Evidence of molecular fragmentation inside the charged droplets produced by electrospray process

Homo- and Heteronuclear Group 12 Metallothionein Type B Cluster Analogs: Synthesis, Structure, 1H NMR and ESI-MS

Metallothioneins (MTs) are a ubiquitous class of small cysteine-rich metal-binding proteins involved in metal homeostasis and detoxification with highly versatile metal binding properties. Despite the long-standing association of MT with M₃S₃ and M₄S₅ metal clusters, synthetic complexes with these core architectures are exceptionally rare. Here, we demonstrate an approach to synthesizing and characterizing aggregates of group 12 metal ions with monocyclic M₃S₃ cores in acetonitrile solution without the protection of a protein. Multidentate monothiol ligand N,N-bis(2-pyridylmethyl)-2-aminoethanethiol (L1H) provided [Cd₃(L1)₃](ClO₄)₃ (1), the first structurally characterized nonproteinaceous aggregate with a metallothionein-like monocyclic Cd₃S₃ core. In addition, [Zn₃(L1)₃](ClO₄)₃·4CH₃CN (2·4CH₃CN) was characterized by X-ray crystallography. The complex cations of 1 and 2 had comparable structures despite being nonisomorphic. Variable temperature and concentration ¹H NMR were used to investigate aggregation equilibria of 1, 2, and a precipitate with composition "Hg(L1)(ClO₄)" (3). Cryogenic ¹H NMR studies of 3 revealed a J(¹⁹⁹Hg¹H) coupling constant pattern consistent with an aggregate possessing a cyclic core. ESI-MS was used for gas-phase characterization of 1-3, as well as mixed-metal [M₂M'(L1)₃(ClO₄)₂]⁺ ions prepared in situ by pairwise acetonitrile solution combinations of the group 12 complexes of L1. Access to synthetic variants of metallothionein-like group 12 aggregates provides an additional approach to understanding their behavior.

A novel methodology and strategy to detect low molecular aldehydes in beer based on charged microdroplet driving online derivatization and high resolution mass spectrometry

The concentration of aldehydes is one of the important indicators in the food quality and safety. To efficiently analyze the four aldehydes (methanal, ethanal, propanal and n-butanal) in beer, charged microdroplet driving online derivatization apparatus coupled with high resolution mass spectrometry was firstly developed. Utilizing the high-speed reaction accelerated by microdroplets, the offline derivative of aldehydes with 2,4-dinitrophenylhydrazine in bulk was transferred into online derivatization. The developed method featured acceptable linearities (R² ≥ 0.95), high sensitivities (LODs at ng mL^-1 level) and qualified precisions (RSDs ≤ 8.4 %) for target compounds. Four aldehydes with trace amount were successfully determined in beer. The results indicated that the novel online analytical strategy did not require complex sample preparation and could conduct simple, rapid, sensitive detection of small molecule aldehydes with high throughput in beer or even other food samples.

In-Source Microdroplet Derivatization Using Coaxial Contained-Electrospray Mass Spectrometry for Enhanced Sensitivity in Saccharide Analysis

Online, droplet-based in-source chemical derivatization is accomplished using a coaxial-flow contained-electrospray ionization (contained-ESI) source to enhance sensitivity for the mass spectrometric analysis of saccharides. Derivatization is completed in microseconds by exploiting the reaction rate acceleration afforded by electrospray microdroplets. Significant improvements in method sensitivity are realized with minimal sample preparation and few resources when compared to traditional benchtop derivatizations. For this work, the formation of easily ionizable phenylboronate ester derivatives of several mono-, di-, and oligosaccharides is achieved. Various reaction parameters including concentration and pH were evaluated, and a Design of Experiments approach was used to optimize ion source parameters. Signal enhancements of greater than two orders of magnitude were observed for many mono- and disaccharides using in-source phenylboronic acid derivatization, resulting in parts-per-trillion (picomolar) limits of detection. In addition, amino sugars such as glucosamine, which do not ionize in negative mode, were detected at low parts-per-billion concentrations, and isobaric sugars such as lactose and sucrose were easily distinguished. The new in-source derivatization approach can be employed to expand the utility of ESI-MS analysis for compounds that historically experience limited sensitivity and detectability, while avoiding resource-intensive, bulk-phase derivatization procedures.

Structural determination of four manufacturing impurities of D&C Red No. 33

Four manufacturing impurities of D&C Red No. 33 isolated by counter-current chromatography were analyzed by NMR and ESI mass spectrometry. Three of these impurities were reported previously with minimal details of structural determination. All four are structurally related to the main component of the dye. The fourth exhibited an unusual discrepancy between the NMR structure and its chemical formula suggested by ESI-MS results. Structural determination and assignment of the main component and four impurities are discussed as well as resolution of the discrepancy between the NMR and ESI-MS results of the fourth impurity.

Stoichiometry of Nucleotide Binding to Proteasome AAA+ ATPase Hexamer Established by Native Mass Spectrometry

AAA+ ATPases constitute a large family of proteins that are involved in a plethora of cellular processes including DNA disassembly, protein degradation and protein complex disassembly. They typically form a hexametric ring-shaped structure with six subunits in a (pseudo) 6-fold symmetry. In a subset of AAA+ ATPases that facilitate protein unfolding and degradation, six subunits cooperate to translocate protein substrates through a central pore in the ring. The number and type of nucleotides in an AAA+ ATPase hexamer is inherently linked to the mechanism that underlies cooperation among subunits and couples ATP hydrolysis with substrate translocation. We conducted a native MS study of a monodispersed form of PAN, an archaeal proteasome AAA+ ATPase, to determine the number of nucleotides bound to each hexamer of the WT protein. We utilized ADP and its analogs (TNP-ADP and mant-ADP), and a nonhydrolyzable ATP analog (AMP-PNP) to study nucleotide site occupancy within the PAN hexamer in ADP- and ATP-binding states, respectively. Throughout all experiments we used a Walker A mutant (PANK217A) that is impaired in nucleotide binding as an internal standard to mitigate the effects of residual solvation on mass measurement accuracy and to serve as a reference protein to control for nonspecific nucleotide binding. This approach led to the unambiguous finding that a WT PAN hexamer carried - from expression host - six tightly bound ADP molecules that could be exchanged for ADP and ATP analogs. Although the Walker A mutant did not bind ADP analogs, it did bind AMP-PNP, albeit at multiple stoichiometries. We observed variable levels of hexamer dissociation and an appearance of multimeric species with the over-charged molecular ion distributions across repeated experiments. We posit that these phenomena originated during ESI process at the final stages of ESI droplet evolution.

Spatial localization of charged molecules by salt ions in oil-confined water microdroplets

Cells contain more than 100 mM salt ions that are typically confined to dimensions of 5 to 10 micrometers by a hydrophobic cellular membrane. We found that in aqueous microdroplets having the same size as cells and that are confined in hydrocarbon oil, negatively charged molecules were distributed rather uniformly over the interior of the microdroplet, whereas positively charged molecules were localized at and near the surface. However, the addition of salt (NaCl) to the microdroplet caused all charged molecules to be localized near the oil-water interface. This salt-induced relocalization required less salt concentration in microdroplets compared to bulk water. Moreover, the localization became more prominent as the size of the microdroplet was reduced. The relocatization also critically depended on the type of oil. Our results imply that salt ions and different hydrophobic interfaces together may govern the local distribution of charged biomolecules in confined intracellular environments.

Influence of Inlet Capillary Temperature on the Microdroplet Chemistry Studied by Mass Spectrometry

Often, studies of microdroplet chemistry using electrospray ionization mass spectrometry (MS) either find a negligible effect of the heated inlet capillary of the mass spectrometer on the reaction rate or do not consider its effect. In this context, we studied two reactions in microdroplets, the Pomeranz-Fritsch synthesis of isoquinoline and the Combes quinoline synthesis. The reagents were electrosprayed with methanol and aqueous solutions forming small and large microdroplets at flow rates of 1 and 20 μL/min, respectively. We also varied the inlet capillary temperature from 100 to 350 °C. Contrary to the view that the inlet temperature has little to no influence on the reaction rate, we found that the Pomeranz-Fritsch reaction was markedly accelerated for both solvents and for both droplet sizes on increasing the temperature, whereas the Combes synthesis showed the opposite behavior. We propose that these strikingly different behaviors result from a competition of two effects, the evaporative cooling versus the heating of ejected bare ions from the droplet, both taking place inside the heated inlet. This finding suggests that these phenomena must be taken into account while interpreting the microdroplet reactions studied by electrospray or a similar kind of ambient ionization MS.

Can all bulk-phase reactions be accelerated in microdroplets?

Recent studies have shown that microdroplet reactions are markedly accelerated compared to the corresponding bulk-phase reactions. This raises the question whether all reactions can be sped up by this means. We present a counter example, and we show that the reaction mechanism in microdroplets can differ sharply from that in bulk, especially because of the distinct microdroplet surface environment. This analysis helps to guide us how to choose and control reactions in microdroplets and provides a possible perspective on utilizing microdroplet chemistry to scale up synthesis.

Exploring Chemistry in Microcompartments Using Guided Droplet Collisions in a Branched Quadrupole Trap Coupled to a Single Droplet, Paper Spray Mass Spectrometer

Recent studies suggest that reactions in aqueous microcompartments can occur at significantly different rates than those in the bulk. Most studies have used electrospray to generate a polydisperse source of highly charged microdroplets, leading to multiple confounding factors potentially influencing reaction rates (e.g., evaporation, charge, and size). Thus, the underlying mechanism for the observed enhancement remains unclear. We present a new type of electrodynamic balance-the branched quadrupole trap (BQT)-which can be used to study reactions in microdroplets in a controlled environment. The BQT allows for condensed phase chemical reactions to be initiated by colliding droplets with different reactants and levitating the merged droplet indefinitely. The performance of the BQT is characterized in several ways. Sub-millisecond mixing times as fast as ∼400 μs are measured for low velocity (∼0.1 m/s) collisions of droplets with <40 μm diameters. The reaction of o-phthalaldehyde (OPA) with alanine in the presence of dithiolthreitol is measured using both fluorescence spectroscopy and single droplet paper spray mass spectrometry. The bimolecular rate constant for reaction of alanine with OPA is found to be 84 ± 10 and 67 ± 6 M^-1 s^-1 in a 30 μm radius droplet and bulk solution, respectively, which demonstrates that bimolecular reaction rate coefficients can be quantified using merged microdroplets and that merged droplets can be used to study rate enhancements due to compartmentalization. Products of the reaction of OPA with alanine are detected in single droplets using paper spray mass spectrometry. We demonstrate that single droplets with <100 pg of analyte can easily be studied using single droplet mass spectrometry.

Ionic and Neutral Mechanisms for C-H Bond Silylation of Aromatic Heterocycles Catalyzed by Potassium tert-Butoxide

Exploiting C-H bond activation is difficult, although some success has been achieved using precious metal catalysts. Recently, it was reported that C-H bonds in aromatic heterocycles were converted to C-Si bonds by reaction with hydrosilanes under the catalytic action of potassium tert-butoxide alone. The use of Earth-abundant potassium cation as a catalyst for C-H bond functionalization seems to be without precedent, and no mechanism for the process was established. Using ambient ionization mass spectrometry, we are able to identify crucial ionic intermediates present during the C-H silylation reaction. We propose a plausible catalytic cycle, which involves a pentacoordinate silicon intermediate consisting of silane reagent, substrate, and the tert-butoxide catalyst. Heterolysis of the Si-H bond, deprotonation of the heteroarene, addition of the heteroarene carbanion to the silyl ether, and dissociation of tert-butoxide from silicon lead to the silylated heteroarene product. The steps of the silylation mechanism may follow either an ionic route involving K⁺ and ^tBuO^- ions or a neutral heterolytic route involving the [KO^tBu]₄ tetramer. Both mechanisms are consistent with the ionic intermediates detected experimentally. We also present reasons why KO^tBu is an active catalyst whereas sodium tert-butoxide and lithium tert-butoxide are not, and we explain the relative reactivities of different (hetero)arenes in the silylation reaction. The unique role of KO^tBu is traced, in part, to the stabilization of crucial intermediates through cation-π interactions.

Exploring Flexibility of Progesterone Receptor Ligand Binding Domain Using Molecular Dynamics

Progesterone receptor (PR), a member of nuclear receptor (NR) superfamily, plays a vital role for female reproductive tissue development, differentiation and maintenance. PR ligand, such as progesterone, induces conformation changes in PR ligand binding domain (LBD), thus mediates subsequent gene regulation cascades. PR LBD may adopt different conformations upon an agonist or an antagonist binding. These different conformations would trigger distinct transcription events. Therefore, the dynamics of PR LBD would be of general interest to biologists for a deep understanding of its structure-function relationship. However, no apo-form (non-ligand bound) of PR LBD model has been proposed either by experiments or computational methods so far. In this study, we explored the structural dynamics of PR LBD using molecular dynamics simulations and advanced sampling tools in both ligand-bound and the apo-forms. Resolved by the simulation study, helix 11, helix 12 and loop 895–908 (the loop between these two helices) are quite flexible in antagonistic conformation. Several residues, such as Arg899 and Glu723, could form salt-bridging interaction between helix 11 and helix 3, and are important for the PR LBD dynamics. And we also propose that helix 12 in apo-form PR LBD, not like other NR LBDs, such as human estrogen receptor α (ERα) LBD, may not adopt a totally extended conformation. With the aid of umbrella sampling and metadynamics simulations, several stable conformations of apo-form PR LBD have been sampled, which may work as critical structural models for further large scale virtual screening study to discover novel PR ligands for therapeutic application.

Acceleration of reaction in charged microdroplets

Using high-resolution mass spectrometry, we have studied the synthesis of isoquinoline in a charged electrospray droplet and the complexation between cytochrome c and maltose in a fused droplet to investigate the feasibility of droplets to drive reactions (both covalent and noncovalent interactions) at a faster rate than that observed in conventional bulk solution. In both the cases we found marked acceleration of reaction, by a factor of a million or more in the former and a factor of a thousand or more in the latter. We believe that carrying out reactions in microdroplets (about 1–15 μm in diameter corresponding to 0·5 pl – 2 nl) is a general method for increasing reaction rates. The mechanism is not presently established but droplet evaporation and droplet confinement of reagents appear to be two important factors among others. In the case of fused water droplets, evaporation has been shown to be almost negligible during the flight time from where droplet fusion occurs and the droplets enter the heated capillary inlet of the mass spectrometer. This suggests that (1) evaporation is not responsible for the acceleration process in aqueous droplet fusion and (2) the droplet–air interface may play a significant role in accelerating the reaction. We argue that this ‘microdroplet chemistry’ could be a remarkable alternative to accelerate slow and difficult reactions, and in conjunction with mass spectrometry, it may provide a new arena to study chemical and biochemical reactions in a confined environment.

Syntheses of Isoquinoline and Substituted Quinolines in Charged Microdroplets

A Pomeranz-Fritsch synthesis of isoquinoline and Friedländer and Combes syntheses of substituted quinolines were conducted in charged microdroplets produced by an electrospray process at ambient temperature and atmospheric pressure. In the bulk phase, all of these reactions are known to take a long time ranging from several minutes to a few days and to require very high acid concentrations. In sharp contrast, the present report provides clear evidence that all of these reactions occur on the millisecond timescale in the charged microdroplets without the addition of any external acid. Decreasing the droplet size and increasing the charge of the droplet both strongly contribute to reaction rate acceleration, suggesting that the reaction occurs in a confined environment on the charged surface of the droplet.

Going beyond electrospray: mass spectrometric studies of chemical reactions in and on liquids

Recently developed and applied ionization techniques have brought mass spectrometry to bear on previously inaccessible chemistry. We offer our perspective on this field and its application for studying reaction mechanisms.

There has been a burst in the number and variety of available ionization techniques to use mass spectrometry to monitor chemical reactions in and on liquids. Chemists have gained the capability to access chemistry at unprecedented timescales, and monitor reactions and detect intermediates under almost any set of conditions. Herein, recently developed ionization techniques that facilitate mechanistic studies of chemical processes are reviewed. This is followed by a discussion of our perspective on the judicious application of these and similar techniques in order to study reaction mechanisms.

Microdroplet fusion mass spectrometry for fast reaction kinetics

We investigated the fusion of high-speed liquid droplets as a way to record the kinetics of liquid-phase chemical reactions on the order of microseconds. Two streams of micrometer-size droplets collide with one another. The droplets that fused (13 μm in diameter) at the intersection of the two streams entered the heated capillary inlet of a mass spectrometer. The mass spectrum was recorded as a function of the distance x between the mass spectrometer inlet and the droplet fusion center. Fused droplet trajectories were imaged with a high-speed camera, revealing that the droplet fusion occurred approximately within a 500-μm radius from the droplet fusion center and both the size and the speed of the fused droplets remained relatively constant as they traveled from the droplet fusion center to the mass spectrometer inlet. Evidence is presented that the reaction effectively stops upon entering the heated inlet of the mass spectrometer. Thus, the reaction time was proportional to x and could be measured and manipulated by controlling the distance x. Kinetic studies were carried out in fused water droplets for acid-induced unfolding of cytochrome c and hydrogen-deuterium exchange in bradykinin. The kinetics of the former revealed the slowing of the unfolding rates at the early stage of the reaction within 50 μs. The hydrogen-deuterium exchange revealed the existence of two distinct populations with fast and slow exchange rates. These studies demonstrated the power of this technique to detect reaction intermediates in fused liquid droplets with microsecond temporal resolution.

Exclusive selectivity in the one-pot formation of C-C and C-Se bonds involving Ni-catalyzed alkyne hydroselenation: optimization of the synthetic procedure and a mechanistic study

A unique Ni-catalyzed transformation is reported for the one-pot highly selective synthesis of previously unknown monoseleno-substituted 1,3-dienes starting from easily available terminal alkynes and benzeneselenol. The combination of a readily available catalyst precursor, Ni(acac)2, and an appropriately tuned phosphine ligand, PPh2Cy, resulted in the exclusive assembly of the s-gauche diene skeleton via the selective formation of C-C and C-Se bonds. The unusual diene products were stable under regular experimental conditions, and the products maintained the s-gauche geometry both in the solid state and in solution, as confirmed by X-ray analysis and NMR spectroscopy. Thorough mechanistic studies using ESI-MS revealed the key Ni-containing species involved in the reaction.

Induction of protein conformational change inside the charged electrospray droplet

The behavior of the analyte molecules inside the neutral core of the charged electrospray (ES) droplet is not unambiguously known to date. The possibility of protein conformational change inside the charged ES droplet has been investigated. The ES droplets encapsulating the protein molecules were exposed to the acetic acid vapor in the ionization chamber to absorb the acetic acid vapor. Because of the faster evaporation of water than that of acetic acid, the droplets became enriched with acetic acid and thus altered the solvent environment (e.g. pH and polarity) of the final charged droplets from where the naked charged analytes (proteins) are formed. Thus, the perturbation of the ES droplet solvent environment resulted in the protein conformational change (unfolding) during the short lifespan of the ES droplet and that is reflected by the multimodal charge state distribution in the corresponding mass spectra. Further, the extent of this conformational change inside the ES droplet was found to be related to the structural flexibility of the protein. Although the protein conformational change inside the ES droplet has been driven by using acetic acid vapor in the present study, the results would help in the near future to understand the spontaneity of the conformational change of the analyte on the millisecond timescale of phase transition in the natural way of ES process.

Selective deletion of the internal lysine residue from the peptide sequence by collisional activation

The gas-phase peptide ion fragmentation chemistry is always the center of attraction in proteomics to analyze the amino acid sequence of peptides and proteins. In this work, we describe the formation of an anomalous fragment ion, which corresponds to the selective deletion of the internal lysine residue from a series of lysine containing peptides upon collisional activation in the ion trap. We detected several water-loss fragment ions and the maximum number of water molecules lost from a particular fragment ion was equal to the number of lysine residues in that fragment. As a consequence of this water-loss phenomenon, internal lysine residues were found to be deleted from the peptide ion. The N,N-dimethylation of all the amine functional groups of the peptide stopped the internal lysine deletion reaction, but selective N-terminal α-amino acetylation had no effect on this process indicating involvement of the side chains of the lysine residues. The detailed mechanism of the lysine deletion was investigated by multistage CID of the modified and unmodified peptides, by isotope labeling and by energy resolved CID studies. The results suggest that the lysine deletion might occur through a unimolecular multistep mechanism involving a seven-membered cyclic imine intermediate formed by the loss of water from a lysine residue in the protonated peptide. This intermediate subsequently undergoes degradation reaction to deplete the interior imine ring from the peptide backbone leading to the deletion of an internal lysine residue.

Degradation of Malachite green by Potassium persulphate, its enhancement by 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane nickel(II) perchlorate complex, and removal of antibacterial activity

In this study, degradation of Malachite green (MG) (10mg/L) by Potassium persulphate (KPS) (1 g/L), and KPS in presence of (1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) nickel(II) perchlorate (complex1), (200 μM), was investigated by spectrophotometric and HPLC methods. KPS alone had ability to degrade MG. Interestingly, rate of degradation of MG was enhanced upon addition of complex1. Degradation was effective at pH range of 3-9 and was found to be dependent on initial concentration of KPS, complex1, MG, and pH. Degradation of MG by KPS was not significantly affected in presence of Ni(II) ions whereas in presence of Fe(II) ions degradation was incomplete. Ability of KPS to reduce TOC increased in presence of complex1. Transformation products were analysed by LC-ESI-MS. Finally, treatment of MG with complex1 and KPS resulted in removal of antibacterial activity of MG under in vitro conditions.

Formation of diclofenac molecular ions as the effect of Cu(2+)-π interaction under electrospray ionization mass spectrometry conditions

Solutions containing diclofenac (M) and a copper salt [CuCl2, Cu(ClO4)2, Cu(NO3)2, CuSO4) were analysed by electrospray ionisation mass spectrometry (ESI-MS). Because of the cation-pi interactions in diclofenac-Cu(II) complexes, the diclofenac molecular ion M+* at m/z 295 was formed. It was found that the solvent composition (methanol versus water/methanol) and counter ion strongly affect the M+* ion formation. Formation pathways of ion M+* are discussed.

Electrospray ionization mass spectrometry: a technique to access the information beyond the molecular weight of the analyte

The Electrospray Ionization (ESI) is a soft ionization technique extensively used for production of gas phase ions (without fragmentation) of thermally labile large supramolecules. In the present review we have described the development of Electrospray Ionization mass spectrometry (ESI-MS) during the last 25 years in the study of various properties of different types of biological molecules. There have been extensive studies on the mechanism of formation of charged gaseous species by the ESI. Several groups have investigated the origin and implications of the multiple charge states of proteins observed in the ESI-mass spectra of the proteins. The charged analytes produced by ESI can be fragmented by activating them in the gas-phase, and thus tandem mass spectrometry has been developed, which provides very important insights on the structural properties of the molecule. The review will highlight recent developments and emerging directions in this fascinating area of research.