High Proton Affinity Research Articles

Effect of charge on the conformation of highly basic peptides including the tail regions of histone proteins by ion mobility mass spectrometry

The first systematic and comprehensive study of the charging behaviour and effect of charge on the conformation of specifically constructed arginine-rich peptides and its significance to the N- and C-terminal basic tail regions of histone proteins was conducted using ion mobility mass spectrometry (IM-MS). Among the basic amino acids, arginine has the greatest impact on the charging behaviour and structures of gas phase ions by virtue of its high proton affinity. A close linear correlation was found between either the maximum charge, or most abundant charge state, that the peptides support and their average collision cross section (CCS) values measured by ion mobility mass spectrometry. The calculated collision cross sections for the lowest energy solution state models predicted by the PEP-FOLD algorithm using a modified MOBCAL trajectory method were found to best correlate with the values measured by IM-MS. In the case of the histone peptides, more compact folded structures, supporting less than the maximum number of charges, were observed. These results are consistent with those previously reported for histone dimers where neutralization of the charge at the basic residues of the tail regions did not affect their CCS values.

The adduction behavior of water reactant ions with mobility shift reagents in ion mobility spectrometry is determined by the number of locations for adduction, interaction energies, proton affinities, and steric hindrance of these species

The introduction of mobility shift reagents (SRs) into the buffer gas of mobility spectrometers yields SR-ion clusters that decrease ion mobilities and allow the separation of overlapping ions. With a large amount of papers on the introduction of SRs in ion mobility spectrometry (IMS) few investigations explain the behavior of the adducts of reactant ions with SRs and it is not clear what type of peaks to expect which obscures the interpretation of spectra. Electrospray-ionization IMS was coupled to quadrupole mass spectrometry, and 2-butanol (B), ethyl lactate (L), and methanol were introduced as SRs into the buffer gas. The hybrid functional X3LYP/6–311++(d,p) with Gaussian 09 was used for theoretical calculations of SR-ion interaction energies. Adducts of the reactant ions with B and L presented different behaviors; even at low flow rates, L consumed all sodium, reactant ions, and water by adduction, because a) in the experimental conditions, SRs were more concentrated in the buffer gas than reactant ions, b) L’s high proton affinity and c) L’s three electron-donor oxygens, increases adduction. Therefore, chemical equilibria in the buffer gas were only between L and LnH+, LmH3O+, or LxNa+ adducts and, consequently, these sets of adducts had different mobilities. The lower mobility of LmH3O+ compared to LnH+ was explained on the base of the lower steric hindrance in LH3O+ for attachment of L molecules. The behavior of reactant ions with B was different: BnH3O+ and BnH+ overlapped because the relatively low proton affinity and the single and weaker interaction site in B allowed protons and water to be exchanged between species. Finally, L4H+, L4H3O+, B4H+ and B5H+ ions, not reported before, were seen for large SR concentrations. This study explains two different behaviors of the adducts of SRs with reactant ions using interaction energies, proton affinities, steric hindrance, and the number of locations for adduction.

Deprotonation effect of tetrahydrofuran‐2‐carbonitrile buffer gas dopant in ion mobility spectrometry

When dopants are introduced into the buffer gas of an ion mobility spectrometer, spectra are simplified due to charge competition. We used electrospray ionization to inject tetrahydrofuran-2-carbonitrile (F, 2-furonitrile or 2-furancarbonitrile) as a buffer gas dopant into an ion mobility spectrometer coupled to a quadrupole mass spectrometer. Density functional theory was used for theoretical calculations of dopant-ion interaction energies and proton affinities, using the hybrid functional X3LYP/6-311++(d,p) with the Gaussian 09 program that accounts for the basis set superposition error; analytes structures and theoretical calculations with Gaussian were used to explain the behavior of the analytes upon interaction with F. When F was used as a dopant at concentrations below 1.5mmolm(-3) in the buffer gas, ions were not observed for α-amino acids due to charge competition with the dopant; this deprotonation capability arises from the production of a dimer with a high formation energy that stabilized the positive charge and created steric hindrance that deterred the equilibrium with analyte ions. F could not completely strip other compounds of their charge because they either showed steric hindrance at the charge site that deterred the approach of the dopant (2,4-lutidine, and DTBP), formed intramolecular bonds that stabilized the positive charge (atenolol), had high proton affinity (2,4-lutidine, DTBP, valinol and atenolol), or were inherently ionic (tetraalkylammonium ions). This selective deprotonation suggests the use of F to simplify spectra of complex mixtures in ion mobility and mass spectrometry in metabolomics, proteomics and other studies that generate complex spectra with thousands of peaks. Copyright © 2016 John Wiley & Sons, Ltd.

N-Terminal Derivatization with Structures Having High Proton Affinity for Discrimination between Leu and Ile Residues in Peptides by High-Energy Collision-Induced Dissociation.

De novo sequencing is still essential in the identification of peptides and proteins from unexplored organisms whose sequence information is not available. One of the remaining problems in de novo sequencing is discrimination between Leu and Ile residues. The discrimination is possible based on differences in side chain fragmentation between Leu and Ile under high-energy collision-induced dissociation (HE-CID) conditions. However, this is observed only when basic residues, such as Arg and Lys, are present near the N- or C-terminal end. It has been shown that the charge derivatization at the N-terminal end by a quarternary ammonium or phosphonium moiety facilitates the side chain fragmentation by HE-CID. However, the effective backbone fragmentation by low-energy CID (LE-CID) is often hampered in those derivatives with a fixed charge. Previously, we demonstrated that the N-terminal charge derivatization with the structures having high proton affinity induced the preferential formation of b-ions under LE-CID conditions, allowing straightforward interpretation of product ion spectra. In the present study, we further investigated whether the same derivatization approach is also effective for discrimination between Leu and Ile under HE-CID conditions. Consequently, the side chain fragmentation of Leu and Ile residues was most effectively enhanced by the N-terminal derivatization with 4-(guanidinomethyl)benzoic acid among the tested structures. This derivatization approach, which is compatible with both HE- and LE-CID analysis, offers a straightforward and unambiguous de novo peptide sequencing method.

Saalfrank-type {Fe2L3} host–guest complexes – prediction of ion selectivity by quantum chemical calculations VII

Based on density functional calculations (B3LYP/LANL2DZp (LANL2DZp=LANL2DZ augmented with polarization functions on non-hydrogen atoms)), the selectivity of the {2}-metallocryptand [Fe2L3] (L2− = 2,6-dibutane-1,3-dionylpyridine dianion) was investigated and the experimentally known K+, Sr2+ and Ba2+ cation selectivity rationalized. The rigidity of L2−, visible in the small torsion angles compared to previously investigated classical organic cryptands, can be identified as the origin of this discrimination. Derived from the hosted alkali and alkaline earth cations, the size of the {2}-metallocryptand’s [Fe2L3] cavity is analogous to the cryptand [2.2.phen]. Additionally, the quantum chemical calculations identified the {2}-metallocryptand [Fe2L3] as the first experimentally verified metalloproton sponge (calculated (B3LYP/LANL2DZp) proton affinity −259.8 kcal mol−1). This high gas-phase proton affinity clearly explains why empty [Fe2L3] was never observed, and only [(HN)3Fe2L3]3+ or [MFe2L3]m+ are known.

Simultaneous determination of benzene and phenol in heat transfer fluid by head-space gas chromatography hyphenated with ion mobility spectrometry

The quantitative determination of some compounds such as benzene and phenol in a complex matrix by ion mobility spectrometry (IMS) can be a difficult task, due to the influence of other components present in the matrix and the chemical properties of both compounds, such as their high volatility and low proton affinity. Monitoring of these compounds in a heat transfer fluid (HTF) is essential to check the correct working of a thermosolar plant and for safety and environmental reasons. Benzene and phenol, among other compounds, are produced when HTF is exposed to high temperatures in continuous cycles and their presence can decrease the efficiency of HTF. For the first time, a headspace module coupled to a gas chromatography column in combination with an IMS (with a tritium ionization source) has been optimized and fully validated to simultaneously quantify benzene and phenol in HTF. The limit of detection (LOD) and limit of quantification (LOQ) achieved with the method proposed were 0.011 and 0.038gL−1 and 0.004 and 0.014gL−1 for benzene and phenol respectively. The precision of the method was evaluated in terms of repeatability and reproducibility with all values lower than 9.2% and 13.3%, respectively. Results demonstrated that benzene and phenol were generated in the HTF heating process, and its concentration increased with heating time (approximately 483h). The average concentration values for benzene and phenol in degraded HTF samples were not significantly different to values obtained using a gas chromatography-flame ionization detector instrument. Therefore, IMS is a promising technique for in-field quality control of HTF in a thermosolar plant due to its speed, versatility, sensitivity and selectivity to quantify these degradation compounds.

Design and synthesis of a novel pre-column derivatization reagent with a 6-methoxy-4-quinolone moiety for fluorescence and tandem mass spectrometric detection and its application to chiral amino acid analysis

A new pre-column derivatization reagent with a 6-methoxy-4-quinolone (6-MOQ) moiety for amino acid analysis, 2,5-dioxopyrrolidin-1-yl(2-(6-methoxy-4-oxoquinolin-1(4H)-yl)ethyl) carbonate (6-MOQ-EtOCOOSu), was designed and synthesized. 6-MOQ is a thermo/photostable fluorophore with a high proton-affinity site and sensitive determination could be carried out by a fluorescence detector and also by an electrospray ionization mass spectrometer. Derivatization of amino acids with 6-MOQ-EtOCOOSu was completed within 1min under mild basic conditions at room temperature. The 6-MOQ derivatives of all chiral proteinogenic amino acids were separated using the combination of three enantioselective columns, Chiralpak QN–AX, Chiralpak ZXIX(+), and KSAACSP-001S, with separation factors of higher than 1.07. The present reagent enables the sensitive determination of amino acid enantiomers, and the values of LLOD using a chiral-HPLC–MS/MS system were 0.05–50fmol/injection.

Modeling and Mechanism of the Adsorption of Proton and Copper to Natural Bamboo Sawdust Using the NICA–Donnan Model

Bamboo sawdust represents an attractive low-cost adsorbent for heavy metal removal from contaminated waters. In the present study, the binding of protons and copper (Cu) ions to purified natural bamboo sawdust was investigated through acid–base titrations and Cu adsorption isotherms. The experimental data were modeled with the nonideal competitive adsorption (NICA)–Donnan equation (the NICA–Donnan model) to account for the heterogeneity of binding sites, stoichiometry of ion binding, as well as electrostatic interaction. Our results showed that the NICA–Donnan modeling of acid–base titration curves provided more accurate fitting results than the two-site surface complexation model. The stoichiometry parameters of the adsorption for Cu were both mono-dentate and binuclear, with the latter existing mainly under low ionic strength and high pH conditions. The contribution of low proton affinity and high proton affinity sites to Cu binding was dependent on pH. For high proton affinity sites, the partial inclusion of CuOH+ species adsorption led to an increase in the Cu ion affinity constant, whereas the incorporation of species adsorption decreased the affinity constant.

2-Adamantylidene and its heavier analogues: hyperconjugation versus lone pair stability and electrophilicity versus nucleophilicity.

The structure, bonding and reactivity of 2-adamantylidene and its heavier group-14 analogues, 2X (X10H14, X = C-Sn), have been studied at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. 2-Adamantylidene and its heavier analogues, 2X, have a singlet ground state, where the carbene and silylene bridges in 2C and 2Si are bent similarly to the foiled-type carbenes. The bending of the carbene and silylene bridges in 2C and 2Si as well as their stability are mainly attributed to the Cieplak-type hyperconjugative interaction between a pair of vicinal σ-bonds with the empty p-orbital on the carbon and silicon atoms. However, the inertness of the lone pair of the silicon atom also contributes to the stability of 2Si. The extent of hyperconjugative interactions is significantly less in 2Ge and 2Sn and the inertness of the lone pair is the major factor for their stability. The high proton affinity (PA) and hydride affinity (HA) of 2-adamantylidene and its heavier analogues suggest their ambiphilic nature. The high PA and HA of 2X can be attributed to the stabilization of the protonated adduct, 4X, and the hydride adduct, 5X, by the Cieplak-type and Felkin-Anh-type hyperconjugations, respectively. However, the nucleophilicity of 2X decreases and electrophilicity increases when X changes from C to Sn.

Energetics of Lateral Membrane Proton Diffusion

The diffusion of protons along biological membranes is vitally important for cellular energetics. The weak dependence of both migration speed and span on lipid composition suggests that proton migrate along water (Springer et al., 2011). However, it is unclear how to reconcile the apparently high proton affinity to the phase boundary (Zhang et al., 2012) with the poor proton acceptability of water, i.e. with the low pK(=0) value of water. Here we monitored the diffusion of excess protons along the phosphatidylcholine lipid bilayer/water interface at different temperatures. The kinetics of proton arrival from a distant spot of proton release to lipid-anchored fluorescent pH-sensitive dyes indicated that the in vitro Gibbs activation energy ΔG‡ for proton surface-to-bulk release harbours only a minor enthalpic constituent. We observed that more than 2/3 of ΔG‡ are entropic in origin, which explains the high proton affinity to membranes in the absence of a potent proton acceptor. This work was supported by Grant P25981 from the Austrian Science Fund (FWF) to P.P.1. Springer, A., Hagen, V., Cherepanov, D.A., Antonenko, Y.N., and Pohl, P. (2011). Protons migrate along interfacial water without significant contributions from jumps between ionizable groups on the membrane surface. Proc. Natl. Acad. Sci. U. S. A. 108, 14461-14466.2. Zhang, C., Knyazev, D.G., Vereshaga, Y.A., Ippoliti, E., Nguyen, T.H., Carloni, P., and Pohl, P. (2012). Water at hydrophobic interfaces delays proton surface-to-bulk transfer and provides a pathway for lateral proton diffusion. Proc. Natl. Acad. Sci. U. S. A. 109, 9744-9749.

Amine permeation sources characterized with acid neutralization and sensitivities of an amine mass spectrometer

Abstract. An acid titration method for quantifying amine permeation rates was used to calibrate an Ambient pressure Proton transfer Mass Spectrometer (AmPMS) that monitors ambient amine compounds. The method involves capturing amines entrained in a N2 flow by bubbling it through an acidified solution (~10−5 M HCl), and the amines are quantified via changes in solution pH with time. Home-made permeation tubes had permeation rates (typically tens of pmol s−1) that depended on the type of amine and tubing and on temperature. Calibrations of AmPMS yielded sensitivities for ammonia, methylamine, dimethylamine, and trimethylamine that are close to the sensitivity assuming a gas-kinetic, ion-molecule rate coefficient. The permeation tubes were also designed to deliver a reproducible amount of amine to a flow reactor where nucleation with sulfuric acid was studied. The high proton affinity compound dimethyl sulfoxide (DMSO), linked to oceanic environments, was also studied and AmPMS is highly sensitive to it. AmPMS was deployed recently in two field campaigns and, using these sensitivities, mixing ratios for ammonia and the alkyl amines are derived from the signals. Correlations between these species and with particle formation events are discussed.

Laser-based ion mobility spectrometer for the direct analysis of aromatic compounds in liquids

A laser-based ionization source for the direct analysis of liquid samples in ion mobility (IM) spectrometry is presented and characterized. Ionization of aromatic substances in liquids is achieved, analogous to atmospheric pressure laser ionization (APLI) in mass spectrometry, by vaporizing the liquid and subsequently ionizing the aromatic substances by resonance-enhanced multiphoton ionization (REMPI). The effects of parameters, such as composition and flow rate of the solvent as well as laser wavelength and pulse energy, are systematically investigated. The characterization of the IM spectrometer is carried out by means of selected substances from diverse fields of applications, e.g., polycyclic aromatic hydrocarbons (PAH), pesticides, wood preservatives and drug compounds. Limits of detection (LOD) down to 10 fmol and linear ranges up to three orders of magnitude are established. In addition to direct laser ionization, indirect laser ionization via dopants (toluene) for substances with low ionization efficiencies is investigated. Ionization occurs as a result of proton transfer from toluene radical cations to substances of sufficiently high proton affinities. As a result of indirect laser ionization, LOD could be decreased by up to two orders of magnitude. Ionization products are investigated by means of a combination of IM and mass spectrometer. Depending on the substance investigated primary ions (radical cations) and secondary ions (protonated molecules) resulting from ion molecule reactions are formed.

Processes of (Xe2D)*-excimer complex formation in ternary solid Kr-Xe-D2solutions

The article presents study results of VUV and UV cathodoluminescence spectra, of solid ternary Kr-Xe-D2 solutions, and Xe-D2, Kr-D2, Xe-Kr solid binary cryoalloys. The object of study is the dependence of the spectral band intensity distribution, on the concentration of impurities, and the dosage dependence of luminescence. All emission bands were identified. It is demonstrated that the formation of emitting states is caused by a transfer of energy through the system of its own electron excitation matrices. An emission from (Xe2D)* complexes is observed, if Xe is present in ternary solid solutions. It is shown that the process of forming (Xe2D)*-complexes has multiple steps, and includes the formation of an intermediate [XeD2]* non-stable state at the first stage, the nonradiative decay of which leads to the dissociation of the admixture molecular deuterium. It is suggested that the formation of [XeD2]* complexes occurs due to the high proton affinity of Xe, and its negative affinity to the electron in the D2 molecule, as a result of which, the intermediate complex acquires the features of a charge-transfer complex.

Blue-colored tert-butylamine clathrate hydrate.

Clathrate hydrates preserve active species more stably than the other icy materials and investigation of the behavior of the active species elucidates the physicochemical properties of clathrate hydrates like guest-guest interaction. Color of the tert-butylamine clathrate hydrate changes to blue after gamma irradiation and is bleachable with visible light. The electron spin resonance (ESR) spectrum at 120 K mainly consists of a triplet signal of the C-centered radical NH2C(CH3)2CH2• together with a single signal at g = 2.0008. The latter signal disappears after light exposure. These results indicate that both the blue color and the single ESR signal are derived from trapped electrons in the hydrate. They thermally decay around 140-160 K by the first-order reaction, and the activation energy is 27 kJ/mol. Since tert-butylamine molecules can capture protons due to the high proton affinity, electrons may remain in the hydrate without reacting with protons, making the hydrate blue after gamma irradiation. The long-lived trapped electrons in the tert-butylamine hydrate have an advantage to investigate those in icy materials because tert-butylamine hydrate is nonionic and has a tetra-coordinated host water network like crystalline ice without any substitution for water molecules.

Microplasma discharge vacuum ultraviolet photoionization source for atmospheric pressure ionization mass spectrometry.

In this paper, we demonstrate the first use of an atmospheric pressure microplasma-based vacuum ultraviolet (VUV) photoionization source in atmospheric pressure mass spectrometry applications. The device is a robust, easy-to-operate microhollow cathode discharge (MHCD) that enables generation of VUV photons from Ne and Ne/H(2) gas mixtures. Photons were detected by excitation of a microchannel plate detector and by analysis of diagnostic sample ions using a mass spectrometer. Reactive ions, charged particles, and metastables produced in the discharge were blocked from entering the ionization region by means of a lithium fluoride window, and photoionization was performed in a nitrogen-purged environment. By reducing the output pressure of the MHCD, we observed heightened production of higher-energy photons, making the photoionization source more effective. The initial performance of the MHCD VUV source has been evaluated by ionizing model analytes such as acetone, azulene, benzene, dimethylaniline, and glycine, which were introduced in solid or liquid phase. These molecules represent species with both high and low proton affinities, and ionization energies ranging from 7.12 to 9.7 eV.

Electronic structure and reactivity in water splitting of the iron oxide dimers and their hexacarbonyls: A density functional study

The iron oxide dimers (FeO)2 and their peroxide isomers are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. Among the bare clusters the planar four-member ring structures are more stable than the non-planar ones and the rhombic dioxide Fe2O2 with antiferromagnetically ordered electrons on iron centers is the global minimum. Water adsorption on the bare diiron dioxide is exothermic, but dissociation does not occur. Carbonylation favors a non-planar Fe2O2 ring for both the dioxides and the peroxides and high electron density at the Fe centers is induced, evidenced by the natural charge distribution, the high proton affinity, and the values of global electronegativity and hardness. The iron dioxide hexacarbonyl Fe2O2(CO)6 is diamagnetic in the state of the global minimum. It is separated from the next low-lying triplet state by a small energy gap of 0.22 eV. Time-dependent density functional theory methods were applied to examine electron excitations from the ground state to the low-lying triplet states in the hexacarbonyls and their adsorption complexes with water. Singlet-to-triplet state excitations occur via ligand-to-metal charge transfer in the hexacarbonyls; in the adsorption complexes excitations from the oxygen lone pairs to the adsorption center also occur and they appear in the IR-visible region. The lowest energy singlet and triplet state reaction paths for water splitting were followed. On the singlet potential energy surface (PES), water splitting is spontaneous, while for the triplet PES an activation barrier of 14.1 kJ mol(-1) was determined.

Charge-Directed Peptide Backbone Dissociations of o-TEMPO-Bz-C(O)-Peptides

In the present study, we report that the charge-directed (assisted) peptide dissociation products, such as b- and y-type peptidebackbone fragments, were the major products in MS/MS and MS3 applications of some o-TEMPO-Bz-C(O)-peptide ions, whileradical-driven dissociation products, such as a/x and c/z-type fragments, were previously shown to be the major products in the freeradical initiated peptide sequencing mass spectrometry (FRIPS MS). Those o-TEMPO-Bz-C(O)-peptides share a common feature intheir sequences, that is, the peptides do not include an arginine residue that has the highest proton affinity among free amino acids. The appearance of b- and y-type fragments as major products in FRIPS MS can be understood in terms of the so-called “mobile-protonmodel”. When the proton is highly mobilized by the absence of arginine, the chare-directed peptide dissociation pathways appearto be more competitive than the radical-driven dissociation pathways, in our FRIPS experiments.

Geometric and Electronic Structure of the Mn(IV)Fe(III) Cofactor in Class Ic Ribonucleotide Reductase: Correlation to the Class Ia Binuclear Non-Heme Iron Enzyme

The class Ic ribonucleotide reductase (RNR) from Chlamydia trachomatis (Ct) utilizes a Mn/Fe heterobinuclear cofactor, rather than the Fe/Fe cofactor found in the β (R2) subunit of the class Ia enzymes, to react with O2. This reaction produces a stable Mn(IV)Fe(III) cofactor that initiates a radical, which transfers to the adjacent α (R1) subunit and reacts with the substrate. We have studied the Mn(IV)Fe(III) cofactor using nuclear resonance vibrational spectroscopy (NRVS) and absorption (Abs)/circular dichroism (CD)/magnetic CD (MCD)/variable temperature, variable field (VTVH) MCD spectroscopies to obtain detailed insight into its geometric/electronic structure and to correlate structure with reactivity; NRVS focuses on the Fe(III), whereas MCD reflects the spin-allowed transitions mostly on the Mn(IV). We have evaluated 18 systematically varied structures. Comparison of the simulated NRVS spectra to the experimental data shows that the cofactor has one carboxylate bridge, with Mn(IV) at the site proximal to Phe127. Abs/CD/MCD/VTVH MCD data exhibit 12 transitions that are assigned as d-d and oxo and OH(-) to metal charge-transfer (CT) transitions. Assignments are based on MCD/Abs intensity ratios, transition energies, polarizations, and derivative-shaped pseudo-A term CT transitions. Correlating these results with TD-DFT calculations defines the Mn(IV)Fe(III) cofactor as having a μ-oxo, μ-hydroxo core and a terminal hydroxo ligand on the Mn(IV). From DFT calculations, the Mn(IV) at site 1 is necessary to tune the redox potential to a value similar to that of the tyrosine radical in class Ia RNR, and the OH(-) terminal ligand on this Mn(IV) provides a high proton affinity that could gate radical translocation to the α (R1) subunit.

Gas-Phase and Solution Studies of Three Resorcin[4]Arene Derivatives Using Electrospray Time-of-Flight Mass Spectrometry

Electrospray ionisation mass spectrometry (ESI-MS) has been used to study the relative gas-phase proton and alkali metal (Li, Na, K and Cs) binding affinities of three different resorcin[4]arenes using the kinetic method. Collision-induced dissociation (CID) was used to study the fragmentation of resorcin[4]arene heterodimer sandwich complexes, allowing the relative binding affinity order to be established. All the alkali metal cations have the same gas-phase binding affinity order with the resorcin[4]arene host molecules. At collision energies of > or = 13eV, one of the [resorcin[4]arene+Metal]+, (Metal = Li, Na, K) ions fragmented through break-up of the resorcin[4]arene, whilst the other host resorcin[4]arene remained intact, causing an apparent change in binding affinity at high collision energy. This effect was not observed with caesium, since all complex ions dissociated readily under CID by displacement of the caesium cation. The binding affinity for the protonated resorcin[4]arenes was found to be different from the alkali metal cation binding affinity because of the higher proton affinity of the nitrogen-containing resorcin[4]arenes. It is shown that resorcin[4]arenes containing an oxazine ring can be converted into a ring-opened derivative via an Eschweiler-CLarke reaction in the presence of formic acid. A second ring-opening process also occurs, including a hydrolysis reaction that results in apparent Losses of 12 mass units from the intact resorcin[4]arene. Both these reactions occur in solution before mass spectrometric investigation and cannot be achieved by CID. This observation was confirmed by inducing the Eschweiter-CLarke reaction in a model benzoxazine compound.

PREDICTIONS OF ION PRODUCTION RATES AND ION NUMBER DENSITIES WITHIN THE DIAMAGNETIC CAVITY OF COMET 67P/CHURYUMOV-GERASIMENKO AT PERIHELION

We present a one-dimensional ion chemistry model of the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko, the target comet for the ESA Rosetta mission. We solve the continuity equations for ionospheric species and predict number densities of electrons and selected ions considering only gas-phase reactions. We apply the model to the subsolar direction and consider conditions expected to be encountered by Rosetta at perihelion (1.29 AU) in 2015 August. Our default simulation predicts a maximum electron number density of {approx}8 Multiplication-Sign 10{sup 4} cm{sup -3} near the surface of the comet, while the electron number densities for cometocentric distances r > 10 km are approximately proportional to 1/r {sup 1.23} assuming that the electron temperature is equal to the neutral temperature. We show that even a small mixing ratio ({approx}0.3%-1%) of molecules having higher proton affinity than water is sufficient for the proton transfer from H{sub 3}O{sup +} to occur so readily that other ions than H{sub 3}O{sup +}, such as NH{sub 4} {sup +} or CH{sub 3}OH{sub 2} {sup +}, become dominant in terms of volume mixing ratio in part of, if not throughout, the diamagnetic cavity. Finally, we test how the predicted electron and ion densities are influenced by changesmore » of model input parameters, including the neutral background, the impinging EUV solar spectrum, the solar zenith angle, the cross sections for photo- and electron-impact processes, the electron temperature profile, and the temperature dependence of ion-neutral reactions.« less