Do b‐ions occur from vibrational excitation upon H‐desorption in electron capture dissociation?

Electron capture dissociation (ECD) has recently been shown in some cases to produce abundant N-terminal b-ion peptide fragments. These product ions are usually only observed when activation occurs via vibrational excitation as in collision-induced dissociation (CID). Here, we show that occurrence of b-ions in the ECD spectra of synthetic peptides are correlated with low gas-phase basicity and that the observed b-ion fragments are N-terminal products. Furthermore, all ECD spectra containing b-ions also had abundant losses of hydrogen and ammonia from the charge-reduced species.     

Multitracer study on adsorption of metal ions on α-Fe2O3

pH dependence of the adsorption of Na, Sc, Ga, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, and Pd on -Fe₂O₃ from a 0.1 mol dm^–3 NaCl solution was studied by using a multitracer technique. Desorption of the metal ions from the -Fe₂O₃ with the adsorbed metal ions at pH 11 was also studied by lowering the pH of the suspensions. The desorption curve of each element was in good agreement with the adsorption curve except for Ru and Rh under conditions studied. Adsorption kinetics showed that the adsorption of most metal ions increases with shaking time before an adsorption equilibrium is attained. An increase in the adsorption was also observed with an elevation in temperature for the elements, suggesting that the adsorption is involved in chemisorption.     

Quantitative aspects of analyzing small molecules – monitoring singly or doubly charged ions? A case study of ximelagatran

Precision, reproducibility and lower limit of quantitation (LLOQ) are important characteristics of a quantitative method. We have investigated these properties for Ximelagatran (Xi), which has a high tendency to form doubly charged ions in electrospray ionization (ESI), by studying the percentage of doubly charged species formed when varying the formic acid (FA) concentration, analyte concentration, amount of organic modifier and flow rate. It was found that the percentage of [Xi + 2H]²⁺ can be controlled to be more than 90% or less than 10% by varying the amount of FA present, and that the change between these values is dramatic. Furthermore, the percentage of [Xi + 2H]²⁺ formed decreases with increased analyte concentration and increased flow rate. No apparent relationship with the amount of organic modifier was found. The results have the implication that, by carefully controlling the selected parameters, the LLOQ, precision and reproducibility can be improved. We have compared the fragmentation of the singly and doubly charged species and concluded that the [Xi + 2H]²⁺ ion is more inclined to undergo fragmentation than [Xi + H]⁺. As a consequence, unusual instrumental settings had to be used for the experiments. The fragmentation patterns are to a great extent similar, but the doubly charged species is more inclined to generate low‐mass product ions. Copyright © 2010 John Wiley & Sons, Ltd.     

A study on the mechanism of reaction between BNAH and chloranil:Evidence for electron transfer

The reaction between chloranil and N-benzyldihydronicotinamide(BNAH)in boratebuffer/DMF was investigated.The reaction mixture gave a strong esr signal,which is consistentwith that of chloranil anion radical,and tetrachlorohydrophenol(QH_2)and N-benzylnicotinamide(BNA~+)were obtained as the sole products.When the reaction was run in benzene solution,a greencoloured charge-transfer complex between the reactants could be isolated,which decomposed in polarsolvents to give BNA-+ and QH_2.Based on kinetic studies by esr spectroscopy by the stopped-flowtechnique,a two-step electron-transfer mechanism for the reactionis proposed in contrast to thehydride-transfer mechanism reported in the literature.     

Can a dipole-bound electron form a pseudo-atom? An atoms-in-molecules study of the hydrated electron

Non-nuclear local maxima, or attractors, of electron density are a rare but very interesting feature of the electron density distribution in molecules and solids. Recently, non-nuclear attractors (NNAs) and the corresponding pseudoatoms of electron density have been identified with the quantum theory of atoms in molecules for some anionic clusters formed by several polar solvent molecules and an excess electron bound in either a solvated-electron or dipole-bound fashion. This contribution reports a detailed study of the topology of the electron density for a series of dipole-bound water cluster anions, as calculated with Hartree-Fock, M?ller-Plesset perturbation theory, and coupled-cluster methods together with basis sets augmented with extra diffuse basis functions to accommodate the excess electron. For dipole-bound clusters, electron densities obtained with insufficient inclusion of electron correlation effects and tight basis sets feature a well-pronounced pseudoatom due to the excess electron, which ultimately disappears when a higher level of electronic structure theory and a more diffuse basis set are used. On the other hand, for solvated-electron clusters, where the excess electron is surrounded by solvent molecules, the existence of NNAs does not seem to be an artifact of the method employed, but rather a genuine feature of the electron density distribution. Pseudoatoms of electron density thus appear to be an exclusive feature of confined environments and are unlikely to be found on the tip of a cluster dipole or on solid surfaces.     

Protonation effects on electron transport through diblock molecular junctions:A theoretical study

Diblock oligomers are widely used in molecular electronics. Based on fully self-consistent nonequilib-rium Green's function method and density functional theory, we study the electron transport properties of the molecular junction with a dipyrimidinyl-diphenyl (PMPH) diblock molecule sandwiched between two gold electrodes. Effects of different kinds of molecule-electrode anchoring geometry and protona-tion of the PMPH molecule are studied. Protonation leads to both conductance and rectification en-hancements. However, the experimentally observed rectifying direction inversion is not found in our calculation. The preferential current direction is always from the pyrimidinyl to the phenyl side. Our calculations indicate that the protonation of the molecular wire is not the only reason of the rectification inversion.     

1,3‐O‐Transposition or Trisubstituted Z‐Enol Ester? A Comparative Study of Reactions of Ynones

Ynones are useful substrates for transition‐metal‐mediated synthesis. The Au^I‐catalyzed 1,3‐O‐transposition is an important reaction of ynones. Recently, an efficient Cu^I‐catalyzed synthesis of trisubstituted Z‐enol esters via interrupting the traditional 1,3‐O‐transposition reaction of ynones was reported by Zhu's group. Herein, density functional theory studies disclosed that the hydrogen bond formed by carboxylic acid plays an important role for the reactivity and selectivity in this novel reaction. A qualitative rule was also found to explain the substituent effect in the ynone substrate, and this is consistent with experiments. The Au^I‐catalyst and Cu^I‐catalyst were further compared to interpret the essential cause of why the Au^I‐catalyst prefers the 1,3‐O‐transpostion reaction. These conclusions might be helpful for the rational design of reactions of ynones.     

RAFT‐Mediated Polymerization—A Story of Incompatible Data?

The mechanistic interpretation of kinetic anomalies in reversible addition–fragmentation chain transfer (RAFT)‐mediated polymerization is critically reviewed. The main conclusion of this exercise is that available data do not allow model discrimination between the two prevailing mechanistic schemes, i.e., the slow fragmentation model and the intermediate radical termination model. However, assessment of the rate parameters reveals that the incompatibilities may not be as large as previously reported in literature. Dedicated kinetic studies on model compounds should be performed to shed further light on the seemingly incompatible data that currently exists in literature.

     

A DFT study on the correlation between topology and Bader charges: Part IV,on the change of atomic charges in polymorphic transitions – A case study on CaCl2

Referring to the experimental results of high pressure experiments of Léger et al. (1998) we have calculated the energies of all phases observed for CaCl₂ within the DFT formalism using the VASP package, and we have retrieved enthalpies and transition pressures. All phases can be considerably compressed or dilated without much change in energy. This energetic “softness” could even be quantified. We classify the high temperature TiO₂-type structure and the PbCl₂-type one at highest pressures as the energetically “softest” ones and the SrI₂-type one as the “hardest”. We furthermore discuss the energy density (E/V) of the different phases and redefine it as a fictive cohesive pressure within these structures.Pursuing our earlier approaches we have analysed the charges of the atoms in the different CaCl₂ phases and their change on compression or dilation. On comparing the gradients of the charge curves we define a sort of “charge hardness” which will generally depend on the type of cation–anion pair but also on their topological connection in the respective structures. We speculate that exhausting the “charge softness or hardness” of individual ions in such arrangements may initiate the structural reorganization at the transition pressures.     

Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry—III. The lanthanum modifier and the determination of phosphorus

Morphological studies on graphite surfaces by scanning electron microscopy are presented for platforms made from pyrolytic graphite, and for polycrystalline electrographite tubes with pyrolytic graphite coating in which phosphorus was determined without and with the addition of higher concentrations of lanthanum as the modifier. Lanthanum causes severe pitting and corrosion of the graphite surface already after relatively few determinations, and definite indication was found for the formation of intercalation compounds between lanthanum and graphite. No sign was found, however, for the formation of a dense coating of lanthanum carbide as proposed by several authors. The mechanism for the increase of phosphorus sensitivity is most probably the formation of a thermally stable compound involving lanthanum and phosphorus which leads to vaporization of phosphorus at high enough temperatures to obtain sufficient atomization and useful analytical signals. This is supported by the morphological changes of the graphite surface observed after application of higher lanthanum concentrations, and the resulting increased number of active carbon sites. Phosphorus alone also causes substantial corrosion of graphite, but with a completely different pattern. A very pronounced secondary coating of tube and platform wall is observed in the absence of lanthanum which is most probably supported by the formation and decomposition of compounds between phosphorus and graphite.     

899 — A potentiometric study on the redox properties of hemoglobin from Camelus dromedarius

A potentiometric study of the redox properties of hemoglobin from Camelus dromedarius has been performed, with the aim of having a better insight in the structure-function relationships of this protein; data on human hemoglobin have been included for comparative purposes. Analysis of the experimental data has shown:

• 1.the existence of intermediate conformers (neither R nor T) in the oxidized form of dromedary hemoglobin;
• 2.the amino acid groups involved in the oxidation Bohr effect of dromedary and human hemoglobins are likely to be the same; thus, only invariant amino acid residues seem to be involved in this effect.

     

A comparative adsorption study of β-naphthol on four polymeric adsorbents from aqueous solutions

Adsorptive removal of aromatic compounds from aqueous solutions by polymeric adsorbents has attracted many concerns in recent years. A comparative adsorption study including equilibria, kinetics and column dynamics of β-naphthol from aqueous solutions was carried out using two hypercrosslinked polymeric adsorbents (HJ-1 and TEPA) we developed and two commercial polymeric adsorbents (XAD-4 and XAD-7). The adsorption isotherms could be well described by the Freundlich equation, and the adsorption equilibrium capacities of β-naphthol followed an order of q(e)((TEPA))>q(e)((HJ-1))>q(e)((XAD-4))>q(e)((XAD-7)). The isosteric adsorption enthalpies on HJ-1 and TEPA decreased with increasing adsorption fractional loading, while a constant enthalpy was observed for XAD-4 and XAD-7, implying that HJ-1 and TEPA had a heterogeneous surface while XAD-4 and XAD-7 possessed a homogenous surface. The surface energetic heterogeneity of HJ-1 and TEPA could be well characterized by the Do's model. The adsorption kinetics were fitted by both pseudo-first-order and pseudo-second-order rate equations, and the intra-particle diffusion was found to be the rate-limiting step. The adsorption breakthrough data were well correlated by the Thomas and Clark models, and the dynamic capacities for TEPA, HJ-1, XAD-4 and XAD-7 adsorbents were calculated to be 341.7, 321.6, 268.0 and 173.8 mg/g dry resin, which were within 90% of the corresponding equilibrium capacities obtained in the batch experiments.     

Theoretical study on decomposition of γ-halo allylalkoxide and β-halo acrylate ions

β, γ-Substituted γ-halo allylalkoxide ions decompose to form a halogen ion, formaldehyde, and an alkyne under mild conditions, for example at room temperature. The E isomer does not differ from the Z isomer in terms of activation energy. We attempted to shed light on the mechanism of the reaction by using ab initio molecular orbital calculations. The observed propensity was confirmed by the present calculation on model molecules, γ-chloro allylalkoxide ions. We conducted further calculations and compared the alkoxide results with a similar reaction of β-haloacrylate ions that release carbon dioxide instead of formaldehyde. This similar reaction needs heating as high as 150°C. The activation energy of the acrylate ions (36–39 kcal mol⁻¹) was calculated to be about 10 kcal mol⁻¹ higher than that of the alkoxide ions. The activation energy of the E acrylate ion is smaller by 0.8 kcal mol⁻¹ than that of the Z isomer at the MP2/6-31+G**//RHF/6-31+G* level of theory. This is consistent with experimental results. While the ready deprotonation from the carboxylic group does not activate the acrylate ion very much, the alkoxide ion is destabilized to a great degree in the process of anion formation. The difficulty in deprotonation that proceeds from the neutral molecule is seen in the difference in the activation energies for the decomposition of the corresponding anions. Therefore, the pK _a of a hydroxy or a carboxylic group plays the leading role in determining the magnitude of activation energies of allyl halides with a negatively charged fragment. Received: 2 July 1998 / Accepted: 9 September 1998 / Published online: 8 February 1999     

A new compound in kidney stones? Powder X‐ray diffraction study of calcium glycinate trihydrate

The present identification of a new compound in kidney stones is relevant in clinical practice. Here, poly[[di‐μ‐aqua‐bis(glycinato‐κ²N,O)calcium(II)] monohydrate], {[Ca(C₂H₄NO₂)₂(H₂O)₂]·H₂O}_n, has been identified in a possible kidney concretion, although it could be a `false calculus' associated with Munchausen syndrome. The crystal packing is characterized by an infinite zigzag chain of Ca atoms in [Ca(OW)₄O₂N₂] (OW is a water O atom) square antiprisms, sharing edges formed by water molecules. An uncoordinated water molecule interconnects the parallel chains in a three‐dimensional hydrogen‐bonding scheme. Similarities between the trihydrate and the monohydrate are described.     

Does double electron capture lead to the formation of biradicals? An ECD-SORI-CID study on lacticin 481

We studied lacticin 481, a small lantibiotic with three lanthionine bridges, by electron capture dissociation (ECD) in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Following electron capture, very little fragmentation was observed, but species formed by nondissociative single and multiple electron capture were abundant. Ions formed by double electron capture were subjected to sustained off resonance irradiation collision induced dissociation (SORI-CID) to determine whether stable biradicals were formed. In the SORI-CID spectra of the ions formed by double electron capture, some, but minor, H* radical loss was observed, which was not observed at all for regularly protonated ions. A small part of the ions formed by double electron capture are thus long-lived biradicals. Apart from the observed H* loss, the SORI-CID spectra of ions that captured two electrons was similar to that of regularly protonated ions and quite different from the SORI-CID spectra of radical ions formed by single electron capture. This implies that recombination of the two radical sites is the dominant process in biradical lacticin 481 ions, at least on the time scale of our SORI-CID experiments.     

Is wood pre‐treatment essential for tree‐ring nitrogen concentration and isotope analysis?

Tree-ring nitrogen concentrations and isotope ratios (δ(15)N) are gaining in popularity for environmental research although their use is still debated because of nitrogen mobility in tree stems. Modern studies generally present results on wood that is pre-treated to remove soluble nitrogen compounds and to minimize the impact of radial translocation on tree-ring nitrogen environmental records. However, the necessity to use such pre-treatment has never been fully assessed. Here we compare the nitrogen concentrations and δ(15)N values of two wood preparation protocols applied to beech and red spruce tree rings for the removal of soluble compounds from ring pairs with non pre-treated tree rings. For both tree species, pre-treatment did not minimize the radial patterns of tree-ring nitrogen concentrations and the increasing concentration trends that are coincident with the heartwood-sapwood boundary. Therefore, even if the tree-ring nitrogen concentrations are slightly modified by pre-treatment, these concentrations are considered to reflect internal stem processes rather than environmental conditions in both species. The δ(15)N values were similar for untreated and pre-treated ring pairs, suggesting that wood pre-treatment did not substantially change the δ(15)N values and temporal trends in ring series. In addition, tree-ring δ(15)N series of untreated and pre-treated wood did not show any sign of influence of the heartwood-sapwood boundary in either tree species, indicating that nitrogen translocation did not generate significant isotopic fractionation. We therefore suggest that untreated ring δ(15)N values of beech and red spruce trees can be used for environmental research.