Near-edge x-ray absorption fine structure spectra and specific dissociation of small peptoid molecules.

In this study, the total ion yield near-edge x-ray absorption fine structure spectra of four similar peptoid molecules, which differ in the numbers and positions of methyl groups, were investigated experimentally and theoretically. At each excitation energy, the intensity and branching ratio of each ionic product were measured. At a few resonant excitation energies, a specific dissociation of the C-CO bond at the nitrogen and oxygen K-edges and of the N-CO bond at the carbon K-edge was dominant, which correlated well with the predicted destination antibonding orbitals of the core electron excitation. These specific dissociation mechanisms of small peptoid molecules could provide insights into similar phenomena that occur in peptide molecules.

Catalytic Coupling of Free Radical Oxidation and Electrophilic Chlorine Addition by Phase-Transfer Intermediates in Liquid Aerosols.

While examining the heterogeneous reaction of chlorine atoms with alkenes, in the presence of Cl2, we have observed an unexpectedly large enhancement of reactivity and the predominance of chlorinated reaction products even under high O2 conditions, where Cl atom recycling is expected to be minimal. These observations cannot be explained by known free radical oxidation or cycling mechanisms, but rather we find evidence for the multiphase catalytic coupling of free radical oxidation with electrophilic Cl2 addition. The mechanism entails the production of oxygenated reaction intermediates, which act as gas-liquid phase-transfer catalysts (gl-PTCs) by promoting the accommodation of gas-phase Cl2 by the aerosol, thereby enhancing electrophilic addition. Although the majority of PTCs typically couple chemistry between two immiscible liquid phases (aqueous/organic), there are few examples of PTCs that couple gas-liquid reactions. This work shows how multiphase reaction schemes of aerosols can be reimagined for understanding catalytic reaction mechanisms.

Exploring the Pharmacological Potential of Metformin for Neurodegenerative Diseases.

Metformin, one of the first-line of hypoglycemic drugs, has cardioprotective, anti-inflammatory and anticancer activities, in addition to its proven hypoglycemic effects. Furthermore, the preventive and therapeutic potential of metformin for neurodegenerative diseases has become a topic of concern. Increasing research suggests that metformin can prevent the progression of neurodegenerative diseases. In recent years, many studies have investigated the neuroprotective effect of metformin in the treatment of neurodegenerative diseases. It has been revealed that metformin can play a neuroprotective role by regulating energy metabolism, oxidative stress, inflammatory response and protein deposition of cells, and avoiding neuronal dysfunction and neuronal death. On the contrary, some have hypothesized that metformin has a two-sided effect which may accelerate the progression of neurodegenerative diseases. In this review, the results of animal experiments and clinical studies are reviewed to discuss the application prospects of metformin in neurodegenerative diseases.

Highly Selective Dissociation of a Peptide Bond Following Excitation of Core Electrons.

The controlled breaking of a specific chemical bond with photons in complex molecules remains a major challenge in chemistry. In principle, using the K-edge absorption of a particular atomic element, one might excite selectively a specific atomic entity in a molecule. We report here highly selective dissociation of the peptide bonds in N-methylformamide and N-methylacetamide on tuning the X-ray wavelength to the K-edge absorption of the atoms connected to (or near) the peptide bond. The high selectivity (56-71%) of this cleavage arises from the large energy shift of X-ray absorption, a large overlap of the 1s orbital and the valence π* orbital that is highly localized on a peptide bond with antibonding character, and the relatively low bond energy of the peptide bonds. These characteristics indicate that the high selectivity on bond dissociation following core excitation could be a general feature for molecules containing peptide bonds.

Core excitation, specific dissociation, and the effect of the size of aromatic molecules connected to oxygen: phenyl ether and 1,3-diphenoxybenzene.

Near-edge X-ray absorption fine structure (NEXAFS) spectra of phenyl ether at the carbon K-edge and 1,3-diphenoxybenzene at both the carbon and oxygen K-edges were measured in the total ion yield mode using X-rays from a synchrotron and a reflectron time-of-flight mass spectrometer. Time-dependent density functional theory was adopted to calculate the carbon and oxygen K-edge NEXAFS spectra of phenol, phenyl ether, and 1,3-diphenoxybenzene. The assignments and a comparison of the experimental and calculated spectra are presented. The mass spectra of ionic products formed after X-ray absorption at various excitation energies are also reported. Specific dissociations were observed for the 1s → π* transition of phenyl ether. In comparison with phenol and phenyl ether, the dependence of the fragmentation on the excitation site and destination state was weak in 1,3-diphenoxybenzene, likely as a result of delocalization of the valence electrons and rapid randomization of energy.

Near-edge X-ray absorption fine structure spectra and site-selective dissociation of phenol.

A time-of-flight mass spectrometer with orthogonal acceleration and soft X-rays from synchrotron radiation were utilized to measure near-edge X-ray absorption fine structure (NEXAFS) spectra of carbon and oxygen in phenol and the corresponding ionic fragments following core excitation. The photon energies were in the range of 284-298 eV for the carbon K-edge and 529.5-554.5 eV for the oxygen K-edge. The total ion yield, ion intensity for each ionic fragment, and ion intensity ratio, defined as ion intensity divided by total ion yield, were measured as a function of photon energy. Possible mechanisms of dissociation are proposed and enhancements of specific products of dissociation are reported. In general, the enhancement of these specific products is small in the carbon K-edge region but is clear for some products at the oxygen K-edge. In particular, elimination of the H atom from the hydroxyl group was observed only at the oxygen K-edge. One remarkable result is that an excitation of a core-level electron of oxygen greatly enhanced the cleavage of specific C-C bonds.

The direct observation of secondary radical chain chemistry in the heterogeneous reaction of chlorine atoms with submicron squalane droplets.

The reaction of Cl atoms, in the presence of Cl(2) and O(2), with sub-micron squalane particles is used as a model system to explore how surface hydrogen abstraction reactions initiate chain reactions that rapidly transform the chemical composition of an organic particle. The heterogeneous reaction is measured in a photochemical flow tube reactor in which chlorine atoms are produced by the photolysis of Cl(2) at 365 nm. By monitoring the heterogeneous reaction, using a vacuum ultraviolet photoionization aerosol mass spectrometer, the effective reactive uptake coefficient and the distributions of both oxygenated and chlorinated reaction products are measured and found to depend sensitively upon O(2), Cl(2), and Cl concentrations in the flow reactor. In the absence of O(2), the effective reactive uptake coefficient monotonically increases with Cl(2) concentration to a value of ∼3, clearly indicating the presence of secondary chain chemistry occurring in the condensed phase. The effective uptake coefficient decreases with increasing O(2) approaching a diffusion corrected value of 0.65 ± 0.07, when 20% of the total nitrogen flow rate in the reactor is replaced with O(2). Using a kinetic model it is found that the amount of secondary chemistry and the product distributions in the aerosol phase are controlled by the competitive reaction rates of O(2) and Cl(2) with alkyl radicals. The role that a heterogeneous pathway might play in the reaction of alkyl radicals with O(2) and Cl(2) is investigated within a reasonable range of reaction parameters. These results show, more generally, that for heterogeneous reactions involving secondary chain chemistry, time and radical concentration are not interchangeable kinetic quantities, but rather the observed reaction rate and product formation chemistry depends sensitively upon the concentrations and time evolution of radical initiators and those species that propagate or terminate free radical chain reactions.

Soft ionization of thermally evaporated hypergolic ionic liquid aerosols.

Isolated ion pairs of a conventional ionic liquid, 1-Ethyl-3-Methyl-Imidazolium Bis(trifluoromethylsulfonyl)imide ([Emim(+)][Tf(2)N(-)]), and a reactive hypergolic ionic liquid, 1-Butyl-3-Methyl-Imidazolium Dicyanamide ([Bmim(+)][Dca(-)]), are generated by vaporizing ionic liquid submicrometer aerosol particles for the first time; the vaporized species are investigated by dissociative ionization with tunable vacuum ultraviolet (VUV) light, exhibiting clear intact cations, Emim(+) and Bmim(+), presumably originating from intact ion pairs. Mass spectra of ion pair vapor from an effusive source of the hypergolic ionic liquid show substantial reactive decomposition due to the internal energy of the molecules emanating from the source. Photoionization efficiency curves in the near threshold ionization region of isolated ion pairs of [Emim(+)][Tf(2)N(-)] ionic liquid vapor are compared for an aerosol source and an effusive source, revealing changes in the appearance energy due to the amount of internal energy in the ion pairs. The aerosol source has a shift to higher threshold energy (∼0.3 eV), attributed to reduced internal energy of the isolated ion pairs. The method of ionic liquid submicrometer aerosol particle vaporization, for reactive ionic liquids such as hypergolic species, is a convenient, thermally "cooler" source of isolated intact ion pairs in the gas phase compared to effusive sources.

Airfoil sampling of a pulsed Laval beam with tunable vacuum ultraviolet synchrotron ionization quadrupole mass spectrometry: application to low-temperature kinetics and product detection.

A new pulsed Laval nozzle apparatus with vacuum ultraviolet (VUV) synchrotron photoionization quadrupole mass spectrometry is constructed to study low-temperature radical-neutral chemical reactions of importance for modeling the atmosphere of Titan and the outer planets. A design for the sampling geometry of a pulsed Laval nozzle expansion has been developed that operates successfully for the determination of rate coefficients by time-resolved mass spectrometry. The new concept employs airfoil sampling of the collimated expansion with excellent sampling throughput. Time-resolved profiles of the high Mach number gas flow obtained by photoionization signals show that perturbation of the collimated expansion by the airfoil is negligible. The reaction of C(2)H with C(2)H(2) is studied at 70 K as a proof-of-principle result for both low-temperature rate coefficient measurements and product identification based on the photoionization spectrum of the reaction product versus VUV photon energy. This approach can be used to provide new insights into reaction mechanisms occurring at kinetic rates close to the collision-determined limit.

Molecular cloning and tissue expression patterns of a small conductance calcium-activated potassium channel gene in turbot (Scophthalmus maximus L.).

Calcium-activated potassium channels on plasma membrane enable potassium influx into the cell with ensuing changes in plasma membrane potential and consequent effects on cellular metabolic functions. Recently, this potassium channel was reported to regulate the cellular responses of mammalian immune cells. We have postulated the presence of such a channel in fish immune cells and its potential role in immunoregulation in fish. Employing specific primers and RNA template, we cloned a segment of a novel gene from turbot blood sample and subsequently obtained a full cDNA sequence using RACE approaches. Bioinformatic analysis revealed structural and phylogenetic characteristics of a novel small conductance calcium-activated potassium channel gene, we called TSKCa, which exhibits homologous domains to other species particularly in the transmembrane regions. Full-length TSKCa cDNA is 1698 bp with a 1632 bp open reading frame encoding a protein of 544 amino acids. TSKCa gene is expressed in majority of the tested organs and tissues of turbot. To assess the postulated immune function of TSKCa, we infected turbot with the pathogen Vibrio anguillarum. Here, semi-quantitative RT-PCR analysis demonstrated increased mRNA expression of TSKCa in head kidney, spleen and blood, indicating an important role of TSKCa in these organ tissues that mediate the immune defense response of turbot. In contrast, there was less change in expression in the turbot intestines and liver which were less implicated in the immune response in present study.

Energy transfer of highly vibrationally excited 2-methylnaphthalene: Methylation effects.

The methylation effects in the energy transfer between Kr atoms and highly vibrationally excited 2-methylnaphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at a translational collision energy of approximately 520 cm(-1). Comparison of the energy transfer between naphthalene and 2-methylnaphthalene shows that the difference in total collisional cross section and the difference in energy transfer probability density functions are small. The ratio of the total cross sections is sigma(naphthalene): sigma(methylnaphthalene)=1.08+/-0.05:1. The energy transfer probability density function shows that naphthalene has a little larger probability at small T-->VR energy transfer, DeltaE(u)<300 cm(-1), and 2-methylnaphthalene has a little larger probability at large V-->T energy transfer, -800 cm(-1)

Energy transfer of highly vibrationally excited naphthalene. III. Rotational effects.

The rotational effects in the energy transfer between Kr atoms and highly vibrationally excited naphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity map ion imaging at various translational collision energies. As the initial rotational temperature changes from less than 10 to approximately 350 K, the ratio of vibrational to translational (V-->T) energy transfer cross section to translational to vibrational/rotational (T-->VR) energy transfer cross section increases, but the probability of forming a complex during the collisions decreases. Significant increases in the large V-->T energy transfer probabilities, termed supercollisions, at high initial rotational temperature were observed.

Energy transfer of highly vibrationally excited naphthalene. II. Vibrational energy dependence and isotope and mass effects.

The vibrational energy dependence, H and D atom isotope effects, and the mass effects in the energy transfer between rare gas atoms and highly vibrationally excited naphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at various translational collision energies. Increase of vibrational energy from 16 194 to 18 922 cm(-1) does not make a significant difference in energy transfer. The energy transfer properties also remain the same when H atoms in naphthalene are replaced by D atoms, indicating that the high vibrational frequency modes do not play important roles in energy transfer. They are not important in supercollisions either. However, as the Kr atoms are replaced by Xe atoms, the shapes of energy transfer probability density functions change. The probabilities for large translation to vibration/rotation energy transfer (T-->VR) and large vibration to translation energy transfer (V-->T) decrease. High energy tails in the backward scatterings disappear, and the probability for very large vibration to translation energy transfer such as supercollisions also decreases.

Energy transfer of highly vibrationally excited naphthalene. I. Translational collision energy dependence.

Energy transfer between highly vibrationally excited naphthalene and Kr atom in a series of translational collision energies (108-847 cm(-1)) was studied separately using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. Highly vibrationally excited naphthalene in the triplet state (vibrational energy: 16,194 cm(-1); electronic energy: 21,400 cm(-1)) was formed via the rapid intersystem crossing of naphthalene initially excited to the S(2) state by 266 nm photons. The collisional energy transfer probability density functions were measured directly from the scattering results of highly vibrationally excited naphthalene. At low collision energies a short-lived naphthalene-Kr complex was observed, resulting in small amounts of translational to vibrational-rotational (T-->VR) energy transfer. The complex formation probability decreases as the collision energy increases. T-->VR energy transfer was found to be quite efficient at all collision energies. In some instances, nearly all of the translational energy is transferred to vibrational-rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy. The translational energy gained from vibrational energy extend to large energy transfer (up to 3000 cm(-1)) as the collision energy increases to 847 cm(-1). Substantial amounts of large V-->T energy transfer were observed in the forward and backward directions at large collision energies.

Energy transfer of highly vibrationally excited azulene. III. Collisions between azulene and argon.

The energy transfer dynamics between highly vibrationally excited azulene molecules (37 582 cm(-1) internal energy) and Ar atoms in a series of collision energies (200, 492, 747, and 983 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. The angular resolved collisional energy-transfer probability distribution functions were measured directly from the scattering results of highly vibrationally excited azulene. Direct T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational/rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). Significant amount of energy transfer from vibration to translation was observed at large collision energies in backward and sideway directions. The ratios of total cross sections between T-VR and V-T increases as collision energy increases. Formation of azulene-argon complexes during the collision was observed at low enough collision energies. The complexes make only minor contributions to the measured translational to vibrational/rotational (T-VR) energy transfer.

Energy transfer of highly vibrationally excited azulene. II. Photodissociation of azulene-Kr van der Waals clusters at 248 and 266 nm.

Photodissociation of azulene-Kr van der Waals clusters at 266 and 248 nm was studied using velocity map ion imaging techniques with the time-sliced modification. Scattered azulene molecules produced from the dissociation of clusters were detected by one-photon vacuum ultraviolet ionization. Energy transfer distribution functions were obtained from the measurement of recoil energy distributions. The distribution functions can be described approximately by multiexponential functions. Fragment angular distributions were found to be isotropic. The energy transfer properties show significantly different behavior from those of bimolecular collisions. No supercollisions were observed under the signal-to-noise ratios S/N=400 and 100 at 266 and 248 nm, respectively. Comparisons with the energy transfer of bimolecular collisions in thermal systems and the crossed-beam experiment within detection limit are made.

Generation and characterization of highly vibrationally excited molecular beam.

A simple method to generate and characterize a pure highly vibrationally excited azulene molecular beam is demonstrated. Azulene molecules initially excited to the S4 state by 266-nm UV photons reach high vibrationally excited levels of the ground electronic state upon rapid internal conversion from the S4 electronically excited state. VUV laser beams at 157 and 118 nm, respectively, are used to characterize the relative concentrations of the highly vibrationally excited azulene and the rotationally and vibrationally cooled azulene in the molecular beam. With a laser intensity of 34 mJ/cm2, 75% of azulene molecules absorb a single 266-nm photon and become highly vibrationally excited molecules. The remaining ground-state azulene molecules absorb two or more UV photons, ending up either as molecular cations, which are repelled out of the beam by an electric field, or as dissociation fragments, which veer off the molecular-beam axis. No azulene without absorption of UV photons is left in the molecular beam. The molecular beam that contains only highly vibrationally excited molecules and carrier gas is useful in various experiments related to the studies of highly vibrationally excited molecules.

Energy transfer of highly vibrationally excited azulene: collisions between azulene and krypton.

The energy-transfer dynamics between highly vibrationally excited azulene molecules and Kr atoms in a series of collision energies (i.e., relative translational energies 170, 410, and 780 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. "Hot" azulene (4.66 eV internal energy) was formed via the rapid internal conversion of azulene initially excited to the S4 state by 266-nm photons. The shapes of the collisional energy-transfer probability density functions were measured directly from the scattering results of highly vibrationally excited or hot azulene. At low enough collision energies an azulene-Kr complex was observed, resulting from small amounts of translational to vibrational-rotational (T-VR) energy transfer. T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational-rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). The shapes of V-T energy-transfer probability density functions were best fit by multiexponential functions. We find that substantial amounts of energy are transferred in the backward scattering direction due to supercollisions at high collision energies. The probability for supercollisions, defined arbitrarily as the scattered azulene in the region 160 degrees 2000 cm(-1) is 1% and 0.3% of all other collisions at collision energies 410 and 780 cm(-1), respectively.

Supercollisions and energy transfer of highly vibrationally excited molecules.

Collisional energy-transfer probability distribution functions of highly vibrationally excited molecules and the existence of supercollisions remain as the outstanding questions in the field of intermolecular energy transfer. In this investigation, collisional interactions between ground state Kr atoms and highly vibrationally excited azulene molecules (4.66 eV internal energy) were examined at a collision energy of 410 cm-1 using a crossed molecular beam apparatus and time-sliced ion imaging techniques. A large amount of energy transfer (1000-5000 cm-1) in the backward direction was observed. We report the experimental measurement for the shape of the energy-transfer probability distribution function along with a direct observation of supercollisions.

Time-sliced ion imaging study of I2 and I2+ photolysis at 532 nm.

Photodissociation of I2 and I2+ at 532.10 nm was studied using VUV photoionization/time-sliced velocity map imaging. The dissociation energies D0 of I2(X 1sigma g+) and I2(X 2pi 1/2g) were found to be 12,416 +/- 48 and 16,439 +/- 48 cm(-1), respectively. Absorption of 532.10 nm mainly corresponds to the excitation of I2(X 1sigma g+) to the 1pi(1u) and/or A3pi(1u) states, and the excitation of I2+(X 2pi 1/2g) to the A2pi 1/2u state, with a small fraction to the A2pi 3/2u and 4sigma u+ states. A method for accurate calibration of magnification factor of ion imaging system was presented. The improvement of energy resolution for large ionization volume experiments, like the crossed molecular beam and one-photon VUV photoionization experiments, was discussed.