利用荧光屏探测器质谱方法研究邻位二氯苯的电子贴附解离

我们在自制的负离子速度成像装置上发展了负离子质谱技术。测量获得了电子贴附解离邻位二氯苯产物Cl^-;的0.2至8 eV效率谱,并且在两个峰位1.2和6.0 eV 测量了其切片速度影像。     

State Selectivity and Dynamics in Dissociative Electron Attachment to CF3I Revealed through Velocity Slice Imaging

In light of its substantially more environmentally friendly nature, CF₃I is currently being considered as a replacement for the highly potent global‐warming gas CF₄, which is used extensively in plasma processing. In this context, we have studied the electron‐driven dissociation of CF₃I to form CF₃^? and I, and we compare this process to the corresponding photolysis channel. By using the velocity slice imaging (VSI) technique we can visualize the complete dynamics of this process and show that electron‐driven dissociation proceeds from the same initial parent state as the corresponding photolysis process. However, in contrast to photolysis, which leads nearly exclusively to the ²P_1/2 excited state of iodine, electron‐induced dissociation leads predominantly to the ²P_3/2 ground state. We believe that the changed spin state of the negative ion allows an adiabatic dissociation through a conical intersection, whereas this path is efficiently repressed by a required spin flip in the photolysis process.     

Formation of Temporary Negative Ions and Their Subsequent Fragmentation upon Electron Attachment to CoQ0 and CoQ0H2

Ubiquinone molecules have a high biological relevance due to their action as electron carriers in the mitochondrial electron transport chain. Here, we studied the dissociative interaction of free electrons with CoQ₀, the smallest ubiquinone derivative with no isoprenyl units, and its fully reduced form, 2,3-dimethoxy-5-methylhydroquinone (CoQ₀H₂), an ubiquinol derivative. The anionic products produced upon dissociative electron attachment (DEA) were detected by quadrupole mass spectrometry and studied theoretically through quantum chemical and electron scattering calculations. Despite the structural similarity of the two studied molecules, remarkably only a few DEA reactions are present for both compounds, such as abstraction of a neutral hydrogen atom or the release of a negatively charged methyl group. While the loss of a neutral methyl group represents the most abundant reaction observed in DEA to CoQ₀, this pathway is not observed for CoQ₀H₂. Instead, the loss of a neutral OH radical from the CoQ₀H₂ temporary negative ion is observed as the most abundant reaction channel. Overall, this study gives insights into electron attachment properties of simple derivatives of more complex molecules found in biochemical pathways.     

Electron Attachment to Solvated dGpdG: Effects of Stacking on Base‐Centered and Phosphate‐Centered Valence‐Bound Radical Anions

To explore the nature of electron attachment to guanine‐centered DNA single strands in the presence of a polarizable medium, a theoretical investigation of the DNA oligomer dinucleoside phosphate deoxyguanylyl‐3′,5′‐deoxyguanosine (dGpdG) was performed by using density functional theory. Four different electron‐distribution patterns for the radical anions of dGpdG in aqueous solution have been located as local minima on the potential energy surface. The excess electron is found to reside on the proton of the phosphate group (dGp^H?dG), or on the phosphate group (dGp^.?dG), or on the nucleobase at the 5′ position (dG^.?pdG), or on the nucleobase at the 3′ position (dGpdG^.?), respectively. These four radical anions are all expected to be electronically viable species under the influence of the polarizable medium. The predicted energetics of the radical anions follows the order dGp^.?dG>dG^.?pdG>dGpdG^.?>dGp^H?dG. The base–base stacking pattern in DNA single strands seems unaffected by electron attachment. On the contrary, intrastrand H‐bonding is greatly influenced by electron attachment, especially in the formation of base‐centered radical anions. The intrastrand H‐bonding patterns revealed in this study also suggest that intrastrand proton transfer might be possible between successive guanines due to electron attachment to DNA single strands.     

Novel Pulsed Electron Paramagnetic Resonance Techniques for the Studies of Structure and Dynamics of Photo‐excited Triplet State of Organic Molecules: A Professional Journey

We present a few novel pulsed electron paramagnetic resonance techniques developed in our laboratory for the studies of structure and dynamics of the photo‐excited triplet state of organic molecules. We discuss many aspects of these new techniques and the significances of these measurements: (1) enhancing NMR signal intensity by dynamic nuclear polarization ‐ integrated solid effect, (2) performing magnetic resonance in zero‐field and low‐field by pulsed microwave, (3) mapping molecular motion of organic crystals by pulsed zero‐field and low‐field experiments, (4) probing spin dynamics at level anti‐crossing by fast field switching, (5) measuring hyperfine interaction by electron spin echo envelop modulation and spin‐echo electron nuclear double resonance and (6) detecting spin dynamics, nuclear quantum oscillation, entanglements and new avenues for quantum computer. We have employed the highly electron spin polarized pentacene triplet state as the model system in all of our pulsed EPR experiments. We performed most of our experiments at room temperature. The goals of our studies are aiming to improve spin detectability, to probe molecular dynamics, to determine electronic structures, to measure molecular interaction and motion, and to examine quantum coherence and oscillation which may yield new avenues in the applications of pulsed EPR techniques to quantum computer.     

Frontiers in Rotational Spectroscopy: Shapes and Tunneling Dynamics of the Four Conformers of the Acrylic Acid—Difluoroacetic Acid Adduct

The rotational spectra of four conformers of the acrylic acid—difluoroacetic acid adduct (CH₂=CHCOOH–CHF₂COOH, AA‐DFA) are reported and information on their internal dynamics is supplied. This represents an unprecedented result for the conformational analysis, with microwave spectroscopy, of such a heavy molecular adduct.     

Molecular Dynamics Simulations and Steered Molecular Dynamics Simulations of Glabridin Bound to Wild Type and V30A Mutant Transthyretin: Ligand-linked Perturbation of Tertiary Conformation

Transthyretin(TTR), as a tetrameric protein, functions as a neuroprotector. The native TTR homotetramer dissociates into dimers and monomers. Dimers and monomers self-assemble into amyloid fibrils, and this process can lead to some diseases. Native TTR homotetramer is a widely accepted model for TTR amyloid formation. In this study, simulations using molecular dynamics(MD) and steered MD(SMD) were performed to explore the mechanisms for glabridin(Glab), a specific inhibitor for TTR binding, for V30A mutant and wild-type(WT) TTR. MD simulation results indicate that, compared with Glab binding to WT and V30A mutant, the WT TTR could lead to the collapse of β-strands from Ser52 to His56 at chain A. This phenomenon facilitated the easy dissociation of chains A and C. Calculations of the binding free energy between the two chains showed that the V30A-Glab TTR complex displayed a lower binding energy than other systems(WT TTR and WT-Glab TTR). Then, SMD simulation was performed to explore the unbinding pathway for Glab through the WT and V30A mutant TTR. The results show that Lys15(chain A) produced a hydrogen bond with Glab at the force peak via the WT TTR tunnel. Meanwhile, in the V30A TTR mutant, the hydrogen bond between Lys15(chain A) and Glab was broken at the force peak. This condition was beneficial for Glab to be taken off from the protein. Our theoretical results will be useful in designing a new specific inhibitor of TTR protein to control the TTR homotetramer dissociation.     

Electron attachment to DNA and RNA nucleobases: An EOMCC investigation

We report a benchmark theoretical investigation of both vertical and adiabatic electron affinities of DNA and RNA nucleobases: adenine, guanine, cytosine, thymine, and uracil using equation of motion coupled cluster method. The vertical electron affinity (VEA) values of the first five states of the DNA and RNA nucleobases are computed. It is observed that the first electron attached state is energetically accessible in gas phase. Furthermore, an analysis of the natural orbitals exhibits that the first electron attached states of uracil and thymine are valence‐bound in nature and undergo significant structural changes on attachment of an extra electron, which reflects in the deviation of the adiabatic electron affinity (AEA) than that of the vertical ones. Conversely, the first electron attached states of cytosine, adenine, and guanine are in the category of dipole‐bound anions. Their structure, by and large, remain unaffected on attachment of an extra electron, which is evident from the observed small difference between the AEA and VEA values. VEA and AEA values of all the DNA and RNA nucleobases are found to be negative, which implies that the first electron attached states are not stable rather quasi bound. The results of all previous theoretical calculations are out of track and shows large deviation with respect to the experimentally measured values, whereas, our results are found to be in good agreement. Therefore, our computed values can be used as a reliable standard to calibrate new theoretical methods. © 2015 Wiley Periodicals, Inc.     

Electron Momentum Spectroscopy of Valence Orbitals of Cyclopentene: Nuclear Dynamics and Distorted Wave Effect

We report a measurement of electron momentum distributions of valence orbitals of cyclopentene employing symmetric noncoplanar (e, 2e) kinematics at impact energies of 1200 and 1600 eV plus the binding energy. Experimental momentum profiles for individual ionization bands are obtained and compared with theoretical calculations considering nuclear dynamics by harmonic analytical quantum mechanical and thermal sampling molecular dynamics approaches. The results demonstrate that molecular vibrational motions including ring-puckering of this flexible cyclic molecule have obvious influences on the electron momentum profiles for the outer valence orbitals, especially in the low momentum region. For π*-like molecular orbitals 3a'', 2a'', and 3a', the impact-energy dependence of the experimental momentum profiles indicates a distorted wave effect.     

Allosteric Control of Naphthalene Diimide Encapsulation and Electron Transfer in Porphyrin Containers: Photophysical Studies and Molecular Dynamics Simulation

The complexation processes of N,N’-dibutyl-1,4,5,8-naphthalene diimide ( NDI ) into two types of π-electron-rich molecular containers consisting of two Zn(II)-porphyrins connected by four flexible linkers of two different lengths, were characterized by means of absorption and emission spectroscopies and molecular dynamics simulation. Notably, the addition of NDI leads to a strong quenching of the fluorescence of both cages only when they are in an open conformation suitable for guest encapsulation, a situation triggered by silver(I) ions binding to the lateral triazoles. Molecular dynamics simulations confirm the fast binding of NDI , likely assisted by NDI -silver(I) interactions. Upon NDI complexation, the two porphyrin macrocycles get closer, with an optimized face to face orientation, suggesting an induced-fit mechanism through π–π interactions with the NDI aromatic cycle. Ultrafast transient absorption experiments allowed to identify the process of quenching of the Zn-porphyrin fluorescence as an efficient photoinduced electron transfer reaction between the cage porphyrin and the included NDI guest. The process occurs on fast and ultrafast time scales in the two complexes (1.5 ps and ≤300 fs) leading to a short-lived charge separated state (charge recombination lifetimes in the order of 30–40 ps). The combined computational and experimental approach used here is able to furnish a reliable model of the NDI -cage complexation mechanism and of the corresponding electron transfer reaction, attesting the allosteric control of both processes by the silver(I) ions.     

Uptake of Cell-Penetrating Peptide RL2 by Human Lung Cancer Cells: Monitoring by Electron Paramagnetic Resonance and Confocal Laser Scanning Microscopy

RL2 is a recombinant analogue of a human κ-casein fragment, capable of penetrating cells and inducing apoptosis of cancer cells with no toxicity to normal cells. The exact mechanism of RL2 penetration into cells remains unknown. In this study, we investigated the mechanism of RL2 penetration into human lung cancer A549 cells by a combination of electron paramagnetic resonance (EPR) spectroscopy and confocal laser scanning microscopy. EPR spectra of A549 cells incubated with RL2 (sRL2) spin-labeled by a highly stable 3-carboxy-2,2,5,5-tetraethylpyrrolidine-1-oxyl radical were found to contain three components, with their contributions changing with time. The combined EPR and confocal-microscopy data allowed us to assign these three forms of sRL2 to the spin-labeled protein sticking to the membrane of the cell and endosomes, to the spin-labeled protein in the cell interior, and to spin labeled short peptides formed in the cell because of protein digestion. EPR spectroscopy enabled us to follow the kinetics of transformations between different forms of the spin-labeled protein at a minimal spin concentration (3–16 μM) in the cell. The prospects of applications of spin-labeled cell-penetrating peptides to EPR imaging, DNP, and magnetic resonance imaging are discussed, as is possible research on an intrinsically disordered protein in the cell by pulsed dipolar EPR spectroscopy.     

Electron Spin Echo Study of Molecular Structure and Dynamics: New Approaches Based on Spontaneous Fluctuations of Magnetic Interactions

Modulation phenomena that take place during electron spin echo signal decay have long been used in structural studies of free radicals and their environment. These phenomena are based on coherent dynamic effects, arising from simultaneous excitation (by microwave pulses) of two or more transitions in the EPR spectrum. Recently, a new source of stimulated electron spin echo (ESE) modulation was discovered due to spontaneous changes in the magnetic parameters of radicals during the operation of the pulse sequence. For monoradicals, these changes are caused by intramolecular motions. For radical pairs, additional mechanisms are longitudinal relaxation of spin counterparts and transformations of the paramagnetic partners during chemical reactions. Promising applications of this phenomenon to structural studies of radicals and radical pairs in solids and to investigations of their mobility and chemical transformations are considered.     

Electron transfer to sulfides and disulfides: intrinsic barriers and relationship between heterogeneous and homogeneous electron-transfer kinetics

The electron-acceptor properties of series of related sulfides and disulfides were investigated in N,N-dimethylformamide with homogeneous (redox catalysis) and/or heterogeneous (cyclic voltammetry and convolution analysis) electrochemical techniques. The electron-transfer rate constants were determined as a function of the reaction free energy and the corresponding intrinsic barriers were determined. The dependence of relevant thermodynamic and kinetic parameters on substituents was assessed. The kinetic data were also analyzed in relation to corresponding data pertaining to reduction of diaryl disulfides. All investigated reductions take place by stepwise dissociative electron transfer (DET) which causes cleavage of the C(alkyl)--S or S--S bond. A generalized picture of how the intrinsic electron-transfer barrier depends on molecular features, ring substituents, and the presence of spacers between the frangible bond and aromatic groups was established. The reduction mechanism was found to undergo a progressive (and now predictable) transition between common stepwise DET and DET proceeding through formation of loose radical anions. The intrinsic barriers were compared with available results for ET to several classes of dissociative- and nondissociative-type acceptors, and this led to verification that the heterogeneous and the homogeneous data correlate as predicted by the Hush theory.     

Electron Attachment to Hydrated Oligonucleotide Dimers: Guanylyl‐3′,5′‐Cytidine and Cytidylyl‐3′,5′‐Guanosine

The dinucleoside phosphate deoxycytidylyl‐3′,5′‐deoxyguanosine (dCpdG) and deoxyguanylyl‐3′,5′‐deoxycytidine (dGpdC) systems are among the largest to be studied by reliable theoretical methods. Exploring electron attachment to these subunits of DNA single strands provides significant progress toward definitive predictions of the electron affinities of DNA single strands. The adiabatic electron affinities of the oligonucleotides are found to be sequence dependent. Deoxycytidine (dC) on the 5′ end, dCpdG, has larger adiabatic electron affinity (AEA, 0.90 eV) than dC on the 3′ end of the oligomer (dGpdC, 0.66 eV). The geometric features, molecular orbital analyses, and charge distribution studies for the radical anions of the cytidine‐containing oligonucleotides demonstrate that the excess electron in these anionic systems is dominantly located on the cytosine nucleobase moiety. The π‐stacking interaction between nucleobases G and C seems unlikely to improve the electron‐capturing ability of the oligonucleotide dimers. The influence of the neighboring base on the electron‐capturing ability of cytosine should be attributed to the intensified proton accepting–donating interaction between the bases. The present investigation demonstrates that the vertical detachment energies (VDEs) of the radical anions of the oligonucleotides dGpdC and dCpdG are significantly larger than those of the corresponding nucleotides. Consequently, reactions with low activation barriers, such as those for O? C σ bond and N‐glycosidic bond breakage, might be expected for the radical anions of the guanosine–cytosine mixed oligonucleotides.     

Electron attachment to the DNA bases adenine and guanine and dehydrogenation of their anionic derivatives: Density functional study

Density functional theory (DFT) calculations have been used to explore electron attachment to the purines adenine and guanine and their hydrogen atom loss. Calculations show that the dehydrogenation at the N9 site in the adenine and guanine transient anions is the lowest‐cost channel of hydrogen loss, and the N9? H bond scission has Gibbs free energies of dissociation ΔG° of 8.8 kcal mol^?1 for the anionic adenine and 13.9 kcal mol^?1 for the anionic guanine. The relatively high feasibility of low‐energy electron (LEE)‐induced N9? H bond cleavage in the purine nucleobases arises from high electron affinities of their H‐deleted counterparts. Unlike adenine, other N? H bond dissociations are competitive with the N9? H bond fission in the anionic guanine. The replacement of hydrogen in the ring of purine has a significant effect on the N9? H bond fragmentation. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007