Gas-phase Activation Research Articles

Ion Mobility-Mass Spectrometry Reveals Conformational Changes in Charge Reduced Multiprotein Complexes

Characterizing intact multiprotein complexes in terms of both their mass and size by ion mobility-mass spectrometry is becoming an increasingly important tool for structural biology. Furthermore, the charge states of intact protein complexes can dramatically influence the information content of gas-phase measurements performed. Specifically, protein complex charge state has a demonstrated influence upon the conformation, mass resolution, ion mobility resolution, and dissociation properties of protein assemblies upon collisional activation. Here we present the first comparison of charge-reduced multiprotein complexes generated by solution additives and gas-phase ion-neutral reaction chemistry. While the charge reduction mechanism for both methods is undoubtedly similar, significant gas-phase activation of the complex is required to reduce the charge of the assemblies generated using the solution additive strategy employed here. This activation step can act to unfold intact protein complexes, making the data difficult to correlate with solution-phase structures and topologies. We use ion mobility-mass spectrometry to chart such conformational effects for a range of multi-protein complexes, and demonstrate that approaches to reduce charge based on ion-neutral reaction chemistry in the gas-phase consistently produce protein assemblies having compact, 'native-like' geometries while the same molecules added in solution generate significantly unfolded gas-phase complexes having identical charge states.

Synergy of Homogeneous and Heterogeneous Chemistry Probed by In Situ Spatially Resolved Measurements of Temperature and Composition

The interaction between heterogeneous and homogeneous chemistries is a crucial issue for high-temperature catalytic processes. In particular, the assessment of the main routes that control the selectivity to the desired products is essential for the design and safe operation of reaction units. This assessment is particularly true for the ultrafast conversion of hydrocarbons in short-contact-time (SCT) reactors that play a pivotal role in the effort to cope with the worldwide growing demand for more efficient exploitation of energy andmaterial resources. Examples are the catalytically assisted combustion for gas turbines with ultralow emissions, the catalytic partial oxidation (CPO) of hydrocarbons to H2 or CO/H2 mixtures (i.e., syngas), and the oxidative dehydrogenation (ODH) of light alkanes to olefins. As a common feature, these processes operate in autothermal and compact reactors, with noble-metal catalysts (palladium, rhodium, and platinum). An enormous energy intensity is peculiar to the SCT autothermal conversion of hydrocarbons over noble metals. Strongly exothermic and endothermic reactions proceed on the catalyst surface at extremely high rates. As a consequence, sharp gradients of temperature (up to 200 8Cmm ) and concentration are established within the small reactor volumes. Temperatures ranging from 250 to 1100 8C are generally experienced within a few millimeters. Such a level of severity—in terms of power density and extent of temperature and concentration gradients—is typical of flames and gas-phase oxidation processes in general. For a catalytic process, however, these conditions represent a thoroughly unconventional kinetic regime. To best grasp the intensity and the speed of the involved phenomena, we need to think of SCT conversion of light alkanes as the catalytic equivalent of a flame. This analogy can clearly depict the complexity of the process and emphasize the related scientific issues: To what extent does a catalytic process “stick” to the catalyst surface at these very high temperatures, where the adsorption of species is thermodynamically unfavored? Can the gas-phase activation of C H bonds (e.g., the formation and propagation of radicals) cooperate or compete with the catalytic process? In this respect, it is largely accepted that in the case of CH4 CPO over rhodium the gas-phase paths are negligible at atmospheric pressure and the catalytic route dominates. Conversely, the SCT-ODH of short alkanes over platinum proceeds mainly in the gas phase, thus giving rise to the production of olefins and other hydrocarbon species. On the basis of these examples, we could conclude that either catalytic or gas-phase chemistry governs the SCT conversion of hydrocarbons, depending only on the stability of the C H bond in the gas phase. Herein, by using novel techniques for collecting spatially resolved temperature and concentration profiles, we show that the partial oxidation of short-chain alkanes over rhodium breaks the paradigmatic compartments of heterogeneous processes and gas-phase processes, revealing the real complexity of these “flame-like” processes. Specifically, we examine the reaction of C3H8 CPO (C3H8+ 3/2O2!3CO+ 4H2) as a case study, and we apply novel techniques for collecting spatially resolved gas-phase and solid temperature and concentration profiles within a rhodium-coated honeycomb monolith to monitor the evolution of a propane/air mixture fed at high flow rate. The temperature and the composition of the reacting system were monitored from the inlet reactor section where the mixture was fed to the outlet section of the catalytic unit where the syngas stream was delivered. The results are reported in Figure 1 and Figure 2 as spatially resolved profiles of temperature and molar fraction of reactants and products. In the first 5 mm of the honeycomb, a sharp drop of O2 and C3H8 concentration was observed, accompanied by the formation of total oxidation products (CO2 and H2O) and partial oxidation products (H2 and CO). Correspondingly, a hot spot formed on the catalyst surface (980 8C, measured by the pyrometer) and a steep rise was observed in the gas temperature (up to 945 8C, measured by the thermocouple). In line with the occurrence of the endothermic steam reforming reaction, the evolution of H2O showed a maximum. Moreover, the temperature of the solid surface and in the gas phase decreased toward the exit of the honeycomb. Qualitatively, this behavior is what our and other research groups have observed also in the case of a CH4CPO experiment on rhodium, and can be fully explained by the catalytic production of syngas on rhodium. Thus, integral measurements (i.e., measurements of temperature and composition collected exclusively at the reactor outlet) would have suggested the unique existence of a heterogeneous process. A purely catalytic process was, for instance, [*] Dr. A. Donazzi, D. Livio, Dr. M. Maestri, Prof. A. Beretta, Prof. G. Groppi, Prof. E. Tronconi, Prof. P. Forzatti Laboratory of Catalysis and Catalytic Processes Dipartimento di Energia, Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 Milano (Italy) Fax: (+39)0223993284 E-mail: alessandra.beretta@polimi.it Homepage: http://www.lccp.polimi.it

Gas-phase coupling of reactive surfaces by oscillating reactant clouds

The collective, global behavior of a heterogeneous catalytic system depends on the effective communication of local reactivity variations to distant points in the system. One particularly efficient mode of communication occurs via partial pressure fluctuations in the gas-phase above the reactive surface. Although gas-phase communication has been implicated in a number of heterogeneous systems, the details of this coupling mechanism are lacking due to experimental difficulties in addressing local variations in surface and gas-phase activity simultaneously. Here, we take advantage of a spatially distributed system of isolated chemical oscillators to investigate the details of gas-phase communication in the 10 −3 mbar range. Characterization of local gas-phase oscillations, in parallel with kinetic oscillations on the surface, provides a novel description of the surface/gas-phase interaction under reaction conditions. This analysis further allows for a quantitative estimate of the effective gas-phase coupling length scale observed in surface imaging experiments.

Room-temperature methane activation by a bimetallic oxide clusterAlVO4+

The spin density of AlVO 4 + clusters is localized on one terminal O atom bonded with Al rather than V atom. This cluster can abstract one H-atom from CH 4 at room temperature. Gas-phase room-temperature methane activation by various clusters draws increasing attention. In this Letter, we produce a series of bimetallic oxide clusters which contain both main-group metal aluminum atoms and transition-metal vanadium atoms. We provide solid experimental and theoretical evidence that bimetallic AlVO 4 + cluster can draw a hydrogen atom from methane to produce CH 3 at room temperature. The Al 3 VO 7 + cluster which can be considered as a system formed from Al 2 O 3 + AlVO 4 + does not break C–H bonds of methane. The results enrich the chemistry of gas phase methane activation and provide possible molecular level mechanism to couple with heterogeneous catalysis.

BOREPIN, BORANORBORNADIENE AND BORANORCARADIENE: SUBSTITUENT EFFECTS ON INTERCONVERSIONS AT THEORETICAL LEVELS

Density-functional B3LYP and ab initio HF calculations are used to study three isomeric systems including: borepin I, boranorbornadiene II, and boranorcaradiene III as well as their X-substituted analogues at two different positions ( X = F, Cl, Br, CH3, OCH3, CF3, CN , and NH2 ). Geometries, enthalpies, and energy barriers for two series of interconversions, IIIX → IX and IIIX → IIX, are calculated using 6-311G* basis set. The B3LYP calculated relative stability is in the order: I (0.00 kcal/mol) > II (23.33 kcal/mol) > III (39.07 kcal/mol). Except for NH2 , the electronic effects of the substituents are generally insignificant on the relative stability. At B3LYP, the gas-phase activation enthalpies for III → I and III → II interconversions are estimated to be very small (0.6 and 0.75 kcal/mol, respectively). The reverse conversions, I → III and II → III, have activation energies of 39.67 and 16.49 kcal/mol, respectively. These energies rule out the possibility of a rapid interconversion of the isomers in the gas phase. Again, none of the substituents, X, can change this situation. Two pathways are proposed for the possible [1, 3]-suprafacial sigmatropic shifts of IIIX → IIX. The partial aromatic characters of planar borepins are estimated using magnetic (NICS) and structural criteria (bond length alternation).

Computational Study of Carbonyl Sulphide Formation on Model Interstellar Dust Grains

The formation of carbonyl sulphide, OCS, is investigated computationally on a model carbonaceous grain surface (coronene) using density functional theory. Four reaction pathways for the formation of OCS are investigated: formation from CO + S (on both the singlet and the triplet surfaces) and CO + HS, and formation from CS + O (again on both the singlet and the triplet surfaces) and CS + OH. The Langmuir−Hinshelwood, Eley−Rideal, and hot-atom mechanisms are investigated. Calculations show that all species in the ground state are physisorbed on the surface. However, both sulfur and oxygen in their first excited states chemisorb on coronene. The first reaction pathway, 3OCS formation from CO + 3S, is activated by 18.7 kJ mol−1 in the gas phase. This barrier is much too high for the reaction to occur at a significant rate at the low temperatures (10−20 K) found in dark interstellar clouds. However, calculations show that coronene catalyzes this reaction, lowering the barrier to 15.6 kJ mol−1 for the Langmuir−Hinshelwood reaction and to 7.1 kJ mol−1 for the Eley−Rideal reaction compared with the same reaction in the gas phase. For the similar reaction CS + 3O → 3OCS, the gas-phase activation barrier is negative, and it remains so on a coronene surface. The formation of OCS from CO + HS does not take place in a one-step mechanism. Instead, a stable intermediate (HSCO) is formed on the surface, which can subsequently react with a hydrogen atom to form OCS and H2. Finally, CS + OH can react to form a hot HOCS intermediate, which can either react exothermically to yield H + OCS or be stablised on the surface. In the latter case, reaction with another H atom can yield H2 + OCS.

Ligand and electronic-structure effects in metal-mediated gas-phase activation of methane: A cold approach to a hot problem

Gas-phase ion-molecule reactions, complemented by DFT and wave-function based electronic structure calculations, are presented, which permit rather detailed descriptions of two fundamental processes related to the activation of methane at room temperature. (i) Recent examples are discussed, which shed light on the role of oxygen-centered radicals in the first step of the oxidative dimerization of methane, i.e. the homolytic C-H bond cleavage of methane. (ii) Thermal ligand exchange processes of the type ML(+) + CH(4) --> M(CH(3))(+) + LH are analyzed in detail with an emphasis on L = H and F attached to various transition-metal cations M(+). It will be demonstrated inter alia that similar systems, e.g. MH(+) (M = Ni, Pd, Pt) in their room-temperature reactions with methane, actually exhibit fundamentally different mechanistic scenarios.

Electrospray Ionization Based Methods for the Generation of Polynuclear Oxo- and Hydroxo Group 6 Anions in the Gas-Phase

Electrospray ionization (ESI) of the Lindqvist (n-Bu4N)2[M6O19] (M = Mo, W) polyoxometalates provides a straightforward entry for the generation of an assortment of oxo- and hydroxo anions in the gas-phase. In particular, the series of oxo dianions of general formula [(MO3) n O]2− (n = 2–6; M = Mo, W), monoanions, namely [(MO3) n O]− (n = 1, 2) and [(MO3) n ]− (n = 1, 2), and the hydroxo [(MO3) n (OH)]− (n = 1–6) species can be readily generated in the gas-phase upon varying the solvent composition as well as the ionisation conditions (typically the Uc cone voltage). Complementary tandem mass experiments (collision induced dissociation and ion–molecule reactions) are also used aimed to investigate the consecutive dissociation of these species and their intrinsic gas-phase reactivity towards methanol. Special emphasis is paid to some of the key factors of these group 6 anions related to the gas-phase activation of methanol, such as molecular composition, open vs closed shell electronic nature and cluster size.

Dependence of Gas-Phase Crotonaldehyde Hydrogenation Selectivity and Activity on the Size of Pt Nanoparticles (1.7–7.1 nm) Supported on SBA-15

The selectivity and activity for the hydrogenation of crotonaldehyde to crotyl alcohol and butyraldehyde was studied over a series of Pt nanoparticles (diameter of 1.7, 2.9, 3.6, and 7.1 nm). The nanoparticles were synthesized by alcohol reduction of a Pt salt in the presence of poly(vinylpyrrolidone) (PVP), followed by incorporation into mesoporous SBA-15 silica. The rate of crotonaldehyde hydrogenation and selectivity towards crotyl alcohol both increase with increasing particle size. With an increase in particle size from 1.7 nm to 7.1 nm, the selectivity towards crotyl alcohol increases from 13.7% to 33.9% (8 Torr crotonaldehyde, 160 Torr H2 and 353 K). The turnover frequency increases from 2.1 × 10−2 s−1 to 4.8 × 10−2 s−1 with increasing particle size. Additionally, the decarbonylation pathway to form propene and CO is enhanced over smaller nanoparticles. The apparent activation energy remains constant (~16 kcal mol−1 for the formation of butyraldehyde and ~8 kcal mol−1 for the formation of crotyl alcohol) as a function of particle size as does the reaction order in H2, which is unity. In the presence of 130–260 mTorr CO, the reaction rate decreases for all products with a CO reaction order of −1 to −1.4 for crotyl alcohol and butyraldehyde. Hydrogen reduction at 673–723 K results in increased activity and selectivity relative to reduction at either higher or lower temperature; this is discussed with respect to the organic capping agent, PVP.

Gas-phase activation of methane by ligated transition-metal cations.

Motivated by the search for ways of a more efficient usage of the large, unexploited resources of methane, recent progress in the gas-phase activation of methane by ligated transition-metal ions is discussed. Mass spectrometric experiments demonstrate that the ligands can crucially influence both reactivity and selectivity of transition-metal cations in bond-activation processes, and the most reactive species derive from combinations of transition metals with the electronegative elements fluorine, oxygen, and chlorine. Furthermore, the collected knowledge about intramolecular kinetic isotope effects associated with the activation of C-H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or sigma-bond metathesis.

Screening approaches for gas-phase activity of flame retardants

Molecular beam mass spectrometry (MBMS) and optical diagnostic techniques, two common combustion science diagnostic tools for studying the impact of material on flames, are evaluated as tools for estimating the gas-phase potential of polymer-flame retardant additives. The gas-phase activity of hexabromocyclododecane (HBCD), a widely used commercial flame retardant, was studied and compared via the two combustion diagnostic techniques. MBMS data for HBCD were reviewed and provided identification of gas-phase active species as well as quantitative information on the degree of effectiveness based upon reduction of OH in a premixed CH 4/air/N 2 flame. In contrast, optical chemiluminescence detection of OH ∗ and CH ∗ provided a simpler technique for monitoring the gas-phase potential of flame retardants. Studies of CH ∗ and OH ∗ levels after addition of pyrolyzed products from polystyrene/HBCD blends into a diffusion flame system are compared with MBMS experiments of flames doped with pure HBCD. Comparison of chemiluminescence data with similar data from a small-scale heat release test, the pyrolysis combustion flow calorimeter, indicated that CH ∗ and OH ∗ activity relate to the heat release rate for flaming combustion.

Thermal activation of the co‐chaperonins GroES and gp31 probed by mass spectrometry

Many biological active proteins are assembled in protein complexes. Understanding the (dis)assembly of such complexes is therefore of major interest. Here we use mass spectrometry to monitor the disassembly induced by thermal activation of the heptameric co-chaperonins GroES and gp31. We use native electrospray ionization mass spectrometry (ESI-MS) on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to monitor the stoichiometry of the chaperonins. A thermally controlled electrospray setup was employed to analyze conformational and stoichiometric changes of the chaperonins at varying temperature. The native ESI-MS data agreed well with data obtained from fluorescence spectroscopy as the measured thermal dissociation temperatures of the complexes were in good agreement. Furthermore, we observed that thermal denaturing of GroES and gp31 proceeds via intermediate steps of all oligomeric forms, with no evidence of a transiently stable unfolded heptamer. We also evaluated the thermal dissociation of the chaperonins in the gas phase using infrared multiphoton dissociation (IRMPD) for thermal activation. Using gas-phase activation the smaller (2-4) oligomers were not detected, only down to the pentamer, whereafter the complex seemed to dissociate completely. These results demonstrate clearly that conformational changes of GroES and gp31 due to heating in solution and in the gas phase are significantly different.

Studies of degradation enhancement of polystyrene by flame retardant additives

Experimental methods as well as thermodynamic modeling techniques were utilized to explore potential gas and condensed-phase contributions of various flame retardant (FR) additives with polystyrene polymer. FR additives investigated include hexabromocyclododecane (HBCD), triphenyl phosphine oxide (TPPO), triphenyl phosphate (TPP), triphenyl phosphine sulfide (TPPS), and sulfur. Flame studies of fundamental FR activity were also employed using molecular beam mass spectrometry analysis of FR active species directly in a flame system. The flame studies show direct evidence for active bromine (HBr, Br) species for HBCD and active phosphorous species (HPO 2, PO, PO 2 HPO 3) species for TPPO and TPP which provide high potential for gas-phase activity for these FR additives. Various experimental measurements were also done to assess the degradation species and the degree of degradation of polystyrene by the FR additives. These studies support enhanced degradation of the base polystyrene polymer by the FR additive as a major pathway for condensed FR activity for HBCD and sulfur FR additives. Phosphorous based structures appear to show little enhancement of polystyrene degradation.

Molecular Content and Structure of Aqueous Organic Nanodroplets from the Vapor−Liquid Nucleation Study of the Water/n-Nonane/1-Alcohol Series

Prompted by a previous finding of unusual mixing behavior for the critical clusters involved in the vapor-liquid nucleation of the ternary water/n-nonane/1-butanol mixture, atomistic simulations employing the AVUS-HR technique were carried out to extend such investigations to include both shorter and longer alcohols, namely, the water/n-nonane/CiH2i+1OH mixture with i = 2, 4, 6, and 8. It is clear from this extensive investigation that the miscibility between water and n-nonane can be further improved by increasing the chain length of the alcohol (surfactant). In fact, for the water/n-nonane/1-octanol mixture at an intermediate gas-phase activity composition, the nucleation can proceed via fully mixed critical nuclei containing a roughly equal amount of all three components, which is in contrast to the dominantly binary-like nucleation channels observed for such mixtures involving shorter alcohols. Structural analysis revealed that these mixed nuclei take on a multilayered structural motif of the core-shell (water-alcohol) type with n-nonane distributed outside, forming an additional layer, more or less uniformly, compared to the one-sided deposition found for systems involving shorter alcohols. This structure provides a microscopic origin for the enhanced miscibility of water with n-nonane observed in the presence of 1-alcohol. These results may also have important implications for atmospheric organic aerosols.

Aluminosilicate Surfaces as Promoters for Peptide Bond Formation: An Assessment of Bernal's Hypothesis by ab Initio Methods

The role in prebiotic chemistry that Brønsted and Lewis sites, both present at the surface of common aluminosilicates, may have played in favoring the peptide bond formation has been addressed by ab initio methods within a cluster approach. B3LYP/6-31+G(d,p) free energy potential energy surfaces have been fully characterized for the model reaction glycine + NH3 --> 2-NH2 acetamide (mimicking the true 2 Gly --> GlyGly one) occurring on (i) a Lewis site, (ii) a Brønsted site, and (iii) a combined action of Lewis/Brønsted sites. Compared to the gas-phase (gp) activation free energy of 50 kcal/mol, the Lewis site alone reduces the gp barrier to 41 kcal/mol, whereas the activation by the Brønsted site dramatically reduces the barrier to about 18 kcal/mol. Nevertheless, formation of the prereactant complex in this latter case will rarely occur, since water will easily displace the glycine molecule interacting with the Brønsted site. However, if a realistic feldspar surface with neighboring Brønsted and Lewis sites is considered, the proper prereactant complex is highly stabilized by a simultaneous interaction with the Lewis and the Brønsted sites, in such a way that the Lewis site strongly attaches the glycine molecule to the surface whereas the Brønsted site efficiently catalyzes the condensation reaction, showing that the interplay between Lewis/Brønsted sites is an important issue. The free energy barrier computed for the realistic feldspar surface model is 26 kcal/mol. The role of dispersive interactions on the free energy barrier and the stabilization of the final product, not accounted for by the B3LYP functional, have been estimated and shown to be substantial. Speculations about further elongation of the formed dipeptide have been put forward on the basis of the relatively strong interaction energy of the formed GlyGly dipeptide with the aluminosilicate surface.

Combined QM/MM Molecular Dynamics Study on a Condensed-Phase SN2 Reaction at Nitrogen: The Effect of Explicitly Including Solvent Polarization.

In a previous combined QM/MM molecular dynamics (MD) study from our laboratory on the identity SN2 reaction between a chloride anion and an amino chloride in liquid dimethyl ether (DME), an increase in the free energy activation barrier was observed in the condensed phase when compared to the gas-phase activation energy. Here we reproduce these findings, but when comparing the condensed-phase potential of mean force (PMF) with the free energy profile in the gas phase (obtained from Monte Carlo simulations), we observe a smaller solvent effect on the activation barrier of the reaction. In a next step, we introduce an explicit description of electronic polarization in the MM (solvent) part of the system. A polarizable force field for liquid DME was developed based on the charge-on-spring (COS) model, which was calibrated to reproduce thermodynamic properties of the nonpolarizable model in classical MD simulations. The COS model was implemented into the MNDO/GROMOS interface in a special version of the QM/MM software ChemShell, which was used to investigate the effect of solvent polarization on the free energy profile of the reaction under study. A higher activation barrier was obtained using the polarizable solvent model than with the nonpolarizable force field, due to a better solvation of and a stronger polarization of solvent molecules around the separate reactants. The obtained PMFs were subjected to an energy-entropy decomposition of the relative solvation free energies of the reactant complex along the reaction coordinate, to investigate in a quantitative manner whether the solvent (polarization) effects are mainly due to favorable QM-MM (energetic) interactions.

Computation of Accurate Activation Barriers for Methyl-Transfer Reactions of Sulfonium and Ammonium Salts in Aqueous Solution.

The energetics of methyl-transfer reactions from dimethylammonium, tetramethylammonium, and trimethylsulfonium to dimethylamine were computed with density functional theory, MP2, CBS-QB3, and quantum mechanics/molecular mechanics (QM/MM) Monte Carlo methods. At the CBS-QB3 level, the gas-phase activation enthalpies are computed to be 9.9, 15.3, and 7.9 kcal/mol, respectively. MP2/6-31+G(d,p) activation enthalpies are in best agreement with the CBS-QB3 results. The effects of aqueous solvation on these reactions were studied with polarizable continuum model, generalized Born/surface area (GB/SA), and QM/MM Monte Carlo simulations utilizing free-energy perturbation theory in which the PDDG/PM3 semiempirical Hamiltonian for the QM and explicit TIP4P water molecules in the MM region were used. In the aqueous phase, all of these reactions proceed more slowly when compared to the gas phase, since the charged reactants are stabilized more than the transition structure geometries with delocalized positive charges. In order to obtain the aqueous-phase activation free energies, the gas-phase activation free energies were corrected with the solvation free energies obtained from single-point conductor-like polarizable continuum model and GB/SA calculations for the stationary points along the reaction coordinate.

Comparative gas-phase activation of two similar non-covalent heptameric protein complexes: gp31 and GroES

Using electrospray ionisation, non-covalently bound protein complexes can be transferred intact into the gas-phase and analysed and manipulated in a mass spectrometer. Here, two large (70–80 kDa) similar non-covalent protein complexes, gp31 and GroES, both cochaperonin to GroEL in Escherichia coli, were manipulated and compared inside a mass spectrometer using several gas-phase activation techniques. Nozzle-skimmer dissociation and collision-activated dissociation were performed using a quadrupole time-of-flight mass spectrometer and a Fourier transform ion cyclotron resonance mass spectrometer. Dissociation of these heptameric complexes mainly results in hexamers and monomers. There are no significant differences in gas-phase stability between the two complexes, but their fragmentation pathways exhibit considerable differences. Charge division over the fragments and overall charge losses during dissociation of the complexes differ clearly between GroES and gp31. These effects also differ between the various activation techniques, demonstrating that the different activation techniques yield complementary data. Combined, the different activation techniques are used to elucidate the dissociation mechanism and the degree of unfolding of the ejected monomer from the complex. The different behaviour of the two protein complexes is rationalized to be dependent on the gas-phase structures of gp31, GroES and their fragmentation products.

Gas‐Phase Chemistry of Diphosphate Anions as a Tool To Investigate the Intrinsic Requirements of Phosphate Ester Enzymatic Reactions: The [M1M2HP2O7]− Ions

Experimental studies on gaseous inorganic phosphate ions are practically nonexistent, yet they can prove helpful for a better understanding of the mechanisms of phosphate ester enzymatic processes. The present contribution extends our previous investigations on the gas-phase ion chemistry of diphosphate species to the [M(1)M(2)HP(2)O(7)](-) ions where M(1) and M(2) are the same or different and correspond to the Li, Na, K, Cs, and Rb cations. The diphosphate ions are formed by electrospray ionization of 10(-4) M solutions of Na(5)P(3)O(10) in CH(3)CN/H(2)O (1/1) and MOH bases or M salts as a source of M(+) cations. The joint application of mass spectrometric techniques and quantum-mechanical calculations makes it possible to characterize the gaseous [M(1)M(2)HP(2)O(7)](-) ions as a mixed ionic population formed by two isomeric species: linear diphosphate anion coordinated to two M(+) cations (group I) and [PO(3)M(1)M(2)HPO(4)](-) clusters (group II). The relative gas-phase stabilities and activation barriers for the isomerization I-->II, which depend on the nature of the M(+) cations, highlight the electronic susceptibility of P-O-P bond breaking in the active site of enzymes. The previously unexplored gas-phase reactivity of [M(1)M(2)HP(2)O(7)](-) ions towards alcohols of different acidity was investigated by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS). The reaction proceeds by addition of the alcohol molecule followed by elimination of a water molecule.

Thermal degradation of 2,4,6-tri[(bromo)x anilino]-1,3,5-triazines

The need for more efficient and 'greener' flame retardants for polymeric materials is ever present and of increasing intensity as regulatory agencies continue to display concern about the environmental impact of traditional materials. Compounds capable of multiple modes of action would be particularly desirable. Compounds containing both bromine (for good gas-phase activity) and nitrogen (to promote solid-phase activity) should be good candidates for development as flame retardant agents. A series of 2,4,6-tri[(bromo)xanilino]-1,3,5-triazines have been synthesized and characterized spectroscopically. The degradation characteristics of these compounds have been examined using thermogravimetry. They undergo step-wise decomposition beginning at about 400°C.