APPI-Derived Cyclic Peptide Enhances Aβ42 Aggregation and Reduces Aβ42-Mediated Membrane Destabilization and Cytotoxicity

An amyloid precursor protein inhibitor (APPI) and amyloid beta 42 (Aβ42) are both subdomains of the human transmembrane amyloid precursor protein (APP). In the brains of patients with Alzheimer's disease (AD), Aβ42 oligomerizes into aggregates of various sizes, with intermediate, low-molecular-weight Aβ42 oligomers currently being held to be the species responsible for the most neurotoxic effects associated with the disease. Strategies to ameliorate the toxicity of these intermediate Aβ42 oligomeric species include the use of short, Aβ42-interacting peptides that either inhibit the formation of the Aβ42 oligomeric species or promote their conversion to high-molecular-weight aggregates. We therefore designed such an Aβ42-interacting peptide that is based on the β-hairpin amino acid sequence of the APPI, which exhibits high similarity to the β-sheet-like aggregation site of Aβ42. Upon tight binding of this 20-mer cyclic peptide to Aβ42 (in a 1:1 molar ratio), the formation of Aβ42 aggregates was enhanced, and consequently, Aβ42-mediated cell toxicity was ameliorated. We showed that in the presence of the cyclic peptide, interactions of Aβ42 with both plasma and mitochondrial membranes and with phospholipid vesicles that mimic these membranes were inhibited. Specifically, the cyclic peptide inhibited Aβ42-mediated mitochondrial membrane depolarization and reduced Aβ42-mediated apoptosis and cell death. We suggest that the cyclic peptide modulates Aβ42 aggregation by enhancing the formation of large aggregates─as opposed to low-molecular-weight intermediates─and as such has the potential for further development as an AD therapeutic.

Dopant-assisted atmospheric pressure photoionization Orbitrap mass spectrometry - An approach to molecular characterization of lignin oligomers

Lignin is the second most abundant biopolymer in nature and is considered an important renewable source of aromatic compounds. One of the most promising analytical methods for molecular characterization of lignin is Orbitrap high-resolution mass spectrometry with atmospheric pressure photoionization (APPI), proved itself in the study of lignins of various origins and their depolymerization products. In this work, the photoionization of lignin using acetone, 1,4-dioxane, and THF as solvents for the biopolymer and APPI dopants providing the generation of protonated and deprotonated molecules of lignin oligomers has been studied. The ionization conditions were optimized on the basis of the dependences of the total ion current on temperature and the flow rate of the solution into the ion source. Lignin degradation processes under APPI conditions occur mainly with the cleavage of ether β-O-4 bonds between phenylpropane structural units, demethylation (negative ion mode), as well as the loss of water and formaldehyde (positive ion mode). Negative ion mode APPI provides a higher ionization efficiency in the region of high molecular weights, however, it is characterized by an increased fragmentation of β-O-4 ether bonds compared to APPI(+) leading to a partial depolymerization of lignin in the ion source. The combination of APPI with Orbitrap high-resolution mass spectrometry allows obtaining mass spectra of coniferous and deciduous wood lignins with resolved fine structure and containing signals of up to 3000 oligomers in the mass range of 300-1800 Da. This can be used for comprehensive characterization of lignins at molecular level and tracking changes in biopolymer chemical composition in various processes.

Rapid screening and quantitation of PAHs in water and complex sample matrices by solid-phase microextraction coupled to capillary atmospheric pressure photoionization-mass spectrometry

A capillary atmospheric pressure photoionization (cAPPI) source was used to analyze polycyclic aromatic hydrocarbons (PAHs) in complex matrices like grilled meat extract and urban dust reference material, as well as screening for PAHs in aqueous samples such as tap and lake water. A high-throughput workflow was developed that allowed rapid screening of unknown samples by direct solid-phase microextraction (SPME) coupled with cAPPI-MS, with confirmatory gas chromatography performed only for samples containing trace amounts of PAHs. Extraction times were as low as 15 s, with a total analysis time of 2 min per sample for screening. Limits of detections were in the low pg/ml range and in the subpg/ml range for the direct and chromatographic approach, respectively, with a linear dynamic range between two and three orders of magnitude, as determined for 15 model PAHs. This rapid approach represents an attractive way to screen samples containing nonpolar compounds using an ambient ionization source.

Assessment of PM2.5-bound nitrogen-containing organic compounds (NOCs) during winter at urban sites in China and Korea

In this study, ambient fine particles (PM_2.5) were collected in two urban cities in China and Korea (Beijing and Gwangju, respectively) simultaneously in January 2018. Analysis of the nonpolar and semipolar organic matter (OM) using atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that compounds containing only C, H, and O (CHO) and those containing C, H, O, and N (CHON) accounted for more than 90% of the total intensity of the OM peaks. Higher proportions of CHON compounds were observed during days with abnormally high PM_2.5 concentrations at both sites than on regular or non-event days. The proportion of CHON species at the Beijing site was not correlated with secondary ionic species (i.e., NO₃^-, SO₄^2-, and NH₄⁺) or gaseous components (i.e., O₃, NO₂, and SO₂). In contrast, the proportion of CHON species at the Gwangju site was positively correlated with the concentrations of particulate nitrate and ammonium ions, assuming that ambient ammonium nitrate plays a role in the atmospheric formation of nitrogen-containing organic compounds (NOCs) at the Gwangju site and that Gwangju is more strongly influenced by secondary aerosols than Beijing is. In particular, a significant proportion of the compounds observed at the Beijing site contained only C, H and N (CHN), while negligible amounts of CHN were detected at the Gwangju site. The CHN species in Beijing were identified as quinoline compounds and the corresponding -CH₂ homologous series using complementary GC × GC-TOF MS analysis. These results suggest that NOCs and their -CH₂ homologous series from primary emissions may be significant contributors to nonpolar and semipolar OM during winter in Beijing, while NOCs with high oxidation states, likely formed via ambient-phase nitrate-mediated reactions, may be the dominant OM constituents in Gwangju.

Atmospheric pressure photoionization versus electrospray for the dereplication of highly conjugated natural products using molecular networks

Natural products are sources of inspiration and reservoir of high valuable molecules. Recently, analytical tools based on liquid chromatography coupled to tandem mass spectrometry to generate molecular network became widely employed for dereplication. This strategy greatly accelerates the identification of known and structural hypothesis of unknown. Despite the availability of different ionization sources, alternatives to classical electrospray ionization (ESI), such as atmospheric pressure chemical ionization (APCI) or photoionization (APPI), have been neglected. In particular, APPI has been described for its ionization efficiency on non-polar molecules bearing no acid or basic groups. For that reason, we investigated APPI potential to generate molecular network and compare it to ESI on several criteria that are generation of ion species, sensitivity and signal-to-noise ratio (SNR) for different extracts rich in highly conjugated natural products. We first optimized APPI experimental conditions on crude extract from a fungus, Penicillium sclerotiorum, producing polyketones belonging to the azaphilone family. Then we compared APPI and ESI on different fractions of the fungus and on two plant extracts, French Guyanese Swartzia panacoco (Aubl.) R.S. Cowan (arial parts) and Indian Cassia auriculata L. (leaves) containing phenolic compounds, such as flavonoids. While ESI generated more ion species and displayed a better sensitivity, APPI generated only protonated adduct and better SNR. Comparing ESI and APPI generated species on molecular network reveal that both strategies overlap for the majority of protonated ions.

Quantification of Brominated Polycyclic Aromatic Hydrocarbons in Environmental Samples by Liquid Chromatography Tandem Mass Spectrometry with Atmospheric Pressure Photoionization and Post-column Infusion of Dopant

A sensitive method for the quantification of brominated polycyclic aromatic hydrocarbons (BrPAHs) in environmental samples is yet to be developed. Here, we optimized the analytical conditions for liquid chromatography tandem mass spectrometry with atmospheric pressure photoionization and post-column infusion of dopant (LC-DA-APPI-MS/MS). We then compared the sensitivity of our developed method with that of conventional gas chromatography high-resolution MS (GC-HRMS) by comparing the limits of quantification (LOQs) for a range of BrPAHs. Finally, to evaluate our developed method, 12 BrPAHs in sediments and fish collected from Tokyo Bay, Japan, were analyzed; 9 common PAHs were also targeted. The LOQs of the developed analytical method were 14 - 160 times lower than those of GC-HRMS for the targeted BrPAHs. The developed analytical method is a sensitive approach for determining the concentrations of BrPAHs in sediment and fish samples.

Assessment of molecular diversity of lignin products by various ionization techniques and high-resolution mass spectrometry

Lignin is a highly complex, plant-derived natural biomass component, the analysis of which requires significant demands on the analytical platform. Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) has been shown to be able to readily assess the complexity of lignin and lignin degradation products by assigning tens of thousands of compounds with elemental formulae. Nevertheless, many experimental and instrumental parameters introduce discrimination towards certain components, which limits the comprehensive MS analysis. As a result, a complete characterization of the lignome remains a challenge. The present study investigated a degraded lignin sample using FT-ICR MS and compared several atmospheric pressure ionization methods, e.g., electrospray ionization, atmospheric-pressure chemical ionization, and atmospheric-pressure photoionization. The results clearly show that the number of heteroatoms (e.g., N, S, P) in the sample greatly increases the chemical diversity of lignin, while at the same time also providing potentially useful biomarkers. We demonstrate here that FT-ICR MS was able to directly isolate isotopically pure single components from the ultra-complex mixture for subsequent structural analysis, without the time-consuming chromatographic separation. CAPSULE: Various ionization techniques coupled to FT-ICR MS provide a powerful tool to assess the lignome coverage.

Direct Inlet Probe Atmospheric Pressure Photo and Chemical Ionization Coupled to Ultrahigh Resolution Mass Spectrometry for the Description of Lignocellulosic Biomass

Lignocellulosic biomass, in particular wood, is a complex mixture containing cellulose, hemicellulose, lignin, and other trace compounds. Chemical analysis of these biomasses, especially lignin components, is a challenge. Lignin is a highly reticulated polymer that is poorly soluble and usually requires chemical, enzymatic, or thermal degradation for its analysis. Here, we studied the thermal degradation of lignocellulosic biomass using a direct insertion probe (DIP). The DIP was used with two ionization sources: atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) coupled to ultrahigh-resolution mass spectrometry. Beech lignocellulosic biomass samples were used to develop the DIP-APCI/APPI methodology. Two other wood species (maple and oak) were analyzed after optimization of DIP parameters. The two ionization sources were compared at first and showed different responses toward beech samples, according to the source specificity. APPI was more specific to lignin degradation compounds, whereas APCI covered a larger variety of oxygenated compounds, e.g., fatty acids and polyphenolics compounds, in addition to lignin degradation products. The study of the thermodesorption profile gave information on the different steps of lignocellulosic biomass pyrolysis. The comparison of the three feed sample types (oak, maple, and beech), using principal component analysis (PCA) with DIP-APCI experiments, showed molecular level differences between beech wood pellets and the two other wood species (maple and oak).

ESI outcompetes other ion sources in LC/MS trace analysis

Choosing an appropriate ion source is a crucial step in liquid chromatography mass spectrometry (LC/MS) method development. In this paper, we compare four ion sources for LC/MS analysis of 40 pesticides in tomato and garlic matrices. We compare electrospray ionisation (ESI) source, thermally focused/heated electrospray (HESI), atmospheric pressure photoionisation (APPI) source with and without dopant, and multimode source in ESI mode, atmospheric pressure chemical ionisation (APCI) mode, and combined mode using both ESI and APCI, i.e. altogether seven different ionisation modes. The lowest limits of detection (LoDs) were obtained by ESI and HESI. Widest linear ranges were observed with the conventional ESI source without heated nebuliser gas. In comparison to HESI, ESI source was significantly less affected by matrix effect. APPI ranked second (after ESI) by not being influenced by matrix effect; therefore, it would be a good alternative to ESI if low LoDs are not required. Graphical abstract.

Understanding the atmospheric pressure ionization of petroleum components: The effects of size, structure, and presence of heteroatoms

Understanding the composition of crude oil and its changes with weathering is essential when assessing its provenience, fate, and toxicity. High-resolution mass spectrometry (HRMS) has provided the opportunity to address the complexity of crude oil by assigning molecular formulae, and sorting compounds into "classes" based on heteroatom content. However, factors such as suppression effects and discrimination towards certain components severely limit a truly comprehensive mass spectrometric characterization, and, despite the availability of increasingly better mass spectrometers, a complete characterization of oil still represents a major challenge. In order to fully comprehend the significance of class abundances, as well as the nature and identity of compounds detected, a good understanding of the ionization efficiency of the various compound classes is indispensable. The current study, therefore, analyzed model compounds typically found in crude oils by high-resolution mass spectrometry with atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI), in order to provide a better understanding of benefits and drawbacks of each source. The findings indicate that, overall, APPI provides the best results, being able to ionize the broadest range of compounds, providing the best results with respect to ionization efficiencies, and exhibiting the least suppression effects. However, just like in the other two sources, in APPI several factors have shown to affect the ionization efficiency of petroleum model compounds. The main such factor is the presence or absence of functional groups that can be easily protonated/deprotonated, in addition to other factors such as size, methylation level, presence of heteroatoms, and ring structure. Overall, this study evidences the intrinsic limitations and benefits of each of the three sources, and should provide the fundamental knowledge required to expand the power of crude oil analysis by high-resolution mass spectrometry.

Liquid Chromatography-Tandem Mass Spectrometry: An Emerging Technology in the Toxicology Laboratory

In the last decade, liquid chromatography-tandem mass spectrometry (LC-MS/MS) has seen enormous growth in routine toxicology laboratories. LC-MS/MS offers significant advantages over other traditional testing, such as immunoassay and gas chromatography-mass spectrometry methodologies. Major strengths of LC-MS/MS include improvement in specificity, flexibility, and sample throughput when compared with other technologies. Here, the basic principles of LC-MS/MS technology are reviewed, followed by advantages and disadvantages of this technology compared with other traditional techniques. In addition, toxicology applications of LC-MS/MS for simultaneous detection of large panels of analytes are presented.

Design Study of an Atmospheric Pressure Photoionization Interface for GC-MS

This contribution reports on the development of an atmospheric pressure photoionization (APPI) source interfacing a gas chromatograph (GC) with a bench-top Orbitrap high resolution mass spectrometer (MS). We present efforts on method development aiming at high temperature stability (325°C), constant low impurity levels upon prolonged source operation, and efficient reaction volume irradiation combined with minimum peak broadening. The performance throughout each iterative development step was carefully assessed. The final GC-APPI-MS setup demonstrated femtogram-on-column sensitivity and chromatographic peaks of Gaussian shape with base peak widths <2 s for even the highest boiling compounds present in different EPA standard mixtures. Graphical Abstract ᅟ.

Differentiating Fragmentation Pathways of Cholesterol by Two-Dimensional Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Two-dimensional Fourier transform ion cyclotron resonance mass spectrometry is a data-independent analytical method that records the fragmentation patterns of all the compounds in a sample. This study shows the implementation of atmospheric pressure photoionization with two-dimensional (2D) Fourier transform ion cyclotron resonance mass spectrometry. In the resulting 2D mass spectrum, the fragmentation patterns of the radical and protonated species from cholesterol are differentiated. This study shows the use of fragment ion lines, precursor ion lines, and neutral loss lines in the 2D mass spectrum to determine fragmentation mechanisms of known compounds and to gain information on unknown ion species in the spectrum. In concert with high resolution mass spectrometry, 2D Fourier transform ion cyclotron resonance mass spectrometry can be a useful tool for the structural analysis of small molecules. Graphical Abstract ᅟ.

Liquid chromatography-atmospheric pressure photoionization-Orbitrap analysis of fullerene aggregates on surface soils and river sediments from Santa Catarina (Brazil)

In the present work, a new analytical approach is proposed for the analysis of seven fullerenes (C₆₀, C₇₀, N-methylfulleropyrrolidine, [6,6]-phenyl C₆₁ butyric acid methyl ester, [6,6]-thienyl C61 butyric acid methyl ester, C60 pyrrolidine tris-acid ethyl ester and [6,6]-phenyl C₇₁ butyric acid methyl ester fullerenes) in soils and sediments. This procedure combines an ultrasound-assisted solvent extraction (UAE) with toluene followed by liquid chromatography (LC), using a pyrenylpropyl group bonded silica based column, coupled to a high-resolution mass spectrometer (HRMS) using atmospheric pressure photoionisation (APPI) in negative ion mode. The analytical performance for fullerene separation of the pyrenylpropyl group bonded silica column was compared to the C18 column. For the ultra-trace analysis of fullerenes in complex environmental samples, the use of the APPI source and the use of the electrospray ionisation (ESI) source were compared. Using this approach for the analysis of fullerenes in complex matrices, a series of advantages, in terms of sensitivity and specificity, have been demonstrated. The method limits of detection (MLOD) and the method limits of quantification (MLOQ) in soils and sediments ranged from 0.022 to 0.39 pg/g and from 0.072 to 1.3 pg/g, respectively. Recoveries were between 68 and 106%. The analytical method was applied in order to assess the occurrence of selected fullerenes in 45 soils of Sul Catarinense (Santa Catalina State, Brazil) and 15 sediments from the Tubarão River, presenting different pressures of contamination: a coal-combustion power plant, car exhaust, coal mining industry and wastewater effluents. C₆₀ and C₇₀ fullerenes have been detected at concentrations ranging from the MLOD to 0.150 ng/g. None of the functionalised fullerenes were detected in any of the samples. Combustion processes, in particular car exhaust, were identified as the main source of fullerenes. However, the potential degradation of residual concentrations of engineered fullerenes to more stable forms, such as C₆₀ and C₇₀, should also be considered.

A critique on the structural analysis of lignins and application of novel tandem mass spectrometric strategies to determine lignin sequencing

This review is devoted to the application of MS using soft ionization methods with a special emphasis on electrospray ionization, atmospheric pressure photoionization and matrix-assisted laser desorption/ionization MS and tandem MS (MS/MS) for the elucidation of the chemical structure of native and modified lignins. We describe and critically evaluate how these soft ionization methods have contributed to the present-day knowledge of the structure of lignins. Herein, we will introduce new nomenclature concerning the chemical state of lignins, namely, virgin released lignins (VRLs) and processed modified lignins (PML). VRLs are obtained by liberation of lignins through degradation of vegetable matter by either chemical hydrolysis and/or enzymatic hydrolysis. PMLs are produced by subjecting the VRL to a series of further chemical transformations and purifications that are likely to alter their original chemical structures. We are proposing that native lignin polymers, present in the lignocellulosic biomass, are not made of macromolecules linked to cellulose fibres as has been frequently reported. Instead, we propose that the lignins are composed of vast series of linear related oligomers, having different lengths that are covalently linked in a criss-cross pattern to cellulose and hemicellulose fibres forming the network of vegetal matter. Consequently, structural elucidation of VRLs, which presumably have not been purified and processed by any other type of additional chemical treatment and purification, may reflect the structure of the native lignin. In this review, we present an introduction to a MS/MS top-down concept of lignin sequencing and how this technique may be used to address the challenge of characterizing the structure of VRLs. Finally, we offer the case that although lignins have been reported to have very high or high molecular weights, they might not exist on the basis that such polymers have never been identified by the mild ionizing techniques used in modern MS.

Evaluation of the optimization space for atmospheric pressure photoionization (APPI) in comparison with APCI

The usefulness of atmospheric pressure photoionization (APPI) is difficult to evaluate for unknowns due to the fragmented literature. Specifically, the variation of dopants with a wide set of compounds or the use of APPI in the negative mode have rarely been explored. Thirty compounds were selected that were not suitable for ESI with a wide variety of functional groups and investigated with atmospheric pressure chemical ionization (APCI) and APPI in the positive and negative ion modes. The influence of the mobile phase (eluents containing acetonitrile or methanol) and--for APPI--four different dopants (acetone, chlorobenzene, toluene, and toluene/anisole) were explored. Stepwise variation of the organic mobile phase allowed to elucidate the ionization mechanism. Atmospheric pressure photoionization was especially useful for compounds, where the M(●+) and not the [M + H](+) was formed. The dopants chlorobenzene and anisole promoted the formation of molecular ions M(●+) for about half of the compounds, and its formation was also positively influenced by the use of mobile phases containing methanol. In the negative ion mode, APPI offered no advantage toward APCI. Best results were generally achieved with the dopant chlorobenzene, establishing that this dopant is suitable for a wide set of compounds. For one quarter of the compounds, significantly better results were achieved with mobile phases containing methanol for both APPI and APCI than those with acetonitrile, but only in the positive mode. With either of the methods--APPI or APCI--about 10% of the compounds were not detected. Strategies to get results quickly with difficult unknowns will be discussed.

Dopant-assisted atmospheric pressure photoionization of patulin in apple juice and apple-based food with liquid chromatography-tandem mass spectrometry

A dopant-assisted atmospheric pressure photoionization (APPI) with liquid chromatography tandem mass spectrometry (LC-MS/MS) method was developed to determine patulin in apple juice and apple-based food. Different dopants, dopant flow rates, and LC separation conditions were evaluated. Using toluene as the dopant, the LC-APPI-MS/MS method achieved a linear calibration from 12.5 to 2000 μg/L (r(2) > 0.99). Matrix-dependent limits of quantitation (LOQs) were from 8 μg/L (solvent) to 12 μg/L (apple juice). [(13)C]-Patulin-fortified apple juice samples were directly analyzed by the LC-APPI-MS/MS method. Other apple-based food was fortified with [(13)C]-patulin, diluted using water (1% formic acid), centrifuged, and filtered, followed by LC-APPI-MS/MS analysis. In clear apple juice, unfiltered apple cider, applesauce, and apple-based baby food, average recoveries were 101 ± 6% (50 μg/kg), 103 ± 5% (250 μg/kg), and 102 ± 5% (1000 μg/kg) (av ± SD, n = 16). Using the suggested method, patulin was detected in 3 of 30 collected market samples with concentrations ranging from <LOQ to 18 μg/L. The use of [(13)C]-patulin allowed quantitation using solvent calibration standards with satisfactory precision and accuracy.

Androgen glucuronides analysis by liquid chromatography tandem-mass spectrometry: could it raise new perspectives in the diagnostic field of hormone-dependent malignancies?

Breast and prostate constitute organs of intense steroidogenic activity. Clinical and epidemiologic data provide strong evidence on the influence of androgens and estrogens on the risk of typical hormone-dependent malignancies, like breast and prostate cancer. Recent studies have focused on the role of androgen metabolites in regulating androgen concentrations in hormone-sensitive tissues. Steroid glucuronidation has been suggested to have a prominent role in controlling the levels and the biological activity of unconjugated androgens. It is well-established that serum levels of androgen glucuronides reflect androgen metabolism in androgen-sensitive tissues. Quantitative analysis of androgen metabolites in blood specimens is the only minimally invasive approach permitting an accurate estimate of the total pool of androgens. During the past years, androgen glucuronides analysis most often involved radioimmunoassays (RIA) or direct immunoassays, both methods bearing serious limitations. However, recent impressive technical advances in mass spectrometry, and particularly in high performance liquid chromatography coupled with mass spectrometry (LC-MS/MS), have overcome these drawbacks enabling the simultaneous, quantitative analysis of multiple steroids even at low concentrations. Blood androgen profiling by LC-MS/MS, a robust and reliable technique of high selectivity, sensitivity, specificity, precision and accuracy emerges as a promising new approach in the study of human pathology. The present review offers a contemporary insight in androgen glucuronides profiling through the application of LC-MS/MS, highlighting new perspectives in the study of steroids and their implication in hormone-dependent malignancies.

APCI/APPI for synthetic polymer analysis

Modern mass spectrometry of synthetic polymers involves soft ionization techniques. Whereas matrix-assisted laser desorption/ionization (MALDI) and electrospray (ESI) are employed routinely, atmospheric pressure chemical ionization (APCI) and more recently atmospheric pressure photoionization (APPI) are used to a lesser extent. However, these latter ionization methods coupled to liquid-phase separation techniques create new opportunities for the characterization of polymers, especially for low molecular weight compounds or for the polymers that are poorly ionizable by the usual methods. After a part devoted to the description of classical MS methods employed for polymer analysis (MALDI, ESI, and their use with chromatography), APCI and APPI techniques will be described, discussed, and selected examples will present the interest of these ionization sources (or interfaces for LC/MS) in the field of polymer analysis.