Stabilization of excess charge in isolated adenosine 5'-triphosphate and adenosine 5'-diphosphate multiply and singly charged anions

Multiply charged anions (MCAs) represent highly energetic species in the gas phase but can be stabilized through formation of molecular clusters with solvent molecules or counterions. We explore the intramolecular stabilization of excess negative charge in gas-phase MCAs by probing the intrinsic stability of the [adenosine 5'-triphosphate-2H](2-) ([ATP-2H](2-)), [adenosine 5'-diphosphate-2H](2-) ([ADP-2H](2-)), and H(3)P(3)O(10)(2-) dianions and their protonated monoanionic analogues. The relative activation barriers for decay of the dianions via electron detachment or ionic fragmentation are investigated using resonance excitation of ions isolated within a quadrupole trap. All of the dianions decayed via ionic fragmentation demonstrating that the repulsive Coulomb barriers (RCB) for ionic fragmentation lie below the RCBs for electron detachment. Both the electrospray ionization mass spectra (ESI-MS) and total fragmentation energies for [ATP-2H](2-), [ADP-2H](2-), and H(3)P(3)O(10)(2-) indicate that the multiply charged H(3)P(3)O(10)(2-) phosphate moiety is stabilized by the presence of the adenosine group and the stability of the dianions increases in the order H(3)P(3)O(10)(2-) < [ADP-2H](2-) < [ATP-2H](2-). Fully optimized, B3LYP/6-31+G* minimum energy structures illustrate that the excess charges in all of the phosphate anions are stabilized by intramolecular hydrogen bonding either within the phosphate chain or between the phosphate and the adenosine. We develop a model to illustrate that the relative magnitudes of the RCBs and hence the stability of these ions is dominated by the extent of intramolecular hydrogen bonding.     

Applications of laser ionization/fourier-transform mass spectrometry to the study of metal ions and their clusters in the gas phase

Recent software and hardware modifications of the Nicolet FTMS-1000 Fourier-transform mass spectrometer have made it possible to conduct research in what can be termed a “complete gas-phase chemical laboratory”. Selected ions of interest can be mixed with various reagents and their detailed chemistries monitored through a series of as many as eight reaction sequences. At any point in these sequences, ion structures can be elucidated and fundamental kinetic and thermodynamic parameters of the reactions can be determined. These powerful new techniques have been applied to examine the gas-phase chemistry and photochemistry of metal ions, metal ion clusters, and metal ion complexes, all of which have a bearing on the fundamentals of organometallic chemistry and catalysis.     

Calixarenes and resorcinarenes as scaffolds for supramolecular metallo-enzyme mimicry

Metallo-enzymes are natural catalysts carrying out highly selective and demanding reactions under mild conditions, using readily available metal ions (iron, copper, zinc, etc.). Understanding the factors explaining these performances is thus of fundamental and applicative interest. Classical model complexes displaying significant limitations in reactivity led to the development of supramolecular systems associating a reactive complex to a molecular cavity receptor, in order to not only mimic the first coordination sphere of the metal, but also the supramolecular enzymatic environment (access channel and binding pocket) responsible for their remarkable kinetics and selectivities. Calixarenes and resorcinarenes are particularly suited to this goal due to their wide range of sizes and flexibility. This review illustrates several specific aspects of enzymatic systems that were successfully mimicked with such supramolecular model systems. This toolbox of supramolecular effects could be used for future developments of bioinorganic supramolecular catalysts.     

Metastable ion formation and disparate charge separation in the gas-phase dissection of protein assemblies studied by orthogonal time-of-flight mass spectrometry

The dissection of specific and nonspecific protein complexes in the gas phase is studied by collisionally activated decomposition. In particular, the gas phase dissection of multiple protonated homodimeric Human Galectin I, E. Coli Glyoxalase I, horse heart cytochrome c, and Hen egg Lysozyme have been investigated. Both the Human Galectin I and E. Coli Glyoxalase I enzymes are biologically active as a dimer, exhibiting molecular weights of approximately 30 kDa. Cytochrome c and Lysozyme are monomers, but may aggregate to some extent at high protein concentrations. The gas phase dissociation of these multiple protonated dimer assemblies does lead to the formation of monomers. The charge distribution over the two concomitant monomers following the dissociation of these multiple protonated dimers is found to be highly dissimilar. There is no evident correlation between the solution phase stability of the dimeric proteins and their gas-phase dissociation pattern. Additionally, in the collisionally activated decomposition spectra diffuse ion signals are observed, which are attributed to monomer ions formed via slow decay of the collisionally activated dimer ions inside the reflectron time-of-flight. Although, the formation of these diffuse metastable ions may complicate the interpretation of collisionally activated decomposition mass spectra, especially when studying noncovalent protein complexes, a simple mathematical equation may be used to reveal their origin and pathway of formation.     

An overlooked series of gas phase diatomic metal oxide ions that are long-lived

Although the "Golden" years of spectroscopy and the major studies on ionization processes now are behind us, as with many branches of science, much yet remains to be gleaned from such topics that is both full of interest and of significance to present day research. Presented here is one such overlooked example, an observation that relates to both these fields. An analysis is presented for the periodic table concerning the gas-phase thermochemical nature of MO+ and MO2+ ions. Unexpectedly, a pattern of 18 elements has been identified that exhibit the potential for having long-lived MO+ ions. Normally such molecular ions are expected to decay extremely rapidly by dissociative recombination with electrons, but in particular, 12 of this group behave not like molecules but rather as atomic ions. These are the diatomic oxide ions of Sc, Y, La, Zr, Hf, Ce, Pr, Nd, Pm, Gd, Tb, and Th. In the gas phase, they decay by much slower three-body recombination channels. As may be noted, these elements are located in the first two columns of the transition elements, among the earlier rare earths and an actinide. From all the elements, UO2+ is the only dioxide ion that behaves similarly. These findings now elevate the potential importance of these ions and should facilitate their spectral characterization. Moreover, subsequent comparisons with spectra of well-known isoelectronic and isovalent neutral monoxides and other diatomics will help in the stimulation of further theoretical advances. In addition, once characterized, an ease of spectrally monitoring such ionic states will provide a useful analytical tool.     

Role of nearby charges on the electronic structure of π-conjugated molecules: symmetric versus asymmetric charge distributions in oligo(p-phenyleneethynylene)

Oligo(p-phenyleneethynylene)s (OPEs) are conjugated oligomers of great interest within materials science and molecular electronics on account of their highly applicable electronic and optical properties. Here we use gas-phase action spectroscopy to elucidate how the intrinsic electronic properties of these chromophores are affected by nearby charges. An OPE3 chromophore with two nearby ammonium groups was synthesized. This molecule and a related OPE3 with only one amine protonation site were transferred to the gas phase by electrospray ionization and subjected to action spectroscopy. Ions were bunched in a 14-pole ion trap, accelerated to 50-keV kinetic energies, mass-to-charge selected by a magnet, and photoexcited in a crossed-beam configuration. Fragment ions were finally mass-analyzed by an electrostatic analyzer. The setup enables photodissociation mass spectrometry and action spectroscopy on the microsecond time scale. The gas-phase absorption of the mono- and dication was measured and compared to that of neutral chromophores in solution. Similar absorption was found for neutral chromophores (in solution) and the dication (in gas phase or solution), whereas the monocation absorbs at lower energies in the gas phase. Simple electrostatic considerations lead to an energy difference like the one found from the experiment. The work presented here addresses how the electronic properties of a π-conjugated system are affected by nearby charges, a question of fundamental interest in, for example, molecular electronics.     

Counter-ion perturbation of the fragmentation pathways of multiply charged anions: evidence for exit channel complexes on the fragmentation potential energy surfaces

We report the first low-energy collisional excitation measurements and density functional theory calculations to characterize the ground state potential energy surfaces of contact ion-pair complexes that contain multiply charged anions (MCAs). Excitation of K+.Pt(CN)(4) (2-) and K+.Pt(CN)(6) (2-) result in fragmentation products associated with decay of the isolated constituent dianions, revealing that the ground state ion-pair surfaces are dominated by the intrinsic characteristics of the MCA. This observation is important since it indicates that counter-ion complexation only weakly perturbs the electronic structure of an MCA. For K+.Pt(CN)(4) (2-), where the Pt(CN)(4) (2-) dianion decays with production of two ionic fragments, we observe evidence for the existence of a novel exit-channel complex corresponding to a polar KCN salt unit bound to the Pt(CN)(3) (-) anion. The results described provide a basis for understanding the potential energy surfaces and fragmentation characteristics of other ion-pair complexes that involve MCAs.     

Origin of asymmetric charge partitioning in the dissociation of gas-phase protein homodimers

The origin of asymmetric charge and mass partitioning observed for gas-phase dissociation of multiply charged macromolecular complexes has been hotly debated. These experiments hold the potential to provide detailed information about the interactions between the macromolecules within the complex. Here, this unusual phenomenon of asymmetric charge partitioning is investigated for several protein homodimers. Asymmetric charge partitioning in these ions depends on a number of factors, including the internal energy, charge state, and gas-phase conformation of the complex, as well as the conformational flexibility of the protein monomer in the complex. High charge states of both cytochrome c and disulfide-reduced alpha-lactalbumin homodimers dissociate by a symmetrical charge partitioning process in which both fragment monomers carry away roughly an equal number of charges. In contrast, highly asymmetric charge partitioning dominates for the lower charge states. Cytochrome c dimer ions with eleven charges formed by electrospray ionization from two solutions in which the solution-phase conformation differs dissociate with dramatically different charge partitioning. These results demonstrate that these gas-phase complexes retain a clear "memory" of the solution from which they are formed, and that information about their solution-phase conformation can be obtained from these gas-phase dissociation experiments. Cytochrome c dimer ions formed from solutions in which the conformation of the protein is native show greater asymmetric charge partitioning with increasing ion internal energy. Cytochrome c dimers that are conformationally constrained with intramolecular cross-linkers undergo predominantly symmetric charge partitioning under conditions where asymmetric charge partitioning is observed for cytochrome c dimers without cross-links. Similar results are observed for alpha-lactalbumin homodimers. These results provide convincing evidence that the origin of asymmetric charge partitioning in these homodimers is the result of one of the protein monomers unfolding in the dissociation transition state. A mechanism that accounts for these observations is proposed.     

Meerwein reaction of phosphonium ions with epoxides and thioepoxides in the gas phase

Phosphonium ions are shown to undergo a gas-phase Meerwein reaction in which epoxides (or thioepoxides) undergo three-to-five-membered ring expansion to yield dioxaphospholanium (or oxathiophospholanium) ion products. When the association reaction is followed by collision-induced dissociation (CID), the oxirane (or thiirane) is eliminated, making this ion molecule reaction/CID sequence a good method of net oxygen-by-sulfur replacement in the phosphonium ions. This replacement results in a characteristic mass shift of 16 units and provides evidence for the cyclic nature of the gas-phase Meerwein product ions, while improving selectivity for phosphonium ion detection. This reaction sequence also constitutes a gas-phase route to convert phosphonium ions into their sulfur analogs. Phosphonium and related ions are important targets since they are commonly and readily formed in mass spectrometric analysis upon dissociative electron ionization of organophosphorous esters. The Meerwein reaction should provide a new and very useful method of recognizing compounds that yield these ions, which includes a number of chemical warfare agents. The Meerwein reaction proceeds by phosphonium ion addition to the sulfur or oxygen center, followed by intramolecular nucleophilic attack with ring expansion to yield the 1,3,2-dioxaphospholanium or 1,3,2-oxathiophospholanium ion. Product ion structures were investigated by CID tandem mass spectrometry (MS(2)) experiments and corroborated by DFT/HF calculations.     

Surface diffusion an—Exercise in D = 2 + 1 dynamics

The diffusion of particles on surfaces of solids play important role in understanding variety of phenomena, which are of considerable theoretical and applied interest. For example surface diffusion may alter the mode of the morphological structure via which crystal growth progresses. Study of surface diffusion, and more generally dynamics of many particle systems on solid surfaces may also offer an insight into the more fundamental problems, for example how many particle dynamics is affected by change of the spatial dimensionality.     

Generation of naphthoquinone radical anions by electrospray ionization: solution, gas-phase, and computational chemistry studies

Radical anions are present in several chemical processes, and understanding the reactivity of these species may be described by their thermodynamic properties. Over the last years, the formation of radical ions in the gas phase has been an important issue concerning electrospray ionization mass spectrometry studies. In this work, we report on the generation of radical anions of quinonoid compounds (Q) by electrospray ionization mass spectrometry. The balance between radical anion formation and the deprotonated molecule is also analyzed by influence of the experimental parameters (gas-phase acidity, electron affinity, and reduction potential) and solvent system employed. The gas-phase parameters for formation of radical species and deprotonated species were achieved on the basis of computational thermochemistry. The solution effects on the formation of radical anion (Q(?-)) and dianion (Q(2-)) were evaluated on the basis of cyclic voltammetry analysis and the reduction potentials compared with calculated electron affinities. The occurrence of unexpected ions [Q+15](-) was described as being a reaction between the solvent system and the radical anion, Q(?-). The gas-phase chemistry of the electrosprayed radical anions was obtained by collisional-induced dissociation and compared to the relative energy calculations. These results are important for understanding the formation and reactivity of radical anions and to establish their correlation with the reducing properties by electrospray ionization analyses.     

Recent development in preparation reactivity and biological activity of enaminoketones and enaminothiones and their utilization to prepare heterocyclic compounds

Enaminoketones and esters are gaining increased interest, particularly cyclic‐β‐enaminoesters, which are known as important intermediates for the synthesis of heterocycles and natural products, because the enantioselective preparation of highly functionalized compounds is of central importance in synthetic chemistry. Enaminones are versatile synthetic intermediates that combine the ambident nucleophilicity of enamines with the ambident eletrophilicity of enones. Enaminoketones and enaminonitriles have proven to be versatile building blocks for the synthesis of various heterocycles such as pyridine, pyrimidine and pyrrole deriva tives. Enaminones systems have “enone” character, and may act as acceptors in both 1,2 and 1,4‐additions. In this way the enaminone serves as a scaffold for annulation, and can gain access to systems such as pyrroles indolizidines, quinolizidines and perhydroindoles, all of which are common motifs in alkaloid structures. Enaminones are frequently employed as building blocks for the preparation of a variety of bicyclic compounds of biological interest and have been recently recognized as potential anticonvulsant compounds. Since a large number of developments in the use of enaminones in heterocyclic synthesis have occurred, a review of the recent developments in the synthetic approaches, covering the literature since 1995 until 2004, to these interesting molecules and their useful chemical transformations and biological activity can be considered of considerable value.     

Negative gas-phase ion chemistry of silane: A quadrupole ion trap study

Silicon clusters are of considerable interest for their importance in astrophysics and chemical vapour deposition processes, as well as from a fundamental point of view. Here, we present a quadrupole ion trap study of the self-condensation ion/molecule reactions of anions of silane. In the high-pressure regime, several ion clusters are formed with increasing size: the largest ions detected are Si5Hn- (n = 0-3). Selective ion isolation and storage allowed detection of the main reaction sequences occurring in the reacting system. The most frequent condensation step is followed by single or multiple dehydrogenation, this latter being particularly observed for the high-mass reactant ions. As a consequence, the most abundant ions in the mass spectra are those with a low content of hydrogen, namely Si2H-, Si3H-, and Si4H-. These results are discussed with reference to literature data on silicon cluster anions and related systems.     

Lead‐enhanced gas‐phase stability of multiply charged EDTA anions: a combined experimental and theoretical study

Besides their fundamental importance, multiply charged anions (MCAs) are considered as promising molecular capacitors for which their intrinsic stabilities are of great significance. Herein, the gas‐phase stabilities of ethylenediaminetetraacetic acid (EDTA) anions (i.e. [EDTA‐nH]^n?, n = 1–4) and their Pb(II) complexes (i.e. [EDTA + Pb‐nH]^(2‐n)?, n = 3, 4) have been investigated using an approach that combines extractive electrospray ionization mass spectrometry (EESI‐MS) measurements, Car–Parrinello molecular dynamics simulations and density functional theory/Tao–Perdew–Staroverov–Scuseria calculations. The EESI‐MS data showed that the doubly charged EDTA anions in the form of [EDTA‐2H]^2? and [EDTA + Pb‐4H]^2? were much more abundantly observed than the singly charged species such as [EDTA‐H]^? and [EDTA + Pb‐3H]^?, respectively. The calculation results indicated that [EDTA‐2H]^2? and [EDTA + Pb‐4H]^2? anions were thermodynamically more stable than the [EDTA‐H]^? and [EDTA + Pb‐3H]^? species in the gas phase, respectively. The [EDTA + Pb‐3H]^? anions preferred five‐coordinated structure, whereas [EDTA + Pb‐4H]^2? anions formed either five‐coordinated or six‐coordinated structures. The calculations further revealed that significant electron clouds drifting from the ligand EDTA to the metal Pb(II) ions and the large distances between the carboxylic groups reduced the Coulomb repulsion among the excess electrons of these MCAs. Our data demonstrated that EESI‐MS combined with theoretic calculations were able to provide a deep insight into the fundamental behavior of stability of MCAs in the gas phase and, thus, might be useful tools for studying MCAs for potential molecular capacitors. Copyright © 2012 John Wiley & Sons, Ltd.     

Perturbation of the PET process in fluorophore-spacer-receptor systems through structural modification: transition metal induced fluorescence enhancement and selectivity

Several fluorescent signaling systems are built in the format fluorophore-spacer-receptor with ethylenediamine or N,N-dimethylethylenediamine as the receptor, anthracene as the fluorophore, and a methylene group as the spacer. The receptors are derivatized with different electron-withdrawing groups such as 4-nitrobenzene, 4-nitro-2-pyridine, and 2,4-dinitrobenzene, to perturb the photoinduced intramolecular electron transfer (PET) process from the nitrogen lone-pair to the fluorophore. The photophysical properties of these supramolecular systems and their fluorescence responses toward a number of quenching transition metal ions are reported. It is shown that the PET is highly efficient in the absence of a metal ion. With a metal ion input, the fluorescence can be recovered to a different extent depending on the nature of the metal and on the overall architecture of the system as well. Despite the possibility of strong interaction between the fluorophore and the metal ion, significant fluorescence enhancement is observed with quenching of paramagnetic transition metal ions. The complex stability data show that the stability constants for the metal ions showing fluorescence enhancement are of the order of 10(4) M(-1). This study shows that structurally simple fluorescent signaling systems for quenching transition metal ions can be built by maximizing the PET. It is also shown here that simple structural modification can make these systems highly specific for particular transition metal ions for potential applications in several contemporary areas of research.     

Gas-phase dissociation pathways of a tetrameric protein complex

The gas-phase dissociation of the tetrameric complex transthyretin (TTR) has been investigated with tandem-mass spectrometry (tandem-MS) using a nanoflow-electrospray interface and a quadrupole time-of-flight (Q-TOF) mass spectrometer. The results show that highly charged monomeric product ions dissociate from the macromolecular complex to form trimeric products. Manipulating the pressure conditions within the mass spectrometer facilitates the formation of metastable ions. These were observed for the transitions from tetrameric to monomeric and trimeric product ions and additionally for losses of small molecules associated with the protein complex in the gas phase. These results are interpreted in the light of recent mechanisms for the electrospray process and provide insight into the composition and factors governing the stability of macromolecular ions in the gas phase.     

Flash photolysis study of complexes between salicylic acid and lanthanide ions in water

In the natural environment humic substances (HS) represent a major factor determining the speciation of metal ions, e.g., in the context of radionuclide migration. Here, due to their intrinsic sensitivity and selectivity, spectroscopic methods are often applied, requiring a fundamental understanding of the photophysical processes present in such HS-metal complexes. Complexes with different metal ions were studied using 2-hydroxybenzoic acid (2HB) as a model compound representing an important part of the chelating substructures in HS. In flash photolysis experiments under direct excitation of 2HB in the absence and the presence of different lanthanide ions, the generation and the decay of the 2HB triplet state, of the phenoxy radical, and of the solvated electron were monitored. Depending on the lanthanide ion different intracomplex processes were observed for these transient species including energy migration to and photoreduction of the lanthanide ion. The complexity of the intracomplex photophysical processes even for small molecules such as 2HB underlines the necessity to step-by-step approach the photochemical reactivity of HS by using suitable model compounds.     

Transition metal phosphides: A wonder catalyst for electrocatalytic hydrogen production

Hydrogen evolution from water electrolysis has become an important reaction for the green energy revolution. Traditional precious metals and their compounds are excellent catalysts for producing hydrogen; however, their high cost limits their large-scale practical application. Therefore, the development of affordable electrocatalysts to replace these precious metals is important. Transition metal phosphides(TMPs) have shown remarkable performance for hydrogen evolution and garnered considerable ...     

Combined Chemical Modification and Collision Induced Unfolding Using Native Ion Mobility-Mass Spectrometry Provides Insights into Protein Gas-Phase Structure

Native mass spectrometry is now an important tool in structural biology. Thus, the nature of higher protein structure in the vacuum of the mass spectrometer is an area of significant interest. One of the major goals in the study of gas-phase protein structure is to elucidate the stabilising role of interactions at the level of individual amino acid residues. A strategy combining protein chemical modification together with collision induced unfolding (CIU) was developed and employed to probe the structure of compact protein ions produced by native electrospray ionisation. Tractable chemical modification was used to alter the properties of amino acid residues, and ion mobility-mass spectrometry (IM-MS) utilised to monitor the extent of unfolding as a function of modification. From these data the importance of specific intramolecular interactions for the stability of compact gas-phase protein structure can be inferred. Using this approach, and aided by molecular dynamics simulations, an important stabilising interaction between K6 and H68 in the protein ubiquitin was identified, as was a contact between the N-terminus and E22 in a ubiquitin binding protein UBA2.