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PubMed2003

Thermal ionization ion mobility spectrometry of alkali salts.

Positive and negative thermal ionization ion mobility spectra (TI-IMS) of some sodium and potassium halides are reported here. The data provide the first measurement of the thermal ionization ion mobility spectrometry of inorganic compounds. A thin Nichrome filament was used as a thermionic ionization source. Sample was directly deposited on the filament, where it was heated and ionized. Each salt produced a different ion mobility pattern, but all sodium salts spectra were common in their first peak. This peak differs from the common peak observed in the spectra of potassium salts. The drift time of the second peak in all spectra was found to be linearly dependent on the size of the counteranion of the salt. Negative thermal ionization ion mobility spectra of alkali halides were also observed. An alkali halide salt (MX), in general, produced its own anion as well as some heavier ions that are thought to be hydrated X- (MX)n species. The capability of the method in quantitative analysis was demonstrated by measuring potassium impurity in sodium bromide. A detection limit of 0.01% K+ in NaBr and a linear rage of 3 orders of magnitude were obtained. The results from this study suggest that TI-IMS has potential as a field technique for the detection of some elements in samples.

Journal Article↗
PubMed2005

Atmospheric pressure ionization in a miniature mass spectrometer.

A miniature cylindrical ion trap mass spectrometer featuring an atmospheric pressure interface allowing atmospheric pressure chemical ionization and electrospray ionization is described together with its analytical performance characteristics. The vacuum system, ion optics, mass analyzer, control electronics system, and detection system have all been designed and built in-house. The design is based upon a three-stage, differentially pumped vacuum system with the instrument capable of being interfaced to many types of atmospheric pressure ionization sources. Ions are transferred through home-built ion optics, and instrument control is achieved through custom-designed electronics and LabView control software. Corona discharge ionization and electrospray ionization sources are implemented and used to allow the analysis of both gaseous- and solution-phase samples during the characterization of the instrument. An upper mass/charge limit of approximately 450 Th with unit resolution was achieved using a 2.5-mm-internal radius cylindrical ion trap as the mass analyzer. The specificity of the instrument can be increased by employing the MS/MS capabilities of the ion trap and has been demonstrated for nitrobenzene. Limits of detection for the trace analysis in air of the chemical warfare agent simulant methyl salicylate (1.24 ppb) and for nitrobenzene (629 pptr) are achieved. The dynamic range of the instrument is currently limited to approximately 2 orders of magnitude by saturation of the detection electronics. Isolation and collision-induced dissociation efficiencies in MS/MS experiments both greater than 50% are reported. Electrospray/nanospray data are presented on solutions including 100 microM (D,L)-arginine, 10 microM (-)-ephedrine, and 10 microM lomefloxacin.

Journal Article↗
PubMed2004

A chemical ionization mass spectrometry method for the online analysis of organic aerosols.

A new technique employing chemical ionization mass spectrometry (CIMS) is described that allows the composition of organic particles to be determined on the time scale of seconds. With this Aerosol CIMS technique, particles are vaporized thermally at temperatures up to 480 degrees C, and the resulting vapor is chemically ionized and detected with a quadrupole mass spectrometer. The separation of the vaporization and ionization steps allows greater control and more flexibility for the detection of condensed phases than with other chemical ionization methods. Consequently, composition can be correlated to volatility, providing an additional dimension of information. The use of a variety of positive and negative reagent ions, such as H(+)(H(2)O)(2), H(+)(CH(3)OH)(2), NO(+), O(2)(+), O(2)(-), F(-), and SF(6)(-), offers flexibility in the detection sensitivity and specificity. Furthermore, the degree of fragmentation of the resulting ion can be controlled, providing more straightforward identification and quantification than with other commonly used methods, such as electron impact ionization. Examples are given of the detection of aerosols consisting of organics with various functionalities, including alkanes, alkenes, alcohols, aldehydes, ketones, and carboxylic acids. Applications of this technique to laboratory studies of atmospherically relevant aerosol reactions are discussed.

Journal Article↗
PubMed2005

Tertiary amine appended derivatives of N-(3,5-dinitrobenzoyl)leucine as chiral selectors for enantiomer assays by electrospray ionization mass spectrometry.

Derivatives of the chiral selector N-(3,5-dinitrobenzoyl)leucine were prepared and used as chiral selectors for enantiomer discrimination in single-stage electrospray ionization mass spectrometric experiments. The chiral selectors were designed to remove the ionization site from the sites required for effective chiral recognition. Addition of a chiral analyte to a solution of the two pseudoenantiomeric chiral selectors, which differ in absolute stereochemistry and the length of the tether connecting the tertiary amine site used for ionization via protonation and the rest of the chiral selector, affords selector-analyte complexes in the electrospray ionization mass spectrum where the ratio of these complexes is dependent on the enantiomeric composition of the analyte. The relationship between the ratio of the selector-analyte complexes in the electrospray ionization mass spectrum and the enantiomeric composition of the analyte can be used to relate the extent of enantioselectivity that is being observed and for quantitative enantiomeric composition determinations. Investigations into the scope and limitations of this method, plus a comparison to the enantioselectivities observed by chiral HPLC using a N-(3,5-dinitrobenzoyl)leucine-derived chiral stationary phase, is presented.

Journal Article↗
PubMed2006

Atmospheric pressure photoionization for ionization of both polar and nonpolar compounds in reversed-phase LC/MS.

Atmospheric pressure photoionization can provide high ionization efficiency simultaneously to both polar and nonpolar compounds delivered in reversed-phase solvent. The method to achieve this utilizes toluene as a dopant and simply requires that the solvent flow be limited so that reactions between toluene photoions and the organic component of the solvent are not driven to completion. Under these conditions, toluene photoions remain in the source for ionizing nonpolar compounds via charge exchange (electron transfer), while protonated solvent ions are available for proton-transfer reactions with polar molecules. The reagent ion mixture is then suitable for ionizing a wide range of both polar and nonpolar compounds. The critical effect of solvent flow rate is demonstrated here with results for a test analyte, 9-methylanthracene, which may be ionized by either charge exchange or proton transfer. For a solvent of 50:50 methanol/water (v/v), lowering the flow from 200 to 50 microL min-1 results in a 10x increase in charge exchange ionization efficiency--further flow reductions provide even greater enhancements. This method is compatible with sample delivery by direct infusion and micro- and narrow-bore LC, as well as conventional LC using a flow splitter.

Journal Article↗
PubMed2000

Secondary electrospray ionization ion mobility spectrometry/mass spectrometry of illicit drugs.

A secondary electrospray ionization (SESI) method was developed as a nonradioactive ionization source for ion mobility spectrometry (IMS). This SESI method relied on the gas-phase interaction between charged particles created by electrospray ionization (ESI) and neutral gaseous sample molecules. Mass spectrometry (MS) was used as the detection method after ion mobility separation for ion identification. Preliminary investigations focussed on understanding the ionization process of SESI. The performance of ESI-IMS and SESI-IMS for illicit drug detection was evaluated by determining the analytical figures of merit. In general, SESI had a higher ionization efficiency for small volatile molecules compared with the electrospray method. The potential of developing a universal interface for both GC- and LC-MS with an addition stage of mobility separation was demonstrated.

Electrochemistry↗
PubMed2003

Ionization of a critical adenosine residue in the neurospora Varkud Satellite ribozyme active site.

The Varkud Satellite (VS) ribozyme catalyzes a site-specific self-cleavage reaction that generates 5'-OH and 2',3'-cyclic phosphate products. Other ribozymes that perform an equivalent reaction appear to employ ionization of an active site residue, either to neutralize the negatively charged transition state or to act as a general acid-base catalyst. To test for important base ionization events in the VS ribozyme ligation reaction, we performed nucleotide analogue interference mapping (NAIM) with a series of ionization-sensitive adenosine and cytidine analogues. A756, a catalytically critical residue located within the VS active site, was the only nucleotide throughout the VS ribozyme that displayed the pH-dependent interference pattern characteristic of functional base ionization. We observed unique rescue of 8-azaadenosine (pK(a) 2.2) and purine riboside (pK(a) 2.1) interference at A756 at reduced reaction pH, suggestive of an ionization-specific effect. These results are consistent with protonation and/or deprotonation of A756 playing a direct role in the VS ribozyme reaction mechanism. In addition, NAIM experiments identified several functional groups within the RNA that play important roles in ribozyme folding and/or catalysis. These include residues in helix II, helix VI (730 loop), the II-III-VI and III-IV-V helix junctions, and loop V.

Adenosine↗
PubMed2006

Toward the accurate first-principles prediction of ionization equilibria in proteins.

The calculation of pK(a) values for ionizable sites in proteins has been traditionally based on numerical solutions of the Poisson-Boltzmann equation carried out using a high-resolution protein structure. In this paper, we present a method based on continuous constant pH molecular dynamics (CPHMD) simulations, which allows the first-principles description of protein ionization equilibria. Our method utilizes an improved generalized Born implicit solvent model with an approximate Debye-Hückel screening function to account for salt effects and the replica-exchange (REX) protocol for enhanced conformational and protonation state sampling. The accuracy and robustness of the present method are demonstrated by 1 ns REX-CPHMD titration simulations of 10 proteins, which exhibit anomalously large pK(a) shifts for the carboxylate and histidine side chains. The experimental pK(a) values of these proteins are reliably reproduced with a root-mean-square error ranging from 0.6 unit for proteins containing few buried ionizable side chains to 1.0 unit or slightly higher for proteins containing ionizable side chains deeply buried in the core and experiencing strong charge-charge interactions. This unprecedented level of agreement with experimental benchmarks for the de novo calculation of pK(a) values suggests that the CPHMD method is maturing into a practical tool for the quantitative prediction of protein ionization equilibria, and this, in turn, opens a door to atomistic simulations of a wide variety of pH-coupled conformational phenomena in biological macromolecules such as protein folding or misfolding, aggregation, ligand binding, membrane interaction, and catalysis.

Hydrogen-Ion Concentration↗
PubMed2001

The heaviest group 2 Difluoride, RaF(2): geometry and ionization energy.

The heaviest group 2 difluoride, radium difluoride (RaF(2)), is studied for the first time. A basis set is employed for radium that combines an effective core potential with a large, flexible valence space. This basis set is tested by calculating the first and second ionization energies of Ra, where values in excellent agreement with experiment are obtained. MP2 ab initio calculations are employed to obtain the optimized equilibrium geometry and the harmonic vibrational frequencies for the ground-state neutral and some of the low-lying cationic states. In addition, the optimized geometry of the ground-state neutral is obtained at the CCSD(T) level. The trend in geometry of the group 2 difluorides is confirmed, with an angle FRaF bond angle of 118 degrees being calculated, together with a bond length of 2.30 A. It is concluded that involvement of the Ra 6p orbitals in the valence molecular orbitals of RaF(2) adequately explains its bent geometry. RCCSD(T) calculations are then employed to calculate ionization energies. The first adiabatic ionization energy, corresponding to the ionization (tilde)X(2)Sigma(u)(+) <-- (tilde)X(1)A(1), is calculated to be 10.67 +/- 0.05 eV. The ordering of the ionic states is discussed in the light of the present results. The polarizability of Ra was calculated to be 36.3 A(3), with that of Ra(2+) was calculated to be 1.97 A(3); the latter was employed in a simple ionic model, but this failed to calculate the ionization energy accurately.

Journal Article↗
PubMed2002

Dramatic changes in geometry after ionization: experimental and theoretical studies on the electronic properties of fluorocarbonyl (mono-, di-, and tri-) sulfur compounds.

In this work, we present a complete study on He I photoelectron spectroscopy (PES) for the fluorocarbonyl mono-, di-, and trisulfur compounds FC(O)SCl, FC(O)SSCH(3), and FC(O)SSSC(O)F. After optimizations of the structure for stable conformers at different levels of theory, a complete theoretical study involving the calculation of the ionization energies using orbital valence Green's functional (OVGF) was performed. Calculations of radical-cationic forms were carried out in order to compare their properties with those of the neutral molecules. The first IP values are 10.7, 9.0, and 10.5 eV for FC(O)SCl, FC(O)SSCH(3), and FC(O)SSSC(O)F, respectively. The groups bonded to the S atom mainly influence the ionizations originating from the sulfur lone pairs. A wide electronic delocalization in the FC(O)S moiety can be deduced from experimental and theoretical results, which leads to a strong energetic stabilization of the n' '(S) (sulfur lone pair pi orbital). Other conclusions relate to effects on the substituents attached to the S atom and the importance of the molecular planarity in the orbital stabilization of the FC(O)S moiety for the neutral molecules. It is worthwhile mentioning that FC(O)SCl retains its planar structure after ionization, but drastic changes occur in the geometry of both FC(O)SSCH(3) and FC(O)SSSC(O)F. The FC(O)SSCH(3) molecule adopts a heavy atom planar structure after ionization. The FC(O)SSS moiety becomes a planar form after the ionization of the FC(O)SSSC(O)F molecule, whereas the second C(O)F group maintains its original conformation with respect to the SSS group.

Journal Article↗
PubMed2005

Thermochemical properties of the ammonia-water ionized dimer probed by ion-molecule reactions.

The thermochemical properties of some small clusters such as the (H2O)2*+ dimer have already been investigated by both experimental and theoretical methods. The recent method to selectively prepare the ammonia-water ionized dimer [NH3, H2O]*+ (and not its proton transfer isomer [NH4+, OH*]) allowed us to study its chemical reactivity. This study focuses on the charge and proton transfer pathways: Ion-molecule reactions in the cell of an FT-ICR mass spectrometer were carried out with a range of organic compounds. Examination of the reactivity of the [NH3, H2O]*+ ionized dimer versus ionization energy and proton affinity of the neutral reagents shows a threshold in the reactivity in both instances. This leads to a bracketing of thermochemical properties related to the dimer. From these experiments and in agreement with ab initio calculations, the adiabatic recombination energy of the [NH3, H2O]*+ dimer was evaluated at -9.38 +/- 0.04 eV. The proton affinity bracketing required the reevaluation of two reference gas-phase basicity values. The results, in good agreement with the calculation, lead to an evaluation of the proton affinity of the [NH2*, H2O] dimer at 204.4 +/- 0.9 kcal mol(-1). These two experimental values are respectively related to the ionization energy of NH3*+ and to the proton affinity of NH2* by the difference in single water molecule solvation energies of ionized ammonia, of neutral ammonia, and of the NH2* radical.

Journal Article↗
PubMed2005

Adsorption, ionization, and migration of hydrogen chloride on ice films at temperatures between 100 and 140 K.

Adsorption of hydrogen chloride (HCl) on water ice films is studied in the temperature range of 100-140 K by using Cs+ reactive ion scattering (Cs+ RIS), low energy sputtering (LES), and temperature-programmed-desorption mass spectrometry (TPDMS). At 100 K, HCl on ice partially dissociates to hydronium and chloride ions and the undissociated HCl exists in two distinct molecular states (alpha- and beta-states). Upon heating of the ice films, HCl molecules in the alpha-state desorb at 135-150 K, whereas those in the beta-state first become ionized and then desorb via recombinative reaction of ions at 170 K. An adsorption kinetics study reveals that HCl adsorption into the ionized state is slightly favored over adsorption into the molecular states at 100 K, leading to earlier saturation of the ionized state. Between the two molecular states, the beta-state is formed first, and the alpha-state appears only at high HCl coverage. At 140 K, ionic dissociation of HCl is completed. The resulting hydronium ion can migrate into the underlying sublayer to a depth <4 bilayers, suggesting that the migration is assisted by self-diffusion of water molecules near the surface. When HCl is covered by a water overlayer at 100 K, its ionization efficiency is enhanced, but a substantial portion of HCl remains undissociated as molecules or contact ion pairs. The observation suggests that three-dimensional surrounding by water molecules does not guarantee ionic dissociation of HCl. Complete ionization of HCl requires additional thermal energy to separate the hydronium and chloride ions.

Journal Article↗
PubMed2005

A modified Poisson-Boltzmann model including charge regulation for the adsorption of ionizable polyelectrolytes to charged interfaces, applied to lysozyme adsorption on silica.

The equilibrium adsorption of polyelectrolytes with multiple types of ionizable groups is described using a modified Poisson-Boltzmann equation including charge regulation of both the polymer and the interface. A one-dimensional mean-field model is used in which the electrostatic potential is assumed constant in the lateral direction parallel to the surface. The electrostatic potential and ionization degrees of the different ionizable groups are calculated as function of the distance from the surface after which the electric and chemical contributions to the free energy are obtained. The various interactions between small ions, surface and polyelectrolyte are self-consistently considered in the model, such as the increase in charge of polyelectrolyte and surface upon adsorption as well as the displacement of small ions and the decrease of permittivity. These interactions may lead to complex dependencies of the adsorbed amount of polyelectrolyte on pH, ionic strength, and properties of the polymer (volume, permittivity, number, and type of ionizable groups) and of the surface (number of ionizable groups, pK, Stern capacity). For the adsorption of lysozyme on silica, the model qualitatively describes the gradual increase of adsorbed amount with pH up to a maximum value at pHc, which is below the iso-electric point, as well as the sharp decrease of adsorbed amount beyond pHc. With increasing ionic strength the adsorbed amount decreases (for pH > pHc), and pHc shifts to lower values.

Adsorption↗
PubMed2006

Collision-energy-resolved penning ionization electron spectroscopy of phenylacetylene and diphenylacetylene by collision with He*(2(3)s) metastable atoms.

Penning ionization of phenylacetylene and diphenylacetylene upon collision with metastable He*(2(3)S) atoms was studied by collision-energy-/electron-energy-resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES). On the basis of the collision energy dependence of partial ionization cross-sections (CEDPICS) obtained from 2D-PIES as well as ab initio molecular orbital calculations for the approach of a metastable atom to the target molecule, anisotropy of interaction between the target molecule and He*(2(3)S) was investigated. For the calculations of interaction potential, a Li(2(2)S) atom was used in place of He*(2(3)S) metastable atom because of its well-known interaction behavior with various targets. The results indicate that attractive potentials localize in the pi regions of the phenyl groups as well as in the pi-conjugated regions of the acetylene group. Although similar attractive interactions were also found by the observation of CEDPICS for ionization of all pi MOs localized at the C[triple bond]C bond, the in-plane regions have repulsive potentials. Rotation of the phenyl groups about the C[triple bond]C bond can be observed for diphenylacetylene because of a low torsion barrier. So the examination of measured PIES was performed taking into consideration the change of ionization energies for conjugated molecular orbitals.

Journal Article↗
PubMed2006

Ionization state and structure of l-1,2-dipalmitoylphosphatidylglycerol monolayers at the liquid/air interface.

Phosphatidylglycerols are components of biological membranes. The phase behavior of these phospholipids was extensively investigated. However, there is still no definite picture about the dependence of the ionization state and monolayer structure on subphase composition. The major problem of previous investigations is that none of the methods used allow obtaining the ionization degree directly. In the present work we apply techniques developed in the past decades for Langmuir monolayers: infrared reflection absorption spectroscopy (IRRAS) as well as X-ray diffraction and reflectivity techniques, which provide straightforward information about structure and ionization state of a L-1,2-dipalmitoylphosphatidylglycerol (DPPG) monolayer. The Gouy-Chapman model is applied to evaluate the intrinsic pKa. Therewith, the ionization degree can be determined even at low pH values. The experimental titration curves are in good agreement with theoretical curves based on the Gouy-Chapman model. The obtained instrinic pKa amounts to 1. The ionization degree of a DPPG monolayer is independent of the monovalent cation size. In contrast, the structure of a DPPG monolayer is strongly affected by the type of divalent cations.

Air↗
PubMed2006

A study of cysteamine ionization in solution by Raman spectroscopy and theoretical modeling.

Different cysteamine (H2N-CH2-CH2-SH) ionization forms have been studied by polarized Raman spectroscopy in solutions prepared with H2O and D2O and by DFT calculations at the B3LYP/6-31++G(d,p) level. To account for solvation effects, we employed the integral equation formalism polarizable continuum model (IEFPCM) option and explicit water molecules. Calculated relative energies and Raman spectra revealed that gauche rotamers around the C-C bond are the most stable conformers in solution. The experimental pKa values and Raman spectra of various ionization forms were best predicted by using a model with three explicit water molecules and the IEFPCM option. In general, the use of IEFPCM tends to lower the calculated frequencies for a few bands, but in some cases (S-H stretching mode) this effect is expressed very strongly. Potential energy distribution (PED) analysis of gauche conformers of various cysteamine ionization forms provided the possibility of discriminating spectroscopically methylene groups adjacent to sulfur, (CH2)S, and nitrogen, (CH2)N, sites. In general, stretching and scissoring modes as well as wagging and twisting vibrations of the (CH2)N group were found to be at higher frequencies. The influence of ionization of SH and NH2 groups on the vibrational spectrum is discussed, and Raman markers for further amine group ionization studies are suggested.

Cysteamine↗
PubMed1998

pH-metric logP 10. Determination of liposomal membrane-water partition coefficients of ionizable drugs.

PURPOSE: To investigate a novel approach for the determination of liposomal membrane-water partition coefficients and lipophilicity profiles of ionizable drugs. METHODS: The measurements were performed by using a pH-metric technique in a system consisting of dioleylphosphatidylcholine (DOPC) unilamellar vesicles in 0.15 M KCl at 25 degrees C. The DOPC unilamellar vesicle suspension was prepared via an extrusion process. RESULTS: The liposomal membrane-water partition coefficients of eight ionizable drugs: ibuprofen, diclofenac, 5-phenylvaleric acid, warfarin, propranolol, lidocaine, tetracaine and procaine were determined and the values for neutral and ionized species were found to be in the ranges of approximately 4.5 to 2.4 and 2.6 to 0.8 logarithmic units, respectively. CONCLUSIONS: It has been shown that the liposomal membrane-water partition coefficients as derived from the pH-metric technique are consistent with those obtained from alternative methods such as ultrafiltration and dialysis. It was found that in liposome system, partitioning of the ionized species is significant and is influenced by electrostatic interaction with the membranes. We have demonstrated that the pH-metric technique is an efficient and accurate way to determine the liposomal membrane-water partition coefficients of ionizable substances.

Drug Carriers↗
PubMed1997

Ataxia telangiectasia mutant protein activates c-Abl tyrosine kinase in response to ionizing radiation.

Ataxia telangiectasia (AT) is a rare human autosomal recessive disorder with pleiotropic phenotypes, including neuronal degeneration, immune dysfunction, premature ageing and increased cancer risk. The gene mutated in AT, ATM, encodes a putative lipid or protein kinase. Most of the human AT patient phenotypes are recapitulated in Atm-deficient mice. Cells derived from Atm-/- mice, like those from AT patients, exhibit abnormal response to ionizing radiation. One of the known responses to ionizing radiation is the activation of a nuclear tyrosine kinase encoded by the c-abl proto-oncogene. Ionizing radiation does not activate c-Abl in cells from AT patients or in thymocytes or fibroblasts from the Atm-deficient mice. Ectopic expression of a functional ATM kinase domain corrects this defect, as it phosphorylates the c-Abl tyrosine kinase in vitro at Ser 465, leading to the activation of c-Abl. A mutant c-Abl with Ser 465 changed to Ala 465 is not activated by ionizing radiation or ATM kinase in vivo. These findings identify the c-Abl tyrosine kinase as a downstream target of phosphorylation and activation by the ATM kinase in the cellular response to ionizing radiation.

3T3 Cells↗