Oxidative Splitting of a Pyrimidine Cyclobutane Dimer:A Pulse Radiolysis Study

The oxidative splitting process of cis-syn 1,3-dimethyluracil cyclobutane dimer(DMUD) in aqueous solution was investigated using pulse radiolysis technique.The results indicated that DMUD can be splitted into 1,3-dimethyluracil(DMU) by OH radicals(OH) and Br2 radical anions(Br2^-),but not by azide radicals(N3^).The oxidative mechanisms that an H-abstracted from DMUD for OH oxidative splitting and an electron transfer from DMUD to Br2-,were suggested.Related kinetic parameters were determined.     

On the Formation of Cyclobutane Pyrimidine Dimers in UV‐irradiated DNA: Why are Thymines More Reactive?¶

The reaction pathways for thermal and photochemical formation of cyclobutane pyrimidine dimers in DNA are explored using density functional theory techniques. Although it is found that the thermal [2 + 2] cycloadditions of thymine + thymine (T + T --> T x T), cytosine + cytosine (C + C --> C x C) and cytosine + thymine (C + T --> C x T) all are similarly unfavorable in terms of energy barriers and reaction energies, the excited-state energy curves associated with the corresponding photochemical cycloadditions display differences that--in line with experimental findings--unanimously point to the predominance of T x T in UV-irradiated DNA. It is shown that the photocycloaddition of thymines is facilitated by the fact that the S1 state of the corresponding reactant complex lies comparatively high in energy. Moreover, at a nuclear configuration coinciding with the ground-state transition structure, the excited-state energy curve displays an absolute minimum only for the T + T system. Finally, the T + T system is also associated with the most favorable excited-state energy barriers and has the smallest S2-S0 energy gap at the ground-state transition structure.     

Density Functional Studies of the C?F Bond Activation of CF3 Radical by Bare Co+

The C? F bond activation mechanism of CF₃ radical by bare Co⁺ has been studied by density functional theory. Three local minima and two first‐order saddle points were located for the potential energy surface (PES) of [Co, C, F₃]⁺. The activation barrier involving C? F bond activation was calculated to be only 14.73 kJ/mol, while the largest barrier of 149.29 kJ/mol on the FES involves Co? C bond rupture. The bonding mechanism between Co⁺, C and F atoms were discussed based on Mulliken population. The relevant bond dissociation energy and thermochemistry data were calculated with the limited experimental values, and the results are in good agreement with the experimental findings.     

Self-Assembled Anion-Binding Cryptand for the Selective Liquid–Liquid Extraction of Phosphate Anions

The ligands L¹ and L² form trinuclear self-assembled complexes with Cu²⁺ (i.e. [( L¹ )₂Cu₃]⁶⁺ or [( L² )₂Cu₃]⁶⁺) both of which act as a host to a variety of anions. Inclusion of long aliphatic chains on these ligands allows the assemblies to extract anions from aqueous media into organic solvents. Phosphate can be removed from water efficiently and highly selectively, even in the presence of other anions.     

Molecular Modeling Studies of the β-Cyclodextrin in Monomer and Dimer Form as Hosts for the Complexation of Cholesterol

Molecular modeling studies on the inclusion complex formation ofcholesterol with -cyclodextrin in monomer anddimer form were performed. Monte Carlo docking simulations, moleculardynamics, and non-equilibrium molecular dynamicssimulations were applied to assess the energeticdriving force for the formation of these inclusioncomplexes. Both Monte Carlo docking and moleculardynamics simulations supported the more favorableinclusion complex formation of -cyclodextrindimer. Non-equilibrium molecular dynamics simulationsprovided a direct assessment of the binding force for theinclusion complexes, of which that of dimer form is much greater.     

Synthesis of 1-(2''''-Phenyl)cyclopropyl-2,3-epoxypropan-1-ol as the Radical Precursor for the Kinetic Study of Oxiranylcarbinyl Radical

The oxiranylcarbinyl radical can undergo rapid rearrangement to generate an allyloxy radical (1 2)1. This process has been found application in organic synthesis2. For example, Rawal developed a novel method leading to cis-fused bicyclic compounds based on tandem reactions of oxiranylcarbinyl radical ring opening - intramolecualr H abstraction - radical addition2f. OOk1k-112 On the other hand, the oxiranylcarbinyl radical rearrangement is so rapid that this radical species has never be…     

Radical Cations of Phenyl‐Substituted Aziridines: What Are the Conditions for Ring Opening?

Radical cations were generated from different phenyl-substituted aziridines by pulse radiolysis in aqueous solution containing TlOH.+, N3. or SO4.- as oxidants or in n-butyl chloride, by 60Co gamma radiolysis in Freon matrices at 77 K, and in some cases by flash photolysis in aqueous solution. Depending on the substitution pattern of the aziridines, two different types of radical cations are formed: if the N atom carries a phenyl ring, the aziridine appears to retain its structure after oxidation and the resulting radical cation shows an intense band at 440-480 nm, similar to that of the radical cation of dimethylaniline. Conversely, if the N atom carries an alkyl substituent while a phenyl ring is attached to a C-atom of the aziridine, oxidation results in spontaneous ring opening to yield azomethine ylide radical cations which have broad absorptions in the 500-800 nm range. In aqueous solution the two types of radical cations are quenched by O2 with different rates, whereas in n-butyl chloride, the ring-closed aziridine radical cations are not quenchable by O2. The results of quantum chemical calculations confirm the assignment of these species and allow to rationalize the different effects that phenyl rings have if they are attached in different positions of aziridines. In the pulse radiolysis experiments in aqueous solution, the primary oxidants can also be observed, whereas in n-butyl chloride a transient at 325 nm remains unidentified. In the laser flash experiments, both types of radical cations were also observed.     

Electrochemical Activation of CO2 for the Synthesis of Ethyl Carbanilate under Mild Conditions

A novel electrochemical procedure for the synthesis of ethyl carbanilate from aniline and carbon dioxide was developed via the selective cathodic reduction of carbon dioxide in CO2-saturated DMF solution containing 0.1 mol/L TEABr at room temperature,followed by the addition of EtI as an alkylating agent.The synthesis was carried out under mild[p(CO2)=1.0×105 Pa,t=20 ℃] and safe conditions.Influences of the nature of the electrodes,the current densities,the passed charges during electrolysis,temperatures,and supporting electrolytes on the yield of ethyl carbanilate were studied to optimize the electrolytic conditions.The selectivity of ethyl carbanilate is 100%.     

Power‐to‐Syngas: An Enabling Technology for the Transition of the Energy System?

Power‐to‐X concepts promise a reduction of greenhouse gas emissions simultaneously guaranteeing a safe energy supply even at high share of renewable power generation, thus becoming a cornerstone of a sustainable energy system. Power‐to‐syngas, that is, the electrochemical conversion of steam and carbon dioxide with the use of renewably generated electricity to syngas for the production of synfuels and high‐value chemicals, offers an efficient technology to couple different energy‐intense sectors, such as “traffic and transportation” and “chemical industry”. Syngas produced by co‐electrolysis can thus be regarded as a key‐enabling step for a transition of the energy system, which offers additionally features of CO₂‐valorization and closed carbon cycles. Here, we discuss advantages and current limitations of low‐ and high‐temperature co‐electrolysis. Advances in both fundamental understanding of the basic reaction schemes and stable high‐performance materials are essential to further promote co‐electrolysis.     

Recent Progress in the Transition-Metal-Catalyzed Activation of Si−Si Bonds To Form C−Si Bonds

Disilanes possessing a Si−Si bond are unique element–element species. Transition-metal-catalyzed activation of the Si−Si bond allows many useful transformations that generate diverse organosilanes. This Minireview highlights impressive developments in this field over the past decade, with an emphasis on the formation of vinyl-, aryl-, and acylsilanes by C(sp²)−Si bond formation as well as the formation of allyl- and alkylsilanes by C(sp³)−Si bond formation.     

To What Extent is FAIMS Beneficial in the Analysis of Proteins?

High field asymmetric waveform ion mobility spectrometry (FAIMS), also known as differential ion mobility spectrometry, is emerging as a tool for biomolecular analysis. In this article, the benefits and limitations of FAIMS for protein analysis are discussed. The principles and mechanisms of FAIMS separation of ions are described, and the differences between FAIMS and conventional ion mobility spectrometry are detailed. Protein analysis is considered from both the top-down (intact proteins) and the bottom-up (proteolytic peptides) perspective. The roles of FAIMS in the analysis of complex mixtures of multiple intact proteins and in the analysis of multiple conformers of a single protein are assessed. Similarly, the application of FAIMS in proteomics and targeted analysis of peptides are considered.

Open image in new window

Graphical Abstract ?

     

Silver‐Catalyzed Oxidative Activation of Benzylic C?H Bonds for the Synthesis of Difluoromethylated Arenes

A mild and catalytic method to form difluoromethylated arenes through the activation of benzylic C H bonds has been developed. Utilizing AgNO₃ as the catalyst, various arenes with diverse functional groups undergo activation/fluorination of benzylic C H bonds with commercially available Selectfluor reagent as a source of fluorine in aqueous solution. The reaction is operationally simple and amenable to gram‐scale synthesis.     

Calibration of the X‐Ray Photoelectron Spectroscopy Binding Energy Scale for the Characterization of Heterogeneous Catalysts: Is Everything Really under Control?

Investigations of X‐ray photoelectron spectra from solid samples need corrections for the surface charging effect. For powder samples such as heterogeneous catalysts and their supports, the C ? (C,H) component of the C 1s peak is often used as an internal standard for the calibration of the binding energy scale. Although this method is widely recognized as suitable for the study of heterogeneous catalysts, we show that a significant calibration bias can be encountered upon comparing samples with different bulk composition. In this paper, a series of SiO₂–Al₂O₃ supports and Pd/SiO₂–Al₂O₃ catalysts with various Si/Al ratios were studied. The spectra issued from these samples were processed with the classical calibration method on the basis of the carbon peak. Important discrepancies in the relative position of the photoelectron peaks were noticed. After systematically discarding instrument‐related issues, a true chemical influence of the bulk matrix on the analyzed surface species was evidenced. The extent of this chemical effect was dependent on the composition of the sample and more precisely on its ionicity. Two possible mechanisms for this chemical effect were proposed and discussed. Finally, an alternative calibration method was offered.     

Do complex matrices modify the sorptive properties of polydimethylsiloxane (PDMS) for non-polar organic chemicals?

The partitioning of non-polar analytes into the silicone polydimethylsiloxane (PDMS) is the basis for many analytical approaches such as solid phase microextraction (SPME), stir bar sorptive extraction (SBSE) and environmental passive sampling. Recently, the methods have been applied to increasingly complex sample matrices. The present work investigated the possible effect of complex matrices on the sorptive properties of PDMS. First, SPME fibers with a 30 μm PDMS coating were immersed in 15 different matrices, including sediment, suspensions of soil and humic substances, mayonnaise, meat, fish, olive oil and fish oil. Second, the surface of the fibers was wiped clean, and together with matrix-free control fibers, they were exposed via headspace to 7 non-polar halogenated organic chemicals in spiked olive oil. The fibers were then solvent-extracted, analyzed, and the ratios of the mean concentrations in the matrix-immersed fibers to the control fibers were determined for all matrices. These ratios ranged from 92% to 112% for the four analytes with the highest analytical precision (i.e. polychlorinated biphenyls (PCBs) 3, 28, 52 and brominated diphenyl ether (BDE) 3), and they ranged from 74% to 133% for the other three compounds (i.e. PCBs 101, 105 and γ-hexachlorocyclohexane (HCH)). We conclude that, for non-polar, hydrophobic chemicals, the sorptive properties of the PDMS were not modified by the diverse investigated media and consequently that PDMS is suited for sampling of these analytes even in highly complex matrices.     

Rhodium‐Catalyzed Tandem Cyclization/Si?C Activation Reaction for the Synthesis of Siloles

Siloles represent an important emerging class of photoluminescent materials. Reported herein is a new synthetic strategy involving a tandem cyclization/Si C activation reaction featuring high efficiency, wide substrate scope, and practical utility. This method enabled the first synthesis of benzofuran siloles as well as rapid access to conjugated siloles. During the course of the study we also uncovered an unusual yet general Si C(sp²) activation in the presence of π acids.     

To what extent is the magnitude of the Cole-Cole α of the β-dielectric dispersion of cell suspensions explicable in terms of the cell size distribution?

The Cole-Cole α is a number that is often used to describe the divergence of a measured dielectric dispersion from the ideal dispersion exhibited by a Debye type of dielectric relaxation, and is widely assumed to be related to a distribution of the relaxation times in the system involved. The magnitude and relaxation time of the β-dielectric dispersion due to the charging of the plasma membrane capacitance of cell suspensions depend, inter alia, on the cell radius. An investigation was carried out to determine whether there might therefore be a relationship between the Cole-Cole α of the β-dispersion of yeast cell suspensions and the distribution of cell sizes. Changes in the Cole-Cole α during the batch culture of baker's yeast were recorded, showing an increase in the Cole-Cole α during the exponential phase (more than 0.3) relative to those of the lag phase (about 0.28) and the stationary phase (about 0.2). Although the cell size distribution, measured by flow cytometry, also showed an increase in breadth during the exponential phase, this was not strictly related to the changes in the Cole-Cole α observed. Further, the Cole-Cole α calculated from the measured cell size distribution was significantly smaller than that obtained experimentally. Simulations in which the internal conductivity or membrane capacitance per unit area of individual cells were allowed to vary substantially did not account for the “excessive” Cole-Cole α. Thus the magnitude of the Cole-Cole α of the β-dispersion of yeast cells cannot be ascribed simply to the charging of a static membrane capacitance in cells of differing sizes and/or internal conductivities.