Identification of a Two‐Coordinate Iron(I)–Oxalate Complex

Exotic oxidation states of the first‐row transition metals have recently attracted much interest. In order to investigate the oxidation states of a series of iron–oxalate complexes, an aqueous solution of iron(III) nitrate and oxalic acid was studied by infrared free liquid matrix‐assisted laser desorption/ionization as well as ionspray mass spectrometry. Here, we show that iron is not only detected in its common oxidation states +II and +III, but also in its unusual oxidation state +I, detectable in both positive‐ion and in negative‐ion modes, respectively. Vibrational spectra of the gas phase anionic iron oxalate complexes [Fe^III(C₂O₄)₂]⁻, [Fe^II(C₂O₄)CO₂]⁻, and [Fe^I(C₂O₄)]⁻ were measured by means of infrared photodissociation spectroscopy and their structures were assigned by comparison to anharmonic vibrational spectra based on second‐order perturbation theory.

Liquid phase IR-MALDI and differential mobility analysis of nano- and sub-micron particles

Infrared matrix-assisted desorption and ionization (IR-MALDI) enables the transfer of sub-micron particles (sMP) directly from suspensions into the gas phase and their characterization with differential mobility (DM) analysis. A nanosecond laser pulse at 2940 nm induces a phase explosion of the aqueous phase, dispersing the sample into nano- and microdroplets. The particles are ejected from the aqueous phase and become charged. Using IR-MALDI on sMP of up to 500 nm in diameter made it possible to surpass the 100 nm size barrier often encountered when using nano-electrospray for ionizing supramolecular structures. Thus, the charge distribution produced by IR-MALDI could be characterized systematically in the 50-500 nm size range. Well-resolved signals for up to octuply charged particles were obtained in both polarities for different particle sizes, materials, and surface modifications spanning over four orders of magnitude in concentrations. The physicochemical characterization of the IR-MALDI process was done via a detailed analysis of the charge distribution of the emerging particles, qualitatively as well as quantitatively. The Wiedensohler charge distribution, which describes the evolution of particle charging events in the gas phase, and a Poisson-derived charge distribution, which describes the evolution of charging events in the liquid phase, were compared with one another with respect to how well they describe the experimental data. Although deviations were found in both models, the IR-MALDI charging process seems to resemble a Poisson-like charge distribution mechanism, rather than a bipolar gas phase charging one.

Modular Synthesis of Dipyrroloquinolines: A Combined Synthetic and Mechanistic Study

A straightforward synthesis of [1,2-a][3',2'-c]dipyrroloquinolines has been developed generating up to eight new σ-bonds and five new stereogenic centers in a simple and modular one-pot operation. Generally good to excellent yields and moderate to good stereoselectivities in favor of the all-cis stereoisomer were observed. A detailed investigation combining synthetic studies, analytical measurements, and theoretical calculations has been conducted to elucidate the reaction mechanism using ESI- and liquid-beam IR-laser desorption mass spectrometry as well as DFT calculations. Key steps of this sequential transformation include a Lewis acid-catalyzed vinylogous Mukaiyama-Mannich reaction of bis(silyl) dienediolate 1 and a Brønsted acid-promoted Mannich-Pictet-Spengler reaction cascade reaction to complete the synthesis of the dipyrroloquinoline core of the target compounds.

Liquid Beam Desorption Mass Spectrometry for the Investigation of Continuous Flow Reactions in Microfluidic Chips

In this work, we present the combination of microfluidic chips and mass spectrometry employing laser-induced liquid beam ionization/desorption. The developed system was evaluated with respect to stable beam generation and laser parameters as well as solvent compatibility. The device was exemplarily applied to study a vinylogous Mannich reaction performed in continuous flow on chip. Fast processes can be observed with this technique which in the future could be beneficial for studying intermediates or contribute to the elucidation of reaction mechanisms.

A Complex Catalytic Reaction Caught in the Act: Intermediates and Products Sampling Online by Liquid μ-Beam Mass Spectrometry and Theoretical Modeling

Liquid-beam IR-laser desorption mass spectrometry has been used to monitor the reactants, intermediates, and products of a complex organic signature reaction in real time on multiple timescales directly from the liquid phase. The reaction was chosen because it has advantages in medicinal chemistry applications, and the three-component, modular construction provides a means to generate molecular diversity rapidly. Under Lewis acid catalysis, a vinylogous Mannich reaction was monitored as it generated a δ-amino-α-silyloxy-α,β-unsaturated ester, which upon hydrolysis to the corresponding α-keto ester spontaneously reacted in a [3+2] cycloannulation to the final pyrrolo[2,1-b]benzoxazole. The kinetic data were compared with predictions of quantum chemical calculations to elucidate and verify or exclude reaction pathways and mechanisms for a possible rational optimization of the reaction. The simplicity and rapid response of this approach make it a very powerful technique for online characterization of chemical reactions on timescales spanning several orders of magnitude. This enables full control over chemical reactions, thereby maximizing the product yield. This combined experimental and theoretical approach opens up a new route for the study of novel chemistry in liquid-phase reactions.

The function and performance of aqueous aerosol devices for inhalation therapy

OBJECTIVES

In this review paper, we explore the interaction between the functioning mechanism of different nebulizers and the physicochemical properties of the formulations for several types of devices, namely jet, ultrasonic and vibrating-mesh nebulizers; colliding and extruded jets; electrohydrodynamic mechanism; surface acoustic wave microfluidic atomization; and capillary aerosol generation.

KEY FINDINGS

Nebulization is the transformation of bulk liquids into droplets. For inhalation therapy, nebulizers are widely used to aerosolize aqueous systems, such as solutions and suspensions. The interaction between the functioning mechanism of different nebulizers and the physicochemical properties of the formulations plays a significant role in the performance of aerosol generation appropriate for pulmonary delivery. Certain types of nebulizers have consistently presented temperature increase during the nebulization event. Therefore, careful consideration should be given when evaluating thermo-labile drugs, such as protein therapeutics. We also present the general approaches for characterization of nebulizer formulations.

SUMMARY

In conclusion, the interplay between the dosage form (i.e. aqueous systems) and the specific type of device for aerosol generation determines the effectiveness of drug delivery in nebulization therapies, thus requiring extensive understanding and characterization.

Mechanism of Protein Molecule Isolation by IR Laser Ablation of Droplet Beam

Gas-phase isolation of bovine serum albumin (BSA) from aqueous solutions is performed by IR laser ablation of a droplet beam. Multiply charged BSA ions (positive and negative) were produced by the IR laser irradiation onto a droplet beam of aqueous BSA solutions with various pH values prepared by addition of hydrochloric acid or sodium hydroxide to the solution. The isolation mechanism was discussed based on the charge state of the isolated BSA ions. A nanodroplet model explains the gas-phase charge distribution of the BSA ions. This study provides a fundamental basis for further studies of a wide variety of biomolecules in the gas phase isolated directly from solution.

Charge separation and isolation in strong water droplet impacts

Charge separation in condensed matter after strong impacts is a general and intriguing phenomenon in nature, which is often identified and described but not necessarily well understood in terms of a quantitative mechanistic picture. Here we show that charge separation naturally occurs if water droplets/clusters or ice particles with embedded charge carriers, e.g., ions, encounter a high energy impact with subsequent dispersion - even if the involved kinetic energy is significantly below the molecular ionization energy. We find that for low charge carrier concentrations (c < 0.01 mol L(-1)) a simple statistical Poisson model describes the charge distribution in the resulting molecular "fragments" or aggregates. At higher concentrations Coulomb interactions between the charge carriers become relevant, which we describe by a Monte Carlo approach. Our models are compared to experimental data for strong (laser) impacts on liquid micro beams and discussed for the charge generation in cluster-impact mass spectrometry on cosmic dust detectors where particle kinetic energies are below the plasma threshold. Taken together, a simple and intuitive but quantitative microscopic model is obtained, which may contribute to the understanding of a larger range of phenomena related to charge generation and separation in nature.

Primary steps of pH-dependent insulin aggregation kinetics are governed by conformational flexibility

Insulin aggregation critically depends on pH. The underlying energetic and structural determinants are, however, unknown. Here, we measure the kinetics of the primary aggregation steps of the insulin monomer in vitro and relate it to its conformational flexibility. To assess these primary steps the monomer concentration was monitored by mass spectrometry at various pH values and aggregation products were imaged by atomic force microscopy. Lowering the pH from 3 to 1.6 markedly accelerated the observed aggregation kinetics. The influence of pH on the monomer structure and dynamics in solution was studied by molecular dynamics simulations, with the protonation states of the titrable groups obtained from electrostatic calculations. Reduced flexibility was observed for low pH values, mainly in the C terminus and in the helix of the B chain; these corresponded to an estimated entropy loss of 150 J mol(-1) K(-1). The striking correlation between entropy loss and pH value is consistent with the observed kinetic traces. In analogy to the well-known Phi value analysis, this result allows the extraction of structural information about the rate determining transition state of the primary aggregation steps. In particular, we suggest that the residues in the helix of the B chain are involved in this transition state.

Atmospheric pressure free liquid infrared MALDI mass spectrometry: toward a combined ESI/ MALDI-liquid chromatography interface

A new atmospheric pressure (AP)-MALDI-type interface has been developed based on a free liquid (FL) microbeam/microdroplets and a mid-infrared optical parametric oscillator (mid-IR OPO). The device is integrated into a standard on-line nanoESI interface. The generation of molecular ions in the gas phase is believed to be the result of a fast (explosive) laser-induced evaporative dispersion(not desorption) of the microbeam into statistically charged nanodroplets. Only the lowest charge states appear insignificant abundance in this type of experiment. Mass spectra of some common peptides have been acquired in positive ion mode, and the limit-of-detection of this first prototype (liquid microbeam setup) was evaluated to be 17 fmol per second. To improve the duty cycle and to reduce the sample consumption, a droplet-on-demand system was implemented (generating 100 pL droplets).With this setup, about 20 attomole of bradykinin were sufficient to achieve a signal-to-noise ratio better than five.This setup can be operated at flow rates down to 100 nL/min and represents a liquid MALDI alternative to the nanoESI. Our particular interest was the application of the developed ion source for on-line coupling of liquid chromatography with mass spectrometry. The flow rates(>100 microL/min), required for stable operation of the ion source in continuous liquid microbeam mode, matches perfectly the flow rate range of micro HPLC. Therefore, online LC/MS experiments have been realized, employing a microbore C18 reversed-phase column to separate an artificial peptide mixture and tryptic peptides of bovine serum albumin (performing a peptide mass fingerprint). In the latter case, sequence coverage of more than 90%has been achieved.

A novel approach to analyze membrane proteins by laser mass spectrometry: from protein subunits to the integral complex

A novel laser-based mass spectrometry method termed LILBID (laser-induced liquid bead ion desorption) is applied to analyze large integral membrane protein complexes and their subunits. In this method the ions are IR-laser desorbed from aqueous microdroplets containing the hydrophobic protein complexes solubilized by detergent. The method is highly sensitive, very efficient in sample handling, relatively tolerant to various buffers, and detects the ions in narrow, mainly low-charge state distributions. The crucial experimental parameter determining whether the integral complex or its subunits are observed is the laser intensity: At very low intensity level corresponding to an ultrasoft desorption, the intact complexes, together with few detergent molecules, are transferred into vacuum. Under these conditions the oligomerization state of the complex (i.e., its quaternary structure) may be analyzed. At higher laser intensity, complexes are thermolyzed into subunits, with any residual detergent being stripped off to yield the true mass of the polypeptides. The model complexes studied are derived from the respiratory chain of the soil bacterium Paracoccus denitrificans and include complexes III (cytochrome bc(1) complex) and IV (cytochrome c oxidase). These are well characterized multi-subunit membrane proteins, with the individual hydrophobic subunits being composed of up to 12 transmembrane helices.

Current literature in mass spectrometry

In order to keep subscribers up‐to‐date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (6 Weeks journals ‐ Search completed at 4th. Oct. 2006)

How to make big molecules fly out of liquid water: applications, features and physics of laser assisted liquid phase dispersion mass spectrometry

Applications, features, and mechanistic details of laser assisted liquid phase dispersion mass spectrometry are highlighted and discussed. It has been used in the past to directly isolate charged molecular aggregates from the liquid phase and to determine their molecular weight employing sensitive time-of-flight mass spectrometry. The liquid matrix in this MALDI (matrix assisted laser desorption and ionization) type approach consists of a 10 microm diameter free liquid filament in vacuum (or a free droplet) which is excited with a focused infrared laser pulse tuned to match the absorption frequency of the OH-stretch vibration of bulk water near 2.8 microm. Due to these features we will refer to the approach as free liquid matrix assisted laser dispersion of ions or ionic aggregates (IR-FL-MALDI), although also LILBID ("laser induced liquid beam (bead) desorption and ionization") has been proposed early as a descriptive acronym for the technique and may be used alternatively. Low-charge-state macromolecular adducts are isolated in the gas phase from solution via a yet poorly characterized mechanism which sensitively depends upon the laser intensity and wavelength, and after the gentle liquid-to-vacuum transfer the aggregates are analyzed via time-of-flight (TOF) mass spectrometry (MS). Possible mechanisms for the isolation and charging of biomolecules directly from liquid solution are discussed in the present contribution. Recent technical advances such as minimizing the sample consumption, strategies for high throughput mass spectrometry, and coupling of liquid beam MS with HPLC will be highlighted as well. An interesting feature of IR-FL-MALDI is what we call the linear response, i.e., a surprising linearity of the gas phase mass signal on the solution concentration over many orders of magnitude for a large number of biomolecular systems as well as ions. Due to these features the approach may be regarded as a true solution probing spectroscopy, which enables elegant biokinetic studies. Several experiments in which time resolved IR-FL-MALDI-MS has recently been employed successfully are given. A particular highlight is the possibility to quantitatively detect oxidation states in solution, which clearly distinguishes the present approach from other established MS source concepts. Due to the good matrix tolerance also proteins in complex mixtures can be monitored quantitatively.

The dynamics of water evaporation from partially solvated cytochrome c in the gas phase

The study of evaporation of water from biological macromolecules is important for the understanding of electrospray mass spectrometry experiments. In electrospray ionization (ESI), electrically charged nanoscale droplets are formed from solutions of, for example, proteins. Then evaporation of the solvent leads to dry protein ions that can be analyzed in the mass spectrometer. In this work the dynamics of water evaporation from native cytochrome c covered by a monolayer of water is studied by molecular dynamics (MD) simulations at constant energy. A model of the initial conditions of the process is introduced. The temperature of the protein drops by about 100 K during the 400 picoseconds of the simulations. This sharp drop in temperature causes the water evaporation rate to decrease by about an order of magnitude, leaving the protein with 50% to 90% of the original water molecules, depending on the initial temperature of the simulation. The structural changes of the protein upon desolvation were considered through calculations of the radius of gyration and the root mean square (RMS) of the protein. A variation of 0.4 A in the radius of gyration, together with an RMS value of less than 3 A, indicates only minor changes in the overall shape of the protein structure. The water coordination number of the solvation shell is much smaller than that for bulk water. The mobility of water is high at the beginning of the simulations and drops as the simulation progresses and the temperature decreases. Incomplete desolvation of protein ions was also observed in recent experiments.

Rotational Energy Distributions of Benzene Liberated from Aqueous Liquid Microjets: A Comparison between Evaporation and Infrared Desorption

We have measured the rotational energy distribution of benzene molecules both evaporated and desorbed by an IR laser from a liquid microjet. Analysis of the 601 vibronic band of benzene has shown that the benzene molecules evaporating from the liquid microjet surface have a rotational temperature of 157 ± 7 K. In contrast, the rotational temperature of benzene molecules desorbed from the liquid microjet by a 1.9 μm laser pulse is 82 ± 5 K. However, in both cases careful inspection of the spectral profiles shows that the experimental rotational distributions are non-Boltzmann, displaying an underpopulation of high rotational states and a relative overpopulation of the low rotational states. The non-equilibrium evaporation and desorption spectral profiles are consistent with a model that involves transfer of internal energy into translation upon liberation from the condensed phase.