Evidence for Dust Related X-ray Emission from Comet C/1995 O1 (Hale-Bopp)

We report the discovery of X-ray emission from comet C/1995 O1 (Hale-Bopp) by the LECS instrument on-board BeppoSAX on 1996 September 10--11. The 0.1--2.0 keV luminosity decayed by a factor of 2 on a timescale of ~10 hr with a mean value of 5.10E16 erg s-1. The spectrum is well fit by a thermal bremsstrahlung model with a temperature of 0.29 +/- 0.06 keV, or a power-law with a photon index of 3.1 +{0.6} -{0.2}. The lack of detected C and O line emission places severe constraints on many models for cometary X-ray emission, especially those which involve X-ray production in cometary gas. The luminosity is a factor of at least 3.4 greater than measured by Extreme Ultraviolet Explorer (EUVE) 4 days later. This difference may be related to the emergence from the nucleus on 1996 September 9 of a dust-rich cloud. Over the next few days the cloud continued to expand becoming increasingly tenuous, until it had reached an extent of ~3.10E5 km (or ~2 arcmin) by the start of EUVE observation. We speculate that the observed reduction in X-ray intensity is evidence for dust fragmentation. These results support the view that cometary X-ray emission arises from the interaction between solar X-rays and cometary dust.Comment: 17 pages. 4 postscript figs (2 in color). Accepted for publication in ApJ (Letters

FT-IR SPECTROSCOPY OF MATRIX ISOLATED PHENYLPEROXYL RADICALS

Author Institution: Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215; Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215 and National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401$^{2}A^{\prime\prime}$ phenylperoxyl radicals, \chem{C_6H_5O_2}, have been generated in a cryogenic Ar matrix. In a two pulse experiment, phenyl radicals, \chem{C_6H_5}, are generated via pyrolysis of phenyliodide in Ar through a hyperthermal \chem{SiC} nozzle and deposited on a \chem{CsI} window at 20 K; a second valve deposits a layer of \chem{O_2} in Ar on top of the phenyl radicals. The process is repeated and phenylperoxyl radicals are formed in the matrix. The matrix is analyzed with an IR spectrometer to identify vibrational frequencies of the radical

Selecting and identifying gas-phase protonation isomers of nicotineH+ using combined laser, ion mobility and mass spectrometry techniques

The detection and assignment of protonation isomers, termed protomers, of gas-phase ions remains a challenge in mass spectrometry. With the emergence of ion-mobility techniques combined with tuneable-laser photodissociation spectroscopy, new experimental combinations are possible to now meet this challenge. In this paper, the differences in fragmentation and electronic spectroscopy of singly protonated (S)-nicotine (nicH+) ions are analysed using action spectroscopy in the ultraviolet region and field asymmetric ion mobility spectrometry (FAIMS). Experiments are supported by quantum chemical calculations (DFT, TD-DFT and CC2) of both spectroscopic and thermochemical properties. Electrospray ionisation (ESI) of (S)-nicotine from different solvents leads to different populations of two nicH+ protomers corresponding to protonation on the pyridine nitrogen and pyrrolidine nitrogen, respectively. FAIMS gives partial resolution of these protomers and enables characteristic product ions to be identified for each isomer as verified directly by analysis of product-ion specific action spectroscopy. It is shown that while characteristic, these product ions are not exclusive to each protomer. Calculations of vertical electronic transitions assist in rationalising the photodissociation action spectra. The integration of photodissociation action spectroscopy with FAIMS-mass spectrometry is anticipated to be a useful approach for separating and assigning protonation isomers of many other small molecular ions

Structure elucidation of cyclohexene (9Z)-octadec-9-enyl ethers isolated from the leaves of Uvaria cherrevensis (Annonaceae)

The phytochemical investigation of the leaf extract of Uvaria cherrevensis (Annonaceae) yielded three new cyclohexene (9Z)-octadec-9-enyl ethers, cherrevenols M-O (1-3), and a known fatty ester derivative (4). The structures of the isolated compounds were elucidated by spectroscopic and computer-aided molecular modelling methods. Ozone Induced Dissociation (OzID) mass spectrometry was employed to determine the C-9 position of the side chain olefinic double bonds, while 13 C NMR spectroscopy indicated their (Z)-configurations. All isolated compounds were evaluated for their antimalarial and cytotoxic activities; all were inactive

EVIDENCE FOR DISSOCIATION FROM THE S\textit{S}0{_0} GROUND STATE OF ACETALDEHYDE TO THE RADICAL PRODUCTS CH3_3 and HCO

P. L. Houston and S. H. Kable, PNASB. R. Heazlewood et al, submittedS.-H. Lee and I.-C. Chen, Chem. Phys.Author Institution: School of Chemistry, University of Sydney, Sydney NSW 2006, AustraliaRecent experiments and theory have implicated a "roaming" mechanism as being important in the photodissociation of \chem{CH_3CHO} into the molecular products \chem{CH_4} + \chem{CO}.}, \textbf{103}, 16079 (2006).}$^{,}$} (2006).} As much as 80\% of the flux for this chemical channel was attributed to roaming; the conventional transition state mechanism is a minor contribution. Quasi-classical trajectory calculations reveal that many of these roaming trajectories can be described as a methyl group roaming around the \chem{HCO} core, before intramolecularly abstracting the formyl \chem{H} atom. A crucial element to this mechanism is that the simple, barrierless, C-C bond cleavage to radical products must be open at the wavelengths used in previous experiments. While there is no doubt that the radical channel is open in an energetic sense, \chem{HCO} and \chem{CH_3} have never been observed from the ground state ($\textit{S}$${_0}$) surface. In this seminar, we will summarize the evidence for roaming in \chem{CH_3CHO} and then present new experimental evidence that \chem{HCO} and \chem{CH_3} are indeed formed on the ground state. Pump/probe experiments were performed on acetaldehyde seeded in a supersonic expansion of helium. \chem{HCO} products were probed via laser-induced fluorescence ($\tilde{B}\leftarrow\tilde{X}$) at a range of pump wavelengths (308 - 330 nm). When the pump energy was above the ($\textit{T}$${_1}$) barrier for dissociation ($\lambda$ $\sim$320 nm),}, \textbf{220}, 175 (1997).} the \chem{HCO} product state distribution is characteristic of a reaction proceeding over a barrier. When the dissociation energy is lower than the triplet barrier, \chem{HCO} was still observed, which must then arise from reaction on the $\textit{S}$${_0}$ surface. In addition, the \chem{HCO} internal energy distribution was different when dissociating above and below the triplet barrier, thereby confirming the presence of two different mechanistic pathways. The existence of the \chem{CH_3} + \chem{HCO} channel from the \chem{CH_3CHO} ground state supports the previous assignment of "\chem{CH_3} roaming" in \chem{CH_3CHO} photodissociation to \chem{CH_4} + \chem{CO}

Reaction of ionised steryl esters with ozone in the gas phase

Cholesterol is an ubiquitous membrane lipid, that also serves as a precursor to many steroid hormones. The 5,6 carbon-carbon double bond on the tetracyclic carbon backbone of cholesterol is an attractive target for ozone with the reaction giving rise to a wide range of possibly bioactive molecules. Despite this, little is known about the ozonolysis of cholesterol esters, which often possess an additional double bond(s) on the fatty acyl chain. Understanding the intrinsic gas phase reaction of ozone with the two disparate double bond positions on cholesteryl esters can inform our understanding of these processes in vivo, particularly reactions occurring at the air-water interface (e.g., tear film lipid layer) and on the surfaces of the body where these cholesterol and cholesteryl esters may be present (e.g., sebum). In the present work we describe the gas phase ozonolysis of lithium and sodium cations formed from three steryl esters: two isomeric for double bond position (cholestanyl oleate and cholesteryl stearate), and a third with carbon-carbon double bonds present in both the sterol ring system and fatty acyl chain (cholesteryl oleate). We confirm the enhanced reactivity of the endocyclic carbon-carbon double bond with ozone over double bonds present in the acyl chain, and elucidate competitive interactions between the two double bond positions during ozonolysis. Elucidation of the mechanisms underlying this interaction is important for both understanding these processes in vivo and for deploying ozonolysis chemistry in analytical strategies for lipidomics

α-Glucosidase Inhibitory Activity of the Extracts and Major Phytochemical Components of Smilax glabra Roxb

© 2020 Bentham Science Publishers. Background: A therapeutic approach to treat diabetes is to decrease postprandial hyperglycemia. α-Glucosidase inhibitors from plant sources offer an attractive strategy for the control of hyperglycemia. Smilax glabra Roxb is a medicinal plant found in Asia, including Vietnam, which is used in the treatment of chronic diseases. However, the antidiabetic activity and the identification of α-glucosidase inhibitors from this plant have not been intensively investigated. This research was carried out to determine the α-glucosidase inhibitory activity of the extracts and that of the major phytochemical components of Smilax glabra Roxb. This could lead to further studies on the role of these compounds in hyperglycemia control, as well as identify their potential future applications. Methods: Column chromatography combined with crystallization procedures were used to isolate active fractions and two major compounds. The chemical structures of these compounds were determined by analysis of their NMR spectroscopic data, as well as MS data and comparisons made with the literature data. The α-glucosidase inhibitory activity was determined spectrophotometrically using p-nitrophenyl α-D-glucopyranoside as a substrate. The in vitro cytotoxicity of the isolated compounds and fractions was determined using the MTT assay. Results: The two major compounds, astilbin and 5-O-caffeoylshikimic acid together with two very active fractions, F7 and F8, were isolated from the rhizome. The two major compounds had α-glucosidase inhibitory activities with IC50 values of ca. 125 μg/mL and 38 μg/mL, respectively which are about 4 and 13 folds higher activity than the reference compound acarbose (IC50 of ca. 525 μg/mL). Fractions F7 and F8 showed very promising inhibitory activities towards α-glucosidase with IC50 values of 5.5 and 5.8 μg/mL, respectively. Cytotoxicity data on mouse fibroblast NIH3T3 cells indicated that the active compounds and fractions were not toxic at concentrations that are greater than their respective IC50 values. The α-glucosidase inhibitory activity of 5-O-caffeoylshikimic acid and that of the two active fractions are reported here for the first time. Conclusion: The two major isolated compounds and fractions, F7 and F8, significantly contribute to the α-glucosidase inhibitory activity of S. glabra Roxb extract. Further work is needed to clarify their modes of action and potential application

Solvent-Mediated Proton-Transfer Catalysis of the Gas-Phase Isomerization of Ciprofloxacin Protomers

Understanding how neutral molecules become protonated during positive-ion electrospray ionization (ESI) mass spectrometry is critically important to ensure analytes can be efficiently ionized, detected, and unambiguously identified. The ESI solvent is one of several parameters that can alter the dominant site of protonation in polyfunctional molecules and thus, in turn, can significantly change the collision-induced dissociation (CID) mass spectra relied upon for compound identification. Ciprofloxacin-a common fluoroquinolone antibiotic-is one such example whereby positive-ion ESI can result in gas-phase [M + H]+ ions protonated at either the keto-oxygen or the piperazine-nitrogen. Here, we demonstrate that these protonation isomers (or protomers) of ciprofloxacin can be resolved by differential ion mobility spectrometry and give rise to distinctive CID mass spectra following both charge-directed and charge-remote mechanisms. Interaction of mobility-selected protomers with methanol vapor (added via the throttle gas supply) was found to irreversibly convert the piperazine N-protomer to the keto-O-protomer. This methanol-mediated proton-transport catalysis is driven by the overall exothermicity of the reaction, which is computed to favor the O-protomer by 93 kJ mol-1 (in the gas phase). Conversely, gas phase interactions of mobility-selected ions with acetonitrile vapor selectively depletes the N-protomer ion signal as formation of stable [M + H + CH3CN]+ cluster ions skews the apparent protomer population ratio, as the O-protomer is unaffected. These findings provide a mechanistic basis for tuning protomer populations to ensure faithful characterization of multifunctional molecules by tandem mass spectrometry

Elucidating the chemical structure of native 1-deoxysphingosine

The 1-deoxysphingolipids (1-deoxySLs) are formed by an alternate substrate usage of the enzyme, serine-palmitoyltransferase, and are devoid of the C1-OH-group present in canonical sphingolipids. Pathologically elevated 1-deoxySL levels are associated with the rare inherited neuropathy, HSAN1, and diabetes type 2 and might contribute to β cell failure and the diabetic sensory neuropathy. In analogy to canonical sphingolipids, it was assumed that 1-deoxySLs also bear a (4E) double bond, which is normally introduced by sphingolipid delta(4)-desaturase 1. This, however, was never confirmed. We therefore supplemented HEK293 cells with isotope-labeled D3-1-deoxysphinganine and compared the downstream formed D3-1-deoxysphingosine (1-deoxySO) to a commercial synthetic SPH m18:1(4E)(3OH) standard. Both compounds showed the same m/z, but differed in their RPLC retention time and atmospheric pressure chemical ionization in-source fragmentation, suggesting that the two compounds are structural isomers. Using dimethyl disulfide derivatization followed by MS(2) as well as differential-mobility spectrometry combined with ozone-induced dissociation MS, we identified the carbon-carbon double bond in native 1-deoxySO to be located at the (Δ14) position. Comparing the chromatographic behavior of native 1-deoxySO to chemically synthesized SPH m18:1(14Z) and (14E) stereoisomers assigned the native compound to be SPH m18:1(14Z). This indicates that 1-deoxySLs are metabolized differently than canonical sphingolipids