Monitoring binding affinity between drug and α1-acid glycoprotein in real time by Venturi easy ambient sonic-spray ionization mass spectrometry

Caffeine quantification in dietary supplements using high-throughput on-line solid phase extraction coupled to Venturi easy ambient sonic-spray ionization mass spectrometry

Caffeine is present in a large number of beverages and is an additive used in dietary supplements. Therefore, the concern about its quality and safety for consumers has been increasing and hence requires faster and simpler analytical methods to determine the caffeine amount. The high-throughput analysis is an appropriate solution to pharmaceuticals, bioanalysis, forensic and food laboratory routines. In this sense, Venturi easy ambient sonic-spray ionization mass spectrometry (V-EASI-MS), a specific ambient ionization source, is suitable to enable direct analysis of sample solutions in real time and is appropriate to be coupled to liquid chromatography (LC). The development of an on-line solid phase extraction system coupled to V-EASI-MS optimizes the advantages of LC-MS hyphenation by enhancing the figures of merit of the analytical method according to AOAC guidelines and simultaneously minimizing the runtime analysis to 1.5 min per sample, as well as sample preparation steps and solvent consumption, which is currently a challenge for quantitative applications of ambient ionization MS.

Condensed Phase Membrane Introduction Mass Spectrometry: A Direct Alternative to Fully Exploit the Mass Spectrometry Potential in Environmental Sample Analysis

Membrane introduction mass spectrometry (MIMS) is a direct mass spectrometry technique used to monitor online chemical systems or quickly quantify trace levels of different groups of compounds in complex matrices without extensive sample preparation steps and chromatographic separation. MIMS utilizes a thin, semi-permeable, and selective membrane that directly connects the sample and the mass spectrometer. The analytes in the sample are pre-concentrated by the membrane depending on their physicochemical properties and directly transferred, using different acceptor phases (gas, liquid or vacuum) to the mass spectrometer. Condensed phase (CP) MIMS use a liquid as a medium, extending the range to new applications to less-volatile compounds that are challenging or unsuitable to gas-phase MIMS. It directly allows the rapid quantification of selected compounds in complex matrices, the online monitoring of chemical reactions (in real-time), as well as in situ measurements. CP-MIMS has expanded beyond the measurement of several organic compounds because of the use of different types of liquid acceptor phases, geometries, dimensions, and mass spectrometers. This review surveys advancements of CP-MIMS and its applications to several molecules and matrices over the past 15 years.

Cell-Based Ambient Venturi Autosampling and Matrix-Assisted Laser Desorption Ionization Mass Spectrometric Imaging of Secretory Products

A cell-based ambient Venturi autosampling device was established for the monitoring of dynamic cell secretions in response to chemical stimulations in real time with temporal resolution on the order of a second. Detection of secretory products of cells and screening of bioactive compounds are primarily performed on an ambient autosampling probe and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. It takes advantage of the Venturi effect in which the fluid flowing through an inlet capillary tube is automatically fed into a parallel array of multiple outlet capillaries. Cells are incubated inside the inlet capillary tube that is connected with either a syringe pump or liquid chromatography (LC) for the transfer of single compounds or mixtures, respectively. Secretory products were continuously pushed into the outlet capillaries and then spotted into a compressed thin film of the matrix material 9-aminoacridine for MALDI mass spectrometric imaging. In physiological pH, without the use of high voltages and without the use of chemical derivatizations, this platform can be applied to the direct assay of neurotransmitters or other secretory products released from cells in response to the stimulation of individual compounds or LC-separated eluates of natural mixtures. It provides a new way to identify bioactive compounds with a detection limit down to 0.04 fmol/pixel.

Liquid chromatography coupled to Venturi easy ambient sonic spray ionization mass spectrometry

Venturi easy ambient sonic spray ionization (V-EASI) is a soft ambient ionization (AI) source that has the advantages of being suitable to the analysis of samples in solution (differently from the majority of AI sources), performing self-pumping, voltage- and heat-free ionization, and requiring minimum or no sample preparation. Since this ionization technique has not been fully explored, the present study provides a proof of principle of the coupling of liquid chromatography to mass spectrometry (LC-MS) using V-EASI as the interface. In order to test the performance of the developed LC-V-EASI-MS system, a quantification method for bixin, a natural dye from annatto (Bixa Orellana L.), which is known to be sensitive to the high voltage applied for electrospray ionization mass spectrometry (ESI-MS) analysis, was validated according to FDA criteria and tested in real samples. The analytical method was successfully applied and met the validation criteria, providing a detectability 10 times better than methods already reported to the quantification of bixin and no matrix effect was observed. Therefore, this proof of principle contributes to the continuous development of AI sources that represents the last great technological advance in MS towards becoming a miniaturized technique able to analyze samples closer to their actual state.

Study of the noncovalent interactions between phenolic acid and lysozyme by cold spray ionization mass spectrometry (CSI-MS), multi-spectroscopic and molecular docking approaches

Elucidating the recognition mechanisms of the noncovalent interactions between pharmaceutical molecules and proteins is important for understanding drug delivery in vivo, and for the further rapid screening of clinical drug candidates and biomarkers. In this work, a strategy based on cold spray ionization mass spectrometry (CSI-MS), combined with fluorescence, circular dichroism (CD), Fourier transform infrared spectroscopy (FTIR), and molecular docking methods, was developed and applied to the study of the noncovalent interactions between phenolic acid and lysozyme (Lys). Based on the real characterization of noncovalent complex, the detailed binding parameters, as well as the protein conformational changes and specific binding sites could be obtained. CSI-MS and tandem mass spectrometry (MS/MS) technique were used to investigate the phenolic acid-Lys complexes and the structure-affinity relationship, and to assess their structural composition and gas phase stability. The binding affinity was obtained by direct and indirect MS methods. The fluorescence spectra showed that the intrinsic fluorescence quenching of Lys in solution was a static quenching mechanism caused by complex formation, which supported the MS results. The CD and FTIR spectra revealed that phenolic acid changed the secondary structure of Lys and increased the α-helix content, indicating an increase in the tryptophan (W) hydrophobicity near the protein binding site resulting in a conformational alteration of the protein. In addition, molecular docking studies were performed to investigate the binding sites and binding modes of phenolic acid on Lys. This strategy can more comprehensively and truly characterize the noncovalent interactions and can guide further research on the interactions of phenolic acid with other proteins.

Study of the noncovalent interactions of ginsenosides and amyloid-β-peptide by CSI-MS and molecular docking

Noncovalent interactions between drugs and proteins play significant roles for drug metabolisms and drug discoveries. Mass spectrometry has been a commonly used method for studying noncovalent interactions. However, the harsh ionization process in electrospray ionization mass spectrometry (ESI-MS) is not conducive to the preservation of noncovalent and unstable biomolecular complexes compared with the cold spray ionization mass spectrometry (CSI-MS). A cold spray ionization providing a stable solvation-ionization at low temperature is milder than ESI, which was more suitable for studying noncovalent drug-protein complexes with exact stoichiometries. In this paper, we apply CSI-MS to explore the interactions of ginsenosides toward amyloid-β-peptide (Aβ) and clarify the therapeutic effect of ginsenosides on Alzheimer's disease (AD) at the molecular level for the first time. The interactions of ginsenosides with Aβ were performed by CSI-MS and ESI-MS, respectively. The ginsenosides Rg1 bounded to Aβ at the stoichiometries of 1:1 to 5:1 could be characterized by CSI-MS, while dehydration products are more readily available by ESI-MS. The binding force depends on the number of glycosyls and the type of ginsenosides. The relative binding affinities were sorted in order as follows: Rg1 ≈ Re > Rd ≈ Rg2 > Rh2, protopanaxatriol by competition experiments, which were supported by molecular docking experiment. CSI-MS is expected to be a more appropriate approach to determine the weak but specific interactions of proteins with other natural products especially polyhydroxy compounds.

Self-aspiration sampling extractive electrospray ionization mass spectrometry (EESI-MS) for high-throughput analysis of liquid samples

RATIONALE

Extractive electrospray ionization mass spectrometry (EESI-MS) was invented as a typical ambient mass spectrometry method (AMS) and has been used for analyzing complex liquid samples. Here, we designed a Venturi effect-based self-aspiration sampling device and applied it to the EESI-MS for high-throughput analysis of liquid sample.

METHODS

A special concentric nebulizer was designed and employed to produce a suction force for the direct aspiration of liquid samples, followed by ionization and detection. This sample aspiration process was explained and optimized using computational fluid dynamics (CFD) analysis. Experiment data were recorded to exhibit the sensitivity, memory effect, inter-day reproducibility, throughput, and applicability of the self-aspiration sampling EESI-MS.

RESULTS

The limit of detection (LOD) of this method was determined as 4.5 × 10(-10)  g/mL (S/N = 3) for caffeine, and the sample throughput and relative standard deviation (RSD) for full scan mode can reach 0.67 samples/s and 4.76%, respectively. Even for MS/MS mode, the frequency can still be kept at 0.4 samples/s (RSD = 4.71%). Inter-day RSD examined in 1 week was below 10% for the signal of characteristic fragment ions of reserpine. Moreover, based on this method, the amount of caffeine in instant coffee was determined as 4.7%. This device was also proven to be suitable for the protein/peptide analysis.

CONCLUSIONS

These experiment results, especially the amazing results on sample throughput and inter-day RSD, suggest that we provide a valuable device which can be used for the direct high-throughput qualitative/quantitative mass spectrometry analysis of real liquid samples in ambient. Copyright © 2016 John Wiley & Sons, Ltd.