a new direction for developing efficient CO2 adsorbents.

Synergistic Coordination in Cu Single-Atom Catalysts Enhances High-Valent Copper-Oxo Species for Efficient PMS Activation.

In the domain of heterogeneous catalytic activation of peroxymonosulfate (PMS), high-valent metal-oxo (HVMO) species are widely recognized as potent oxidants for the abatement of organic pollutants. However, the generation selectivity and efficiency of HVMO are often constrained by stringent requirements for catalyst adsorption sites and electron transfer efficiency. In this study, a single-atom catalyst, CuSA/CNP&S, is synthesized featuring multiple types (planar/axial) of heteroatom coordination via an H-bond-assisted self-assembly strategy. It isconfirmed that CuN3 active centers with axial S coordination are uniformly distributed in a carbon matrix modified by planar P atoms. CuSA/CNP&S activated PMS to selectively generate Cu(III)═OH species as the primary reactive oxygen species (ROS). The pseudo-first-order kinetic rate for bisphenol A degradation reached 1.51min-1, a 17.57-fold increase compared to the unmodified CuSA/CN catalyst. Additionally, the CuSA/CNP&S catalyst demonstrates high efficiency and durability in removing contaminants from various aqueous matrices. Theoretical calculations and experimental results indicate that the intrinsic electric field generated by distal planar P atoms enhances electron transfer efficiency within the carbon matrix. Meanwhile, axial S coordination elevates the d-band center and tunes the eg * band broadening of Cu, thereby enhancing the adsorption selectivity for the terminal oxygen of PMS. This multitype coordination synergistically mitigates the issues of low selectivity and yield of HVMO species.

Reduction of a two-dimensional crystalline MoO3 monolayer

The atomic structure of MoOx films formed upon a gradual thermal reduction of an ordered MoO3 monolayer on the Pd(100) substrate was explored via surface science characterization techniques and density functional theory (DFT) calculations. Two main reduction stages were identified. First, the initial oxygen excess was gradually eliminated by altering the domain boundary length, orientation, and atomic structure. The films nevertheless remained O-rich, with numerous terminal oxygen atoms (formation of MoO groups), and an elevated work function. Second, multiple ordered O-lean phases were formed, characterized by either very few or no terminal oxygen atoms, and a much smaller surface work function. According to calculations, the positive charging of the Pd substrate stabilizes the oxygen excess during the first stage, but during the second reduction stage, the substrate becomes negatively charged, stabilizing enhanced cation oxidation states. On their basis, the mechanisms underlying the oxygen release from the initial c(2 × 2) domains were disclosed. The experiments showed that the film reduction is perfectly reversible, which highlights the very promising properties of the MoO3/Pd system for heterogeneous catalysis.

Multi-channel electron transfer induced by polyvanadate in metal-organic framework for boosted peroxymonosulfate activation

Catalytic peroxymonosulfate (PMS) activation processes don’t solely rely on electron transfer from dominant metal centers due to the complicated composition and interface environment of catalysts. Herein the synthesis of a cobalt based metal-organic framework containing polyvanadate [V4O12]4− cluster, Co2(V4O12)(bpy)2 (bpy = 4,4’-bipyridine), is presented. The catalyst demonstrates superior degradation activity toward various micropollutants, with higher highest occupied molecular orbital (HOMO), via nonradical attack. The X-ray absorption spectroscopy and density functional theory (DFT) calculations demonstrate that Co sites act as both PMS trapper and electron donor. In situ spectral characterizations and DFT calculations reveal that the terminal oxygen atoms in the [V4O12]4− electron sponge could interact with the terminal hydrogen atoms in PMS to form hydrogen bonds, promoting the generation of SO5* intermediate via both dynamic pull and direct electron transfer process. Further, Co2(V4O12)(bpy)2 exhibits long-term water purification ability, up to 40 h, towards actual wastewater discharged from an ofloxacin production factory. This work not only presents an efficient catalyst with an electron sponge for water environmental remediation via nonradical pathway, but also provides fundamental insights into the Fenton-like reaction mechanism.

Oxygen Atom Stabilization by a Main-Group Lewis Acid: Observation and Characterization of an OBeF2 Complex with a Triplet Ground State.

Terminal oxygen radicals involving p- and d-block atoms are quite common, but s-block compounds with an oxygen radical character remain rare. Here, we report that alkaline-earth metal beryllium atoms react with OF2 to form the oxygen beryllium fluorides OBeF and OBeF2. These species are characterized by matrix-isolation infrared spectroscopy with isotopic substitution and quantum-chemical calculations. The linear molecule OBeF has a 2Π ground state with an oxyl radical character. The 3A2 (C2v) ground state of OBeF2 represents the unusual case of a triplet oxygen atom stabilized by a relatively weak interaction by the Lewis acidic BeF2. The interaction involves both a donor component from oxygen to empty Be orbitals and a back-bonding contribution from fluorine substituents toward oxygen.

(Invited) IrOx Nanoparticle Catalysts on a Unique Microstructure of Doped Tin Oxide to Dramatically Boost the Oxygen Evolution Reaction Activity for PEM Water Electrolysis

Proton exchange membrane water electrolyzers (PEM WEs) has a potential to produce high-purity green hydrogen. PEMWEs have been commercialized with the use of high loadings of the noble metal Ir-based anode catalyst for the oxygen evolution reaction (OER). To lower the Ir loading amount, it is critically important to develop efficient catalysts with increased Ir utilization and higher OER activity. One-dimensional Ir oxide nanorod/nanoparticles catalysts supported on doped SnO2 with a unique microstructure were invented to show their exceptionally high activity in the OER catalysis, with the Ir-mass specific activity being 10 times higher than that of commercial IrOx catalyst at 1.5 V vs. RHE.[1] The experiment also found that the OER activation energy was greatly lower than that of commercial Ir oxide. The interaction between Ir oxide nanorod/nanoparticles and doped SnO2 support occurs a lower degree of Ir oxidation which allows the surface terminal oxygens to be closer together while allowing facile desorption of nonreactive oxygenated species. The electronic state of Ir oxide nanorod/nanoparticles catalysts could play an important role in facilitating the crucial step of the OER, thereby enhancing the OER activity. These priorities of the catalysts would promise the reduction of the Ir usage in PEMWEs.Reference[1] G. Shi, T. Tano, D.A. Tryk, T. Uchiyama, A. Iiyama, M. Uchida, K. Terao, M. Yamaguchi, K. Tamoto, Y. Uchimoto and K. Kakinuma, ACS Catalysis, 13 (2023) 12299. Figure 1

Bulk or supported tungstophosphates? Antioxidant and antimicrobial activities following pesticide removal

The study evaluates caesium, potassium, silver, and zinc tungstophosphates synthesized in bulk and Y zeolite-supported forms through a two-step process. Spectral investigation reveals the impact of cation size on tungstophosphates formation. The large cations form ion-ion interaction with the Keggin ion, while smaller cations form hydrogen bonds between the cation hydration sphere and terminal oxygens in the Keggin ion. Supported salts formation proceeds without Keggin ion distortion. Zeta potential showed the absence of particle aggregation for caesium and potassium tungstophosphate. Nicosulfuron removal by the supported salt exhibits enhanced retention, with the exception observed for zinc tungstophosphate suggesting a decomposition mechanism. Antimicrobial evaluations reveal silver salt's potency, especially in zeolite-supported form, emphasizing the role of zeolite support. In the presence of pesticide molecules, the antimicrobial activity of salts lowers, with the exception seen for fungus strain. The antioxidant assessments demonstrate superior inhibition for insoluble bulk salts, with caesium tungstophosphate exhibiting the highest inhibition, while supported silver salt enhances bulk salt performance. The presence of pesticides affects both antimicrobial and antioxidant activities, while a complex relationship with radical scavenging ability in bulk and supported salts is independent of their pesticide adsorption capacities. The study broadens the range of the versatile applications of tungstophosphates, highlighting their specific interactions with pesticides and their impact on bioactivity and environmental remediation.

Structure of vanadia-titania catalysts doped with alkali metals according to solid-state NMR, ESR spectroscopies and DFT

In this work, V/M/TiO2 catalysts (M = Li, Na, K, Rb and Cs) were investigated with Solid-state nuclear magnetic resonance (SSNMR), electron paramagnetic resonance (ESR) and first-principles calculations. The total vanadium content corresponds to half monolayer (4 V atoms·nm−2 of TiO2 support surface), with V/M atomic ratio being 4/0.6. Static 51V and MAS 1H, 7Li, 51V, 23Na, 133Cs NMR spectra were acquired. Surface moieties of vanadium oxide on (001) anatase surface were modeled using density functional theory (DFT); their 51V NMR parameters were calculated with the Gauge-Including Projected Augmented Wave (GIPAW) method and compared to experimental data obtained with 51V SSNMR spectroscopy. It was found that mainly strongly bound vanadium sites (V3) are formed on anatase samples, located on TiO2 terminal oxygen atoms. Weakly bound vanadium sites (V1, V2) are formed on bridged anatase oxygen atoms. Supported alkali metals (Li, Na, K, Rb and Cs) on TiO2 were found to interact with protons located on the terminal and bridged TiO2 oxygen atoms. The relative ratio of weakly bound vanadium sites (V1, V2) increased on alkali-containing samples. Calculations performed showed lithium cation to prefer V-O-Ti or V-O-V bonds over VO, unsystematically deshielding vanadium nuclei. The presence of V3+ is likely to cause a loss of intensity in 51V NMR spectra.

Reaction dynamics simulation of MoO2 cluster precursor with melting, deoxygenation and sulfuration for MoS2 growth by chemical vapor deposition

Chemical vapor deposition (CVD) is a promising strategy for achieving fascinating large-scale production of MoS2. During CVD, MoO2 exhibits high reactivity and can effectively inhibit intermediate phase formation. Although, the experiments qualitatively demonstrate the MoO2 sulfuration mechanism, the reaction dynamics process on the atomic level is not clear. Herein, according to our previous experimental results of pre-deposited MoO2 vapor–liquid-solid growth MoS2, we investigate microscopic reaction events, including melting, deoxygenation, and sulfuration by reactive molecular dynamics simulation as follow: Terminal oxygen in pre-melting zone on the cluster surface has high reactivity, leading to deoxygenation occurring during melting. The unsaturated coordination Mo atoms are more accessible to sulfuration. With a long simulation time, MoO2 is transformed into MoO0.27S1.95 in which MoS2-like clusters account for up to 92.45 % after annealing. We hope that our work will provide theoretical support for the controlled CVD synthesis of MoS2 and other transition metal dichalcogenides.

Structural Elucidation of N2O Clusters at Low Temperatures: Exemplary Framework Stabilized by π-Hole-Driven N···O and N···N Pnicogen Bonding Interactions.

N2O is a classic prototype, in which central nitrogen is sufficiently electropositive with a positive potential of 20 kcal mol-1 in magnitude to qualify it as a possible pnicogen. This was applied to a test with N2O clusters using ab initio calculations in association with various molecular topographic tools. The structure of the energetically dominant and N2O dimer was in favor of a perpendicular geometry, where the central nitrogen atom of the N2O submolecule assumed a near 90° angle with the adjacent N═O and/or N═N moiety, which provides the affirmation of central nitrogen as a possible π-hole-driven pnicogen. The terminal nitrogen and oxygen atoms of N2O continue to act as conventional electron donors (Lewis bases) with a negative potential. Overall, predominant π-hole-driven N···O and N···N pnicogen bonding interactions were observed to stabilize N2O clusters. Furthermore, N2O clusters (dimers and trimers) were synthesized at low temperatures in an Ar matrix using molecular beam (effusive and supersonic expansion) experiments. The geometries of these clusters were characterized by probing infrared spectroscopy with corroboration from ab initio computational methods. In addition to our previously investigated nitromethane and nitrobenzene systems, N2O also makes it to a pnicogen bonder's club with the central nitrogen as a π-hole-driven pnicogen.

Crystal Face-Dependent Behavior of Single-Atom Pt: Construct of SA-FLP Dual Active Sites for Efficient NO2 Detection.

The strong potential of platinum single atom(PtSA) in gas sensor technology is primarily attributed to its high atomic economy. Nevertheless, it is imperative to conduct further exploration to understand the impact of PtSAon the active sites. In this study,the evolution of PtSAon (100)CeO2and (111)CeO2 is examined, revealing notable disparities in the position and activity of surface PtSAon different crystal planes.The PtSAin (100)CeO2surfacecan enhancethe stability of Ce3+and construct a frustrated Lewis pair (FLP) to form a double active site by combining the steric hindrance effect of oxygen vacancies, whichincreases the response value from 1.8 to 27 and reducethe response-recovery time from 140-192s to 25-26s toward five ppm NO2at room temperature.Conversely,PtSAtends to bind to terminal oxygen on the surface of (111)CeO2and become an independent reaction site.The response valueofPtSA-(111)CeO2surface onlyincreased from 1.6 to 3.8.This research underscores the correlation between single atoms and crystal plane effects, laying the groundwork for designing and synthesizing ultra-stable and efficient gas sensors.

Boosting efficient removal of organic pollutants by sulfur modified hollow MnFe2O4 nanocage: Synergist of three-dimensional structure and reactive species

Bimetal-based peroxymonosulfate advanced oxidation processes (PMS-AOPs) present an enticing avenue for water treatment. The challenge of interference effects persists in addressing complex wastewater, including the occlusion of active sites by natural organic matters (NOMs) and the quenching of free radicals by competitive constituents. A sulfur-modified hollow MnFe2O4 nanocage (S-MnFe2O4) was introduced through a facile Prussian blue analog pyrolysis. The nanocage exhibited high interference resistance in terms of both active sites and reactive species. The hollow architecture, featuring a smaller pore diameter (2–50 nm) than NOMs (255.7 nm), effectively sequestered extraneous macromolecular interferences. Sulfur doping orchestrated a transformation in the PMS activation pathway via expediting the preferential adsorption of terminal oxygen in PMS and reducing the energy barrier to a non-radical pathway dominance (1O2), contributing to a highly discerning removal of target phenol. The performance for target degradation was sustained in the presence of multiple negative factors, including humic acid, competitive organic, and inorganic, due to the synergy between size exclusion and 1O2. Even in natural waters (e.g. tap, lake, river, etc.) or continuous flow systems, the S-MnFe2O4/PMS combination showcased a remarkable removal effect. Co-regulation strategy unfolds a complete perspective to eliminate targets in the interference of other compounds.

Efficient simultaneous removal of diesel particulate matter and hydrocarbons from diesel exhaust gas at low temperatures over Cu–CeO2/Al2O3 coupling with dielectric barrier discharge plasma

Diesel particulate matter (DPM) and hydrocarbons (HCs) emitted from diesel engines have a negative affect on air quality and human health. Catalysts for oxidative removal of DPM and HCs are currently used universally but their low removal efficiency at low temperatures is a problem. In this study, Cu-doped CeO2 loaded on Al2O3 coupled with plasma was used to enhance low-temperature oxidation of DPM and HCs. Removals of DPM and HCs at 200 °C using the catalyst were as high as 90% with plasma but below 30% without plasma. Operando plasma diffuse reflectance infrared Fourier transform spectroscopy coupled with mass spectrometry was conducted to reveal the functional mechanism of the oxygen species in the DPM oxidation process. It was found that Cu–CeO2 can promote the formation of adsorbed oxygen ( – ) and terminal oxygen (M=O), which can react with DPM to form carbonates that are easily converted to gaseous CO2. Our results provide a practical plasma catalysis technology to obtain simultaneous removals of DPM and HCs at low temperatures.

Selective separation of Fe and Ga from Al-containing HNO3 leach liquor using cupferron: Efficiency, mechanism, and regeneration

Coal gangue and fly ash are typical coal-based solid waste and can be considered as Al and Ga resources. The HNO3 leaching technology was proposed by our team for the economical and efficient co-extraction of Al and Ga, whereas the separation and recovery of metal elements became a challenge due to the variety of co-existing ions in the leach liquor. In this work, a Ga-Fe iso precipitation strategy with cupferron (C6H9N3O2) as a selective precipitant followed by regeneration and circulation of cupferron was proposed. The feasibility of the technique was assessed by thermodynamic analysis. The systematic precipitation experiments showed that 99.05 % of Ga, 99.89 % of Fe, and 4.74 % of Al were transferred into the complex precipitation product at optimal conditions. And the separation factor of Ga vs. Al and Fe vs. Al were as high as 2101.74 and 17713.93, respectively. The precipitation mechanism was revealed via a suite of characterization methods to be the chelation of metal ions and cupferron, with the terminal oxygen (N = O) and hydroxyl oxygen (O–H) as ligating atoms. The regeneration rate of cupferron reached 81.28 % with an ammonium hydroxide (NH3·H2O) concentration of 8 mol/L. And the circulation experiment demonstrated the excellent performance of the regenerated cupferron. This work provides a new insight into the deep removal of Fe and the enrichment of Ga from the acidic solution.

Unraveling pH-Dependent Changes in Adsorption Structure of Uranyl on Alumina (012).

Mitigating uranium transport in groundwater is imperative for ensuring access to clean water across the globe. Here, in situ resonant anomalous X-ray reflectivity is used to investigate the adsorption of uranyl on alumina (012) in acidic aqueous solutions, representing typical UVI concentrations of contaminated water near mining sites. The analyses reveal that UVI adsorbs at two distinct heights of 2.4-3.2 and 5-5.3 Å from the surface terminal oxygens. The former is interpreted as the mixture of inner-sphere and outer-sphere complexes that adsorb closest to the surface. The latter is interpreted as an outer-sphere complex that shares one equatorial H2O with the terminal surface oxygen. With increasing pH, we observe an increasing prevalence of these outer-sphere complexes, indicating the enhanced role of the hydrogen bond that stabilizes adsorbed uranyl species. The presented work provides a molecular-scale understanding of sorption of uranyl on Al-based-oxide surfaces that has implications for environmental chemistry and materials science.

QM/MM Study Into the Mechanism of Oxidative C=C Double Bond Cleavage by Lignostilbene-α,β-Dioxygenase.

The enzymatic biosynthesis of fragrance molecules from lignin fragments is an important reaction in biotechnology for the sustainable production of fine chemicals. In this work we investigated the biosynthesis of vanillin from lignostilbene by a nonheme iron dioxygenase using QM/MM and tested several suggested proposals via either an epoxide or dioxetane intermediate. Binding of dioxygen to the active site of the protein results in the formation of an iron(II)-superoxo species with lignostilbene cation radical. The dioxygenase mechanism starts with electrophilic attack of the terminal oxygen atom of the superoxo group on the central C=C bond of lignostilbene, and the second-coordination sphere effects in the substrate binding pocket guide the reaction towards dioxetane formation. The computed mechanism is rationalized with thermochemical cycles and valence bond schemes that explain the electron transfer processes during the reaction mechanism. Particularly, the polarity of the protein and the local electric field and dipole moments enable a facile electron transfer and an exergonic dioxetane formation pathway.