Dimerization Reaction Research Articles

Evidence of Self-Association and Conformational Change in Nisin Antimicrobial Polypeptide Solutions: A Combined Raman and Ultrasonic Relaxation Spectroscopic and Theoretical Study

The polypeptide Nisin is characterized by antibacterial properties, making it a compound with many applications, mainly in the food industry. As a result, a deeper understanding of its behaviour, especially after its dissolution in water, is of the utmost importance. This could be possible through the study of aqueous solutions of Nisin by combining vibrational and acoustic spectroscopic techniques. The velocity and attenuation of ultrasonic waves propagating in aqueous solutions of the polypeptide Nisin were measured as a function of concentration and temperature. The computational investigation of the molecular docking between Nisin monomeric units revealed the formation of dimeric units. The main chemical changes occurring in Nisin structure in the aqueous environment were tracked using Raman spectroscopy, and special spectral markers were used to establish the underlying structural mechanism. Spectral changes evidenced the presence of the dimerization reaction between Nisin monomeric species. The UV/Vis absorption spectra were dominated by the presence of π → π* transitions in the peptide bonds attributed to secondary structural elements such as α-helix, β-sheets and random coils. The analysis of the acoustic spectra revealed that the processes primarily responsible for the observed chemical relaxations are probably the conformational change between possible conformers of Nisin and its self-aggregation mechanism, namely, the dimerization reaction. The activation enthalpy and the enthalpy difference between the two isomeric forms were estimated to be equal to ΔH1* = 0.354 ± 0.028 kcal/mol and ΔH10 = 3.008 ± 0.367 kcal/mol, respectively. The corresponding thermodynamic parameters of the self-aggregation mechanism were found to be ΔH2* = 0.261 ± 0.004 kcal/mol and ΔH20 = 3.340 ± 0.364 kcal/mol. The effect of frequency on the excess sound absorption of Nisin solutions enabled us to estimate the rate constants of the self-aggregation mechanism and evaluate the isentropic and isothermal volume changes associated with the relaxation processes occurring in this system. The results are discussed in relation to theoretical and experimental findings.

Dimerization reactions with oxidized brominated thiophenes

Cycloadditions are observed after oxidation of brominated thiophenes depending on the position and number of bromines on the ring.

Synthesis of 2,5-dimethylhexene by isobutene dimerization with H2S co-feeding.

This study investigates the effect of hydrogen sulfide (H2S) co-feeding on the synthesis of 2,5-dimethyl-1-hexene, 2,5-dimethyl-2-hexene, and 2,5-dimethylhexane (2,5-DMHs), useful compounds, using the dimerization of isobutene under mild pressure conditions. The dimerization of isobutene did not proceed in the absence of H2S, whereas the desired products of 2,5-DMHs were produced under H2S co-feeding conditions. The effect of reactor size on the dimerization reaction was then examined, and the optimal reactor was discussed. To enhance the yield of 2,5-DMHs, we changed the reaction conditions of the temperature, molar ratio of isobutene to H2S (iso-C4[double bond, length as m-dash]/H2S) in the feed gas, and the total feed pressure. The optimum reaction condition was at 375 °C and 2/1 of iso-C4[double bond, length as m-dash]/H2S. The product of 2,5-DMHs monotonously increased by an increment of total pressure from 1.0 to 3.0 atm with a fixed iso-C4[double bond, length as m-dash]/H2S ratio at 2/1.

What do we know about bifunctional cage-like T8 silsesquioxanes? Theory versus lab routine.

In this article, we explore theoretical validations of experimental findings pertaining to the classical corner-capping reactions of a commercially available heptaisobutyltrisilanol cage to mono-substituted phenylhepta(isobutyl)-POSS cages. Additionally, the process of opening a fully condensed cage is tracked to assess the possibility of isolating and separating the resulting isomers. The corner-capping reactions of potential silanotriols, both as monomers and dimers, and the impact of these structural motifs on their closing to bifunctional POSS cages are also investigated. Our studies highlight that analyzing experimental results alone, without incorporating complex theoretical investigations, does not offer a clear understanding of the reactions involving multiple simultaneously reacting substrates, which may also undergo further transformations, potentially complicating the conventional pathways of classic corner-opening/capping reactions.

Hydrolysis and condensation of monobutyltin chloride: reaction process analysis with DFT.

As the initial process of preparing transparent conductive oxide materials from monobutyltin chloride (MBTC) to tin oxide, the hydrolysis and condensation of MBTC to form a dimer Sn2 play a critical role. However, the specific mechanism of this process is still unclear. Here we develop a step-by-step searching method based on density functional theory calculation and empirical chemical criteria to determine possible reaction pathways and reveal the most likely reaction mechanism. The wave function analyses of various intermediate species provide more insights into the changes of atomic charge population, chemical bond strength, and coordination situation of central tin in the reaction process. Further investigation on the ring-containing Sn2 reveals the existence of unique three-center four-electron (3c-4e) interactions to stabilize the four-membered Sn2O2 ring structure, which serves as the true driving force for dimerization reaction. These results provide a more detailed understanding of the hydrolysis and condensation process of MBTC and would be helpful for the future optimization of the preparation process of tin oxide films.

One-Pot Solvent-Involved Synthesis of 5-O-Substituted 5H-Chromeno[2,3-b]pyridines.

Chromeno[2,3-b]pyridines are substances demanded in medicinal and material chemistry. PASE (pot, atom, and step economy) and in particular one-pot approaches are key green chemistry techniques that are applied for the synthesis of heterocyclic compounds. In this case, the PASE approach was extended with 'component economy', as solvent was used also as reactant (solvent-involved reaction). This approach was adopted for the one-pot synthesis of previously unknown O-substituted 5-alkoxy-5H-chromeno[2,3-b]pyridines via two-step transformation, namely the reaction of salicylaldehydes and malononitrile dimer, with the subsequent addition of alcohol. The mechanistic studies revealed the possibility of concurrent reaction. The studies aided in optimizing the reaction conditions for the best yields (77-93%). Thus, the one-pot reaction proceeds efficient and quickly, and the work-up procedure (only simple filtering) is very convenient. The structure of synthesized chromeno[2,3-b]pyridines was confirmed by 2D NMR spectroscopy.

Unveiling the Mechanisms of Hydrolytic Ring-Opening Polymerization of Caprolactam and Amino-Assisted Ring Opening of Cyclic Dimers: A DFT Study

At the M06-2X/6-31+G(d,p) computational level, the reactions hydrolytic ring-opening polymerization of caprolactam to form cyclodimers and amino-assisted ring opening of the cyclic caprolactam dimers were studied in detail. For the above-mentioned reactions, free energy barriers were calculated and the possible reaction pathways were discussed. Our calculated results showed that the hydrolytic ring-opening polymerization reaction includes four stages. The first three stages contain two steps: nucleophilic addition and proton transfer. After finishing stage 3, a cyclic caprolactam dimer is formed by the rate-determining step─backbiting of the amino end groups. For the amino-assisted ring-opening reaction of the cyclic caprolactam dimer, the rate-determining step is stage 3 due to its highest free energy barrier. This stage can take place via a concerted or stepwise pathway. Under the experimental conditions, hydrolytic ring-opening polymerization and amino-assisted ring opening of the cyclic caprolactam dimer can be easily carried out. This study provides a theoretical basis for the effective removal of caprolactam oligomers.

Organic molecule-modified copper catalyst enables efficient electrochemical reduction of CO2-to-methane

Renewable electricity powered CO2 electroreduction to methane is promising for producing highly specific combustion fuels. However, the reduction process is severely limited by poor selectivity and chemical activity at high electrolysis reaction rates. Herein, we produce tandem electrocatalysts by combining organic small molecules (perylene-3,4,9,10-tetracarboxylic dianhydride and pentacene) with copper catalysts supported on microporous polytetra fluoroethylene with enhanced *CO protonation and weakened hydrogen evolution. Experimental results show that the tandem catalysts effectively suppress the two-electron reduction process associated with hydrogen (H2) and carbon monoxide (CO) production while inhibiting the multielectron conversion process associated with ethylene production (C2H4). As a result, the tandem catalysts enable an efficient eight-electron transfer for electrochemical reduction of CO2 and demonstrate a ∼ 50 % (±2%) Faradaic efficiency for CO2-to-methane conversion at a partial current density of 220 mA cm−2 in 1 M KHCO3 solution. Notably, the selectively of CO2-to-CH4 is 2.7 times higher than that of a copper catalyst supported on microporous polytetra fluoroethylene which is known to produce C2H4 with ∼ 50 % Faradaic efficiency. This work paves the way for tuning *CO dimerization and hydrogen evolution reaction to maximize CO2 methanation by using abundant organic molecular materials.

Deactivation Mechanism and Regeneration of the CuCl/Activated Carbon Catalyst for Gas-Solid Acetylene Dimerization.

Acetylene dimerization is necessary to the coal chemical industry for producing monovinylacetylene, while the deactivation mechanism and regeneration of catalysts have not been studied in detail, which is crucial to the design of high-efficiency catalysts for acetylene dimerization. Herein, the deactivation mechanism and regeneration methods of CuCl/activated carbon catalysts in gas-solid acetylene dimerization were studied in detail. The catalysts with different reaction times were analyzed by temperature-programmed desorption of ammonia (NH3-TPD), Fourier transform infrared (FT-IR), thermogravimetry (TG), pyridine-FTIR, and X-ray photoelectron spectroscopy (XPS) analyses. NH3-TPD results demonstrated that as the time went on, the strong acid in the samples was enhanced, while the weak acid was weakened. Similarly, pyridine-FTIR results indicated that both Brönsted and Lewis acids in the samples were decreased. TG and XPS results showed that the reasons for deactivation for acetylene dimerization in the gas-solid reaction were significantly affected by coke deposition and the change of Cu valence. The more the content of Cu+, the higher the acetylene conversion rate, implying that Cu+ may be the active center of the acetylene dimerization reaction. Thus, removing carbon deposition through calcining and increasing the content of Cu+ was an effective way of regenerating the catalyst. This work strengthened the understanding of the deactivation behavior and provides a practicable regeneration method for the catalyst in gas-solid acetylene dimerization.

Photoionization of poly-nitrosobenzenes

The electronic structures and valence ionizations of seven nitrosobenzene derivatives have been studied using high-level ab initio methods: electron-propagator theory (EPT) and complete active space self-consistent field method/N-electron valence state perturbation theory (CASSCF/NEVPT2). We describe the valence electronic structure as dependent on the number and position of nitroso substituents. We also discuss the electronic structures of azodioxide geometric isomers and their monomers in connection with photodissociation and dimerization reactions.

Laccase catalyses phytophenol furanocyclic dimerization: Substrate specificity and diastereoselective process

Lacasse can transform phytophenol into furanocyclic dimers, including coumaran linkage dimer and 3,7-dioxabicyclo[3.3.0]octane linkage dimer. However, the substrate specificity and diastereoselective process remain unknown until now. The study therefore selected a series of monomeric phytophenol isomers or analogues as substrates to interact with laccase from Trametes versicolor (Lac TV). The products were rapidly identified by ultra-performance liquid chromatography coupled with high-resolution mass-spectrometry. On this basis, 9 dimerization reactions were established for a systematical structure–function relationship analysis. Through analysis, it can be concluded that, phytophenol furanocyclic dimerization requires a specific substrate structure, i.e., a C=C bond sitting at the -OH para-position and also conjugating with the phenolic core. The coumaran linkage dimerization presents “7,8 diastereoselectivity” character; while the 3,7-dioxabicyclo[3.3.0]octane linkage dimerization shows not only “7,8 diastereoselectivity” character but also “cis- 8H,8′H” character. The former character is attributed to the intramolecular nucleophilic addition which always prevents the Re-attack. The latter character however is caused by a substantial energy gap between normal envelop conformation and twist envelop conformation. All these findings can help to efficiently utilize Lac TV tool for phytophenol biosynthesis in chemistry fields.

On the road to stable, isolable [4]cumulenes: Reactivity and cyclization reactions

AbstractThe design and synthesis of persistent tetraaryl[4]cumulenes are reported. Derivatives with phenyl endgroups ([4]Ph and [4]Ph/Ar*) are susceptible to reactions during synthesis and/or purification that complicate isolation of the desired product, particularly intermolecular dimerization reactions. Incorporation of (3,5‐di‐tert‐butyl)phenyl endgroups (Ar*) provides increased stability, culminating in the stable and isolable [4]cumulene [4]Ar*. Cumulene [4]Ar* remains, however, susceptible to dimerization under pressing conditions, and [4]Ar* also readily reacts with electrophiles under mild conditions that lead to intramolecular cyclization. Given the structural complexity of the side products in these studies (compounds 3, 10, 14, 17, 18), X‐ray crystallography plays a vital role in their identification.

An Ab Initio Investigation on Relevant Oligomerization Reactions of Toluene Diisocyanate (TDI).

2,4- and 2,6-isomers of toluene diisocyanates (2,4-TDI and 2,6-TDI) are important raw materials in the polyurethane industry. These reactive compounds associate even under ambient conditions to form oligomers, changing the physicochemical properties of the raw material. Kinetically and thermodynamically relevant dimerization reactions were selected based on G3MP2B3 calculations from all possible dimers of phenyl isocyanate using these isocyanates as proxies. As it turned out, only the formation of the diazetidine-2,4-dione ring (11-dimer, uretdione) resulted in a species having an exothermic enthalpy of formation (−30.4 kJ/mol at 298.15 K). The oxazetidin-2-one ring product (1-2-dimer) had a slightly endothermic standard enthalpy of formation (37.2 kJ/mol at 298.15 K). The mechanism of the relevant cyclodimerization reactions was investigated further for 2,4-TDI and 2,6-TDI species using G3MP2B3 and SMD solvent model for diazetidine as well as oxazetidin-2-one ring formation. The formation of the uretdione ring structures, from the 2,4-TDI dimer with both NCO groups in the meta position for each phenyl ring and one methyl group in the para and one in the meta position, had the lowest-lying transition state (Δ#E0 = 94.4 kJ/mol) in the gas phase. The one- and two-step mechanisms of the TDI cyclotrimerization were also studied based on the quasi-G3MP2B3 (qG3MP2B3) computational protocol. The one-step mechanism had an activation barrier as high as 149.0 kJ/mol, while the relative energies in the two-step mechanism were significantly lower for both transition states in the gas phase (94.7 and 60.5 kJ/mol) and in ODCB (87.0 and 54.0 kJ/mol).

OXIDATIVE DIMERIZATION OF METHANE: KINETICS OF THE REACTION

In this article, the textural properties of catalysts and the process of catalytic dimerization of methane using catalysts based on salts of alkali and alkaline-earth metals containing molybdenum and manganese oxides were studied. It was found that the catalytic conversion of methane takes place on the surface of a catalyst that adsorbs oxygen to form an active complex [ZOCH3]. It has been shown that the catalytic conversion of methane can be carried out simultaneously with the formation of ethane and ethylene. At the same time, the percentage of ethylene in the free volume of the reactor increases compared to ethane, which helps the reaction of ethylene formation without oxygen and catalyst. Phenomenological equations for the kinetics of the catalytic reaction of methane dimerization are formulated and rate constants and Arrhenius parameters are calculated.

Features of heterogeneous catalytic transformations of strained carbocyclic compounds of the norbornene series

Objectives. Catalytic processes involving norbornene (NBN) and norbornadiene (NBD) offer exceptional opportunities for the synthesis of a wide range of hard-to-reach polycyclic hydrocarbons. The problems of selectivity and manufacturability of these reactions are fundamentally important for their practical implementation. The aim of this review is to summarize the latest advances in the field of designing heterogeneous catalysts for the preparation and transformation of promising NBN- and NBD-derivatives with the maintenance of a strained carbocyclic framework in isomerization and dimerization reactions of these compounds.Results. Various strategies for the selection of catalysts and prospects for the development of heterogeneous catalysis for syntheses based on NBN and NBD derivatives were considered. The possibility of selective cyclic dimerization and isomerization of NBN and NBD was shown. The factors that affect the direction of the reactions and make it possible to maintain the strained norbornane structure were discussed.Conclusions. An analysis of the current state of this problem showed that at present, the technological parameters of the conversion of NBD and NBN derivatives with the participation of heterogeneous catalysts are significantly inferior to homogeneous systems. In order to improve the productivity of these processes and design catalyst regeneration, further investigations are required. However, some progress in these areas has already been made. In a number of processes, it is possible not only to maintain the strained carbocyclic framework, but also to establish ways to control regio- and stereo-selectivity. In some cases, the use of heterogeneous catalysts allows the process to be direct into a completely new path, which has no analogues for homogeneous systems.

The influence of phenyl substitutions on the dimerization of highly substituted iron(III) porphyrin hydroxo complexes

The modeling of iron porphyrins is essential in the study of heme proteins. These complexes readily undergo autooxidation and dimerization; however, this dimerization process can be inhibited by introducing phenyl groups. Herein, the dimerization of highly substituted iron(III) porphyrins is investigated. The stability of water-insoluble iron(III) hydroxo complexes was monitored via thin-layer chromatography and was found to increase with the number of phenyl groups. Dodecaphenyl-substituted complexes formed hydroxo complexes, tetraphenyl and hexaphenyl complexes formed [Formula: see text]-oxo dimers, and octaphenyl and decaphenyl complexes existed as hydroxo complex-[Formula: see text]-oxo dimer mixtures. Crystals of the dodecaphenyl-substituted iron(III) hydroxo complexes were successfully isolated. The decaphenyl-substituted complex was unique in that the respective crystals of both the hydroxo complexes and [Formula: see text]-oxo dimer could be isolated from the chloride complexes. Furthermore, the synthesis and proton equilibria of the new porphyrin iron(III) water-soluble complexes were investigated. Under neutral to basic conditions, at equilibrium, dodecaphenyl-substituted complexes exclusively formed hydroxo complexes, whereas octaphenyl, hexaphenyl, and tetraphenyl complexes exclusively formed [Formula: see text]-oxo dimers, and decaphenyl complexes existed as hydroxo complex-[Formula: see text]-oxo dimer mixtures. Additionally, the order of the dimerization reaction was examined using the initial-rate and half-life methods, which confirmed that the dimerization is a second-order reaction that follows a mechanism wherein two hydroxo complexes form a dihydroxo-bridged intermediate. This result was supported by the dimerization of water-soluble iron(III) hydroxo complexes. The new hydroxo complexes with non-planar porphyrin cores synthesized in this study are expected to have a significant impact on heme protein reaction modeling.

Organo-rhodium and -iridium complexes containing the thiosulfate (S2O32−) ligand

Reaction of the complexes [M2(μ-Cl)2(cod)2] (M = Rh, Ir) with sodium thiosulfate pentahydrate gives the dinuclear thiosulfate complexes [M2(μ-S2O3)2(cod)2]2−, which were isolated as their Ph3PCH2Ph+ salts. An X-ray structure determination on the Rh complex confirms the dimeric nature of the complex, with the two Rh(I) centres bridged by the terminal sulfurs of the thiosulfate ligands. Using ESI mass spectrometry, the dimer [M2(μ-S2O3)2(cod)2]2− and monomer [M(S2O3)(cod)]− are both observed, depending on the metal and capillary exit voltage. Reaction of sodium thiosulfate pentahydrate with the complexes trans-[RhCl(CX)(PPh3)2] (X = O or S) gave the anionic complexes Na[Rh(S2O3)(CX)(PPh3)], which were characterised by 31P{1H} NMR and IR spectroscopies, ESI MS, and in the case of the thiocarbonyl complex, by elemental analysis. Pentamethylcyclopentadienyl (Cp*) rhodium(III) and iridium(III) thiosulfate complexes [(Cp*M)2(μ -S2O3)3]2− were also synthesised by reaction of the chloride-bridged dimers [Cp*MCl2]2 with sodium thiosulfate, and isolated as their Ph3PCH2Ph+ salts. The complexes gave the expected parent dianions in ESI mass spectra, which underwent fragmentation of a thiosulfate ligand at higher capillary exit voltages to give ions [(Cp*M)2(S2O3)2S + H]−.

Phytophenol Dimerization Reaction: From Basic Rules to Diastereoselectivity and Beyond.

Phytophenol dimerization, which is a radical-mediated coupling reaction, plays a critical role in many fields, including lignin biosynthesis. To understand the reaction, 2,2-diphenyl-1-picrylhydrazyl radical was used to initiate a series of phytophenol dimerization reactions in methanol. The products were identified using ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UHPLC-ESI-Q-TOF-MS/MS) analysis in situ. The identified products mainly included biphenols, magnolol, honokiol, gingerol 6,6′-dimers, 3,6-dimethoxylcatechol β,β′ dimer, euphorbetin, bis-eugenol, dehydrodiisoeugenol, trans-ε-viniferin, (+) pinoresinol, and (−) pinoresinol. Structure–function relationship analysis allowed four basic rules to be defined: meta-excluded, C–C bonding domination, ortho-diOH co-activation, and exocyclic C=C involvement. The exocyclic C=C involvement, however, required conjugation with the phenolic core and the para-site of the -OH group, to yield a furan-fused dimer with two chiral centers. Computational chemistry indicated that the entire process was completed via a radical coupling reaction and an intramolecular conjugate addition reaction. Similar results were also found for the horseradish peroxidase (HRP)-catalyzed coniferyl alcohol dimerization, which produced (+) and (−) pinoresinols (but no (−) epipinoresinol), suggesting that the HRP-catalyzed process was essentially an exocyclic C=C-involved phytophenol dimerization reaction. The reaction was highly diastereoselective. This was attributed to the intramolecular reaction, which prohibited Re-attack. The four basic rules and diastereoselectivity can explain and even predict the main products in various chemical and biological events, especially oxidase-catalyzed lignin cyclization.

Integral equation-based simulation of transient experiments for an [formula omitted] mechanism: Beyond the steady state simplification

Transient electrochemical experiments are usually described theoretically by systems of reaction–diffusion partial differential equations. Converting them to integral equations is a classical and valuable modelling approach. Unfortunately, if any reaction–diffusion equation contains nonlinear reaction rate terms, up to now such a conversion has only been possible by assuming a steady state for the equation. Consequently, only sufficiently fast homogeneous reactions could be handled. In this work a novel integral equation-based modelling approach is described. The steady state assumption is replaced by a two-term singular perturbation expansion of the concentration–flux relationship, recently published by the authors. The expansion is valid for homogeneous reactions of (integer) order m≥1, occurring at planar electrodes. An example simulation of cyclic voltammetry for an EC2 reaction mechanism involving a second order dimerization reaction is performed. It is found that in this way the voltammograms can be satisfactorily simulated for homogeneous reaction rate constants smaller by about one order of magnitude than was previously possible.

Design and synthesis of waterborne light-responsive cellulose nanocrystal/fluorinated polyacrylate films toward oil/water repellent and self-healing properties

The development of environmentally friendly waterborne polymeric materials is becoming critical today. In this study, a kind of self-healing cellulose nanocrystal/fluorinated polyacrylate based on the reversible photo-induced dimerization of coumarin groups was prepared via RAFT-mediated Pickering emulsion polymerization using cellulose nanocrystal (CNC) particles modified with light-responsive amphiphilic copolymers as Pickering stabilizers. Emulsion polymerization and latex film properties were significantly varied in dependence on the amount of 7-(2-methacryloyloxyethoxy)-4-methylcoumarin (CMA) monomer. Hydrophobic, oleophobic and mechanical properties of the latex film were dynamic in response to UV light. The maximum water contact angle and CH2I2 contact angle increased by 6.1° and 2.3°on the irradiation of UV light at 365 nm, respectively. The tensile strength of latex film increased initially with increasing amount of CMA increased, from 0 up to 4 wt%, and then decreased. The maximum tensile value increased from 3.0 MPa to 6.4 MPa after the dimerization reaction activated by 365 nm UV light. Results from atomic force microscope (AFM) and energy dispersive X-ray spectrometer (EDX) revealed that latex film possessed a roughness structure and the fluorine had a remarkable enrichment on the film-air interface. Moreover, latex films showed an excellent surface scratch self-healing ability owing to the reversible dimerization reaction of the coumarin groups, and the reversible dimerization reaction of coumarin groups within cellulose nanocrystal/fluorinated polyacrylate polymer was extensively investigated via FT-IR spectrometer and differential scanning calorimetry (DSC). The fruitful outcomes indicated that light-responsive cellulose nanocrystal/fluorinated polyacrylate exhibited a greatly potential application as a smart material.