The mechanism of the Stille reaction investigated by electrospray ionization mass spectrometry

On-line monitoring of Stille reactions was performed via direct infusion electrospray ionization mass spectrometry (ESI-MS) and its tandem version (ESI-MS/MS). When operated in the positive ion mode, ESI(+)-MS was able to transfer, directly from solution to the gas phase, the species involved in all main steps of a Stille reaction, that is, the catalytically active palladium species Pd(PPh3)2, in its molecular ion form as well as the key cationic Pd(II) intermediates, including cyclic IPd-(CH2CH)Sn species. When searching for anionic species, ESI(-)-MS monitoring showed I- as the only anion detectable in the reaction medium. A detailed catalytic cycle for a Stille reaction was elaborated in which reaction intermediates and the previously elusive catalytically active Pd(0) species are shown in association with the respective ionic species intercepted by ESI-MS and further characterized by ESI-MS/MS.     

Determination of micelle mass by electrospray ionization mass spectrometry

By electrospray ionization (ESI) mass spectrometry, micelle solutions of sodium cholate were investigated in detail in the presence and absence of ethanol. The average aggregation number could be evaluated from the spectra acquired under conditions where soft collisions adequate to measure the micelle solution were induced, and the value agreed well with that obtained previously by other methods. From the dependence on ethanol content, it was also found that the average aggregation number in aqueous solution without organic solvent could be reliably estimated. The ESI method proved to be a useful tool for determining the micelle mass in the original aqueous phase.     

Real‐time reaction monitoring by probe electrospray ionization mass spectrometry

Probe electrospray ionization (PESI) is a modified version of the electrospray ionization (ESI), where the capillary for sampling and spraying is replaced by a solid needle. High tolerance to salts and direct ambient sampling are major advantages of PESI compared with conventional ESI. In this study, PESI‐MS was used to monitor some biological and chemical reactions in real‐time, such as acid‐induced protein denaturation, hydrogen/deuterium exchange (HDX) of peptides, and Schiff base formation. By using PESI‐MS, time‐resolved mass spectra and ion chromatograms can be obtained reproducibly. Real‐time PESI‐MS monitoring can give direct and detailed information on each chemical species taking part in reactions, and this is valuable for a better understanding of the whole reaction process and for the optimization of reaction parameters. PESI‐MS can be considered as a potential tool for real‐time reaction monitoring due to its simplicity in instrumental setup, direct sampling with minimum sample preparation and low sample consumption. Copyright © 2010 John Wiley & Sons, Ltd.     

Synchronized dual-polarity electrospray ionization mass spectrometry

This work describes the synchronized dual-polarity (DP) electrospray ionization (ESI) method and demonstrates the first DP ESI mass spectra obtained using two mass spectrometers. Stable double Taylor cones were produced by applying two counter electric voltages with opposite polarities to one electrosprayer. The development of double Taylor cones required higher extraction voltages than conventional ESI, but DP ESI worked effectively at liquid flow rate range three times wider than conventional ESI. Using pure methanol, the emission currents of the two cones were neutralized and no current was drawn from the sprayer. Synchronized DP mass spectra were obtained using electrospray calibrants dissolved in methanol solution of low water content. For bovine insulin with conventional electrospray solution, the gas-assisted electrospray delivered satisfactory sensitivity and stability for routine mass analyses.     

Identification of bismuth-thiolate-carboxylate clusters by electrospray ionization mass spectrometry

The known thermal and hydrolytic stability of bismuth-sulfur bonds indicates that biological targets for bismuth likely involve thiol or thiolate functionalities, such as in L-cysteine. Complexes of bismuth with cysteine or other thiol-carboxylic acid ligands have been isolated and characterized providing a preliminary view of the potential participation of these functional groups in the biochemical mechanisms involving bismuth. A broader assessment of bismuth-thiolate interactions has been possible using electrospray ionization mass spectrometry (ESI-MS). A wide range of complexes has been observed containing mercaptosuccinic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, and/or 2-amino-3-mercaptopropionic acid (cysteine). The identification of various multibismuth multiligand cluster ions defines new chemistry for bismuth.     

Fragmentation studies of sartans by electrospray ionization mass spectrometry

Sartans and related analogues with 5‐oxo‐l, 2, 4‐oxadiazole ring and tetrazole ring are investigated in detail using collision‐induced dissociation (CID) method in positive ion mode by electrospray ionization tandem mass spectrometry (ESI‐MSⁿ). It is found that the protonated sartans and related analogues tend to form the N‐substituted‐3‐substituted phenanthridin‐6‐amine ion which has a large conjugative structure. The possible fragmentation pathways were proposed for the first time, and the key structure of product ions was confirmed by high resolution tandem mass spectrometry and theoretical calculation. It is very helpful for understanding the intriguing roles of sartans analogues in fragmentation reactions and enriching the knowledge of the gas‐phase chemistry of the oxadiazole and tetrazole ring. Copyright © 2017 John Wiley & Sons, Ltd.     

Intact skin analysis by desorption electrospray ionization mass spectrometry

In vivo skin analysis by Desorption Electrospray Ionization was characterized on healthy human volunteers by directing pneumatically assisted electrospray directly onto their fingertips. In order to eliminate the risk of electric shock, a high ohmic resistor was built into the system. Positive ion DESI-MS analysis yields low intensity spectra, while negative ion spectra feature a number of various biogenic carboxylic acids. Compounds of external origin and excreted molecules were found to have different analysis kinetics, with the exception of highly hydrophobic species. The difference was demonstrated in the case of nicotine and cotinine. Pharmacokinetic studies were performed using a rat animal model. The kinetics of the anesthetic ketamine was followed by DESI, and results were in agreement with off-line HPLC-MS blood analysis. Using a similar approach for N,N'-dimethylthiourea (DMTU), a novel method was developed for the real-time quantification of oxidative stress. DMTU was administered to the animals, and the ratio of the molecule and its oxidized form was monitored from the skin surface. The ratio was found to be highly sensitive to experimentally induced diabetes mellitus type I and angiotensin-induced chronic oxidative stress. It was concluded that the method has a number of potential applications in the fields of forensics, pharmacology and clinical chemistry.     

High-throughput quantitative analysis by desorption electrospray ionization mass spectrometry

A newly developed high-throughput desorption electrospray ionization (DESI) source was characterized in terms of its performance in quantitative analysis. A 96-sample array, containing pharmaceuticals in various matrices, was analyzed in a single run with a total analysis time of 3 min. These solution-phase samples were examined from a hydrophobic PTFE ink printed on glass. The quantitative accuracy, precision, and limit of detection (LOD) were characterized. Chemical background-free samples of propranolol (PRN) with PRN-d₇ as internal standard (IS) and carbamazepine (CBZ) with CBZ-d₁₀ as IS were examined. So were two other sample sets consisting of PRN/PRN-d₇ at varying concentration in a biological milieu of 10% urine or porcine brain total lipid extract, total lipid concentration 250 ng/μL. The background-free samples, examined in a total analysis time of 1. 5 s/sample, showed good quantitative accuracy and precision, with a relative error (RE) and relative standard deviation (RSD) generally less than 3% and 5%, respectively. The samples in urine and the lipid extract required a longer analysis time (2. 5 s/sample) and showed RSD values of around 10% for the samples in urine and 4% for the lipid extract samples and RE values of less than 3% for both sets. The LOD for PRN and CBZ when analyzed without chemical background was 10 and 30 fmol, respectively. The LOD of PRN increased to 400 fmol analyzed in 10% urine, and 200 fmol when analyzed in the brain lipid extract.     

Analyzing glycerol-mediated protein oligomerization by electrospray ionization mass spectrometry

Glycerol is widely used as protein stabilizer, in both local and commercial preparations, so it has become necessary to develop methods for mass spectrometric analysis of protein preparations in the presence of glycerol. However, this stabilizing agent may cause signal suppression when present in high concentrations, and is also known to induce protein supercharging even at low concentrations. This work reports the use of electrospray ionization (ESI) mass spectrometry to characterize glycerol-mediated protein oligomerization. This phenomenon seems to involve the formation of strong non-covalent interactions between protein and glycerol involving close contact between the monomers, leading to formation of protein oligomers adducted with glycerol molecules under the characteristic analytical conditions of the ESI interface. At high orders of oligomerization a lower number of glycerol molecules is required to maintain the high oligomeric states than for the dimers and trimers, and it is possible that for the higher oligomers the monomers become so close to one another that non-covalent bonds between the side chains of the amino acid residues in the proteins may be established.     

Antisense peptide interactions studied by electrospray ionization mass spectrometry

Non-covalent interactions between met- and leu-enkephalins and their antisense peptides were studied by electrospray ionization mass spectrometry. Mixtures of sense and antisense peptides gave both the corresponding homodimers and heterodimers. The relative abundance ratios of the heterodimer to that of the homodimer of the sense peptide and the relative stability constants of the heterodimers were compared with the corresponding values from mixtures of the sense peptides and a control peptide. The results show that there is a preferential interaction between the sense and antisense peptides compared with that between the sense and control peptides.     

Infrared laser-assisted desorption electrospray ionization mass spectrometry

We have used an infrared laser for desorption of material and ionization by interaction with electrosprayed solvent. Infrared laser-assisted desorption electrospray ionization (IR LADESI) mass spectrometry was used for the direct analysis of water-containing samples under ambient conditions. An ion trap mass spectrometer was modified to include a pulsed Er:YAG laser at 2.94 microm wavelength coupled into a germanium oxide optical fiber for desorption at atmospheric pressure and a nanoelectrospray source for ionization. Analytes in aqueous solution were placed on a stainless steel target and irradiated with the pulsed IR laser. Material desorbed and ablated from the target was ionized by a continuous stream of charged droplets from the electrosprayed solvent. Peptide and protein samples analyzed using this method yield mass spectra similar to those obtained by conventional electrospray. Blood and urine were analyzed without sample pretreatment to demonstrate the capability of IR LADESI for direct analysis of biological fluids. Pharmaceutical products were also directly analyzed. Finally, the role of water as a matrix in the IR LADESI process is discussed.     

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this month's Special Feature. We particularly hope that younger scientists will find value in this month's Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.     

Detection of intermediates for the Eschweiler-Clarke reaction by liquid-phase reactive desorption electrospray ionization mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) has been developed dramatically as a powerful tool for the rapid analysis of samples in their native environment. Here a novel application of DESI-MS was demonstrated for direct probing of the reactive intermediates in the liquid-phase Eschweiler-Clarke reaction, a reductive amination reaction whereby a primary (or secondary) amine is successively N-methylated using excess formaldehyde and formic acid. The intermediates ion species of sodiated amino alcohol ([I + Na](+)) and iminium ([II](+)), along with the corresponding protonated molecules of amine reactant ([M + H](+)) and end product ([III + H](+)), were simultaneously and unambiguously characterized by the positive ion DESI-MS in the native liquid-phase reactive condition. The operating variables were optimized for better analytical performance including the spray solvent composition (such as formic acid concentration, proportion of methanol-water), voltage applied, spray spatial distance and incident angle. The feasibility of the reactive DESI-MS detection of acid-formaldehyde methylations was further validated using amines of a large variety of chemical types (2 primary and 3 secondary amines). Thus, the liquid-phase reactive DESI-MS technique allows the direct analysis of reaction intermediates occurring in complex liquid solutions without sample preparation to provide a valuable insight into chemical reaction mechanisms.     

Monitoring of unfolding of metallo-proteins by electrospray ionization mass spectrometry

An electrospray ionisation (ESI) mass spectrometric method for the determination of the equilibrium constant and free energy (DeltaG) of protein unfolding was used to monitor the denaturation process at different pH of three metallo-proteins, i.e. wild-type copper azurin, zinc azurin and wild-type amicyanin. The time course of the unfolding process was followed by dissolving the proteins under denaturing conditions (methanol-water (1 : 1, v/v)) at different pH (2.5, 3.0, 3.5) and recording ESI spectra at time intervals. The spectra showed two series of peaks, corresponding to the native holo-protein and the unfolded apo-protein. From the intensity ratio of these two series of peaks at increasing time and at equilibrium, the equilibrium constants for the unfolding process for the three proteins could be determined. From these equilibrium constants a DeltaG degrees derivation was attempted. The DeltaG degrees values obtained decrease with decrease in pH, in agreement with the expected reduction of conformational stability of proteins at lower pH. The results obtained confirm that ESI-MS can be used for monitoring of unfolding process and to derive quantitative thermodynamic data.     

Analysis of oxidizer salt mixtures by electrospray ionization mass spectrometry

Binary aqueous mixtures of NaNO3, KNO3 and NaClO4 oxidizers were analyzed using electrospray ionization mass spectrometry. Sodium nitrate solutions were observed to form doubly charged clusters of the type [(NaNO3)n2Na]2+ and [(NaNO3)n2NO3]2-, where n = 11, 13, 15, etc., in addition to singly charged cluster ions that have been reported previously. The identity of the doubly charged clusters was determined by tandem mass spectrometry. Two-component NaNO3-KNO3 salt solutions were observed to form cluster ions of the type [(NaNO3)i(KNO3)jNO3]- in the negative ion mode and [(NaNO3)i(KNO3)jNa]+ and [(NaNO3)i(KNO3)jK]+ in the positive ion mode, where i + j = 1, 2, 3 ... 10. Two-component solutions of KNO3-NaClO4 formed ions of the type [(KNO3)i(NaClO4)j(KClO4)k(NaNO3)lK](+) and [(KNO3)i(NaClO4)j(KClO4)k(NaNO3)lNa]+ in the positive ion mode, where i + j + k + l = 1, 2, 3 ... 10. Similar clusters containing excess nitrate and perchlorate to provide the charge are formed in the negative ion mode. In each case, the maximum number of spectral lines for a cluster of size n can be calculated as the number of combinations of n(th) order (where n = i + j) of N different cation-anion pairs taken with replication and without regard for the ordering of the N cation-anion pairs. The actual number of lines observed may be reduced due to degeneracy of nominal m/z values for some ions.