Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent

Exploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.

Pitfalls in Using Isotopic Distributions for Structural Interpretation by Mass Spectrometry

This brief cautionary note reports a failure in a common and useful assumption, namely, that the isotopes of the elements occur in their natural abundance ratios in commercially sourced organic compounds. Some commercial sources of tris(pentafluorophenyl)borane, B(C6F5)3, show severely depleted 10B, while materials from other suppliers display natural isotopic abundances. The depletion varies from lot-to-lot, and it was confirmed by inductively coupled plasma (ICP) mass spectrometry. The isotope 10B is used in the nuclear power industry, as a neutron absorber in the power control rods. It is speculated that the residual 11B generated when preparing 10B-enriched boron carbide for control rod use, provides 11B-rich raw material that is then used for commercial B(C6F5)3 synthesis.

Two-Dimensional Tandem Mass Spectrometry for Biopolymer Structural Analysis

Biopolymer analysis, including proteomics and glycomics, relies heavily on the use of mass spectrometry for structural elucidation, including sequence determination. Novel methods to improve sample workup, instrument performance, and data analysis continue to be developed to address shortcomings associated with sample preparation, analysis time, data quality, and data interpretation. Here, we present a new method that couples in-source collision-induced dissociation (IS-CID) with two-dimensional tandem mass spectrometry (2D MS/MS) as a way to simplify proteomics and glycomics workflows while also providing additional insight into analyte structures over traditional MS/MS experiments. Specifically, IS-CID is employed as a gas-phase digestion method, i.e., to break down intact full-length polysaccharide or peptide ions prior to mass analysis. The resulting mixtures of oligomeric ions are analyzed by 2D-MS/MS, a technique that allows association of product ions with their precursor ions without isolation of the latter. A novel data analysis strategy is introduced to leverage the second dimension of 2D MS/MS spectra, in which stairstep patterns, representing outputs of a molecule's MSn scans, are extracted for structural interconnectivity information on the oligomer. The results demonstrate the potential applicability of 2D MS/MS strategies to the modern omics workflow and structural analysis of various classes of biopolymers.

Oxazolone mediated peptide chain extension and homochirality in aqueous microdroplets

Peptide formation from amino acids is thermodynamically unfavorable but a recent study provided evidence that the reaction occurs at the air/solution interfaces of aqueous microdroplets. Here, we show that i) the suggested amino acid complex in microdroplets undergoes dehydration to form oxazolone; ii) addition of water to oxazolone forms the dipeptide; and iii) reaction of oxazolone with other amino acids forms tripeptides. Furthermore, the chirality of the reacting amino acids is preserved in the oxazolone product, and strong chiral selectivity is observed when converting the oxazolone to tripeptide. This last fact ensures that optically impure amino acids will undergo chain extension to generate pure homochiral peptides. Peptide formation in bulk by wet-dry cycling shares a common pathway with the microdroplet reaction, both involving the oxazolone intermediate.

Machine-Learning Classification of Bacteria Using Two-Dimensional Tandem Mass Spectrometry

Biothreat detection has continued to gain attention. Samples suspected to fall into any of the CDC's biothreat categories require identification by processes that require specialized expertise and facilities. Recent developments in analytical instrumentation and machine learning algorithms offer rapid and accurate classification of Gram-positive and Gram-negative bacterial species. This is achieved by analyzing the negative ions generated from bacterial cell extracts with a modified linear quadrupole ion-trap mass spectrometer fitted with two-dimensional tandem mass spectrometry capabilities (2D MS/MS). The 2D MS/MS data domain of a bacterial cell extract is recorded within five s using a five-scan average after sample preparation by a simple extraction. Bacteria were classified at the species level by their lipid profiles using the random forest, k-nearest neighbor, and multilayer perceptron machine learning models. 2D MS/MS data can also be treated as image data for use with image recognition algorithms such as convolutional neural networks. The classification accuracy of all models tested was greater than 99%. Adding to previously published work on the 2D MS/MS analysis of bacterial growth and the profiling of sporulating bacteria, this study demonstrates the utility and information-rich nature of 2D MS/MS in the identification of bacterial pathogens at the species level when coupled with machine learning.

Quantitative Determination of Water in Organic Liquids by Ambient Mass Spectrometry

This study uses a rapid tandem mass-spectrometry method to determine water content in complex organic solutions. Emphasis is placed on trace-water analysis by a fast and accurate alternative to the Karl-Fischer method. In this new method, water is captured by a charge-labeled molecular probe. Water binds strongly with high specificity to the strongly electrophilic aldehyde site in a charge-labelled molecule (N-methylpyridinium); competitive binding by other analytes is effectively discriminated against in the mass-measurement step. Quantitative determinations are made over a wide concentration range, 0.001 % (10 ppm) to 99 %, with better than 10 % relative standard deviation, along with short (1 min) analysis times using small sample volumes (several μL). Applications include water measurement in simple organic solvents, for example, deuterated solvents, as well as in complex mixtures, for example, organic reaction mixtures. Additionally, this method allows for water monitoring in levitated droplets. Mechanistic investigations into the impact of water on important chemical processes in organic synthesis and environmental science are reported.

Re-Imagining Drug Discovery using Mass Spectrometry

It is argued that each of the three key steps in drug discovery, (i) reaction screening to find successful routes to desired drug candidates, (ii) scale up of the synthesis to produce amounts adequate for testing, and (iii) bioactivity assessment of the candidate compounds, can all be performed using mass spectrometry (MS) in a sequential fashion. The particular ionization method of choice, desorption electrospray ionization (DESI), is both an analytical technique and a procedure for small-scale synthesis. It is also highly compatible with automation, providing for high throughput in both synthesis and analysis. Moreover, because accelerated reactions take place in the secondary DESI microdroplets generated from individual reaction mixtures, this allows either online analysis by MS or collection of the synthetic products by droplet deposition. DESI also has the unique advantage, amongst spray-based MS ionization methods, that complex buffered biological solutions can be analyzed directly, without concern for capillary blockage. Here, all these capabilities are illustrated, the unique chemistry at droplet interfaces is presented, and the possible future implementation of DESI-MS based drug discovery is discussed.

Characterization of a Low-Temperature Plasma (LTP) Ambient Ionization Source Using Temporally Resolved Monochromatic Imaging Spectrometry

The low-temperature plasma (LTP) probe is a common plasma-based source used for ambient desorption-ionization mass spectrometry (MS). While the LTP probe has been characterized in detail with MS, relatively few studies have used optical spectroscopy. In this paper, two-dimensional (2D) imaging at selected wavelengths is used to visualize important species in the LTP plasma jet. First, 2D steady-state images of the LTP plume for N2+ (391.2 nm), He I (706.5 nm), and N2 (337.1 nm) emissions were recorded under selected plasma conditions. Second, time-resolved 2D emission maps of radiative species in the LTP plasma jet were recorded through the use of a 200 ns detection gate and varying gate delays with respect to the LTP trigger pulse. Emission from He I, N2+, and N2 in the plasma jet region was found to show a transient behavior (often referred to as plasma bullets) lasting only a few microseconds. The N2+ and He I maps were highly correlated in spatial and temporal structure. Further, emission from N2 showed two maxima in time, one before and one after the maximum emission for N2+ and He I, due to an initial electronic excitation wave and ion-electron recombination, respectively. Third, the interaction of the LTP probe with a sample substrate and an electrically grounded metallic needle was studied. Emission from a fluorophore on the sample substrate showed an initial photon-induced excitation from plasma-generated photons followed by electronic excitation by other plasma species. The presence of a grounded needle near the plasma jet significantly extended the plasma jet lifetime and also generated a long-lived corona discharge on the needle. The effect of LTP operating parameters on emission spectra was correlated with mass-spectral results including reagent-ion signals. Lastly, five movies provide a side-by-side comparison of the temporal behavior of emitting species and insights into the interactions of the emission clouds with a sample surface as well as an external needle. Temporally and spatially resolved imaging provided insights into important processes in the LTP plasma jet, which will help improve analyte ion sampling in LTP-MS.

High-Throughput Diversification of Complex Bioactive Molecules by Accelerated Synthesis in Microdroplets

Late-stage diversification of drug molecules is an important strategy in drug discovery that can be facilitated by reaction screening using high-throughput experimentation. Here we present a rapid method for functionalizing bioactive molecules based on accelerated reactions in microdroplets. Reaction mixtures are nebulized at throughputs better than 1 reaction/second and the accelerated reactions occurring in the microdroplets are followed by desorption electrospray ionization mass spectrometry (DESI-MS). Because the accelerated reactions occur on the millisecond timescale, they allow an overall screening throughput of 1 Hz working at the low nanogram scale. Using this approach, an opioid agonist (PZM21) and an antagonist (naloxone) were diversified using three reactions important in medicinal chemistry: sulfur fluoride exchange (SuFEx) click reactions, imine formation reactions, and ene-type click reactions. Some 269 functionalized analogs of naloxone and PZM21 were generated and characterized by tandem mass spectrometry (MS/MS) after screening over 500 reactions.

Miniature mass spectrometer-based point-of-care assay for measuring phosphatidylethanol in blood

We demonstrate proof-of-concept for point-of-care assessment of long-term alcohol consumption by measuring phosphatidylethanol in blood/dried blood spots with nano-electrospray ionization and MS/MS using a miniature mass spectrometer. 'Abstinence', 'moderate', and 'chronic' consumption could be distinguished rapidly for both sample types, and quantitative performance was obtained with blood (LoQ-100 ng mL^-1).

Accelerated and Concerted Aza-Michael Addition and SuFEx Reaction in Microdroplets in Unitary and High-Throughput Formats

The sulfur fluoride exchange (SuFEx) reaction is significant in drug discovery, materials science, and chemical biology. Conventionally, it involves installation of SO₂ F followed by fluoride exchange by a catalyst. We report catalyst-free Aza-Michael addition to install SO₂ F and then SuFEx reaction with amines, both occurring in concert, in microdroplets under ambient conditions. The microdroplet reaction is accelerated by a factor of ∼10⁴ relative to the corresponding bulk reaction. We suggest that the superacidic microdroplet surface assists SuFEx reaction by protonating fluorine to create a good leaving group. The reaction scope was established by performing individual reactions in microdroplets of 18 amines in four solvents and confirmed using high-throughput desorption electrospray ionization experiments. The study demonstrates the value of microdroplet-assisted accelerated reactions in combination with high-throughput experimentation for characterization of reaction scope.

High-throughput analysis of tissue microarrays using automated desorption electrospray ionization mass spectrometry

Tissue microarrays (TMAs) are commonly used for the rapid analysis of large numbers of tissue samples, often in morphological assessments but increasingly in spectroscopic analysis, where specific molecular markers are targeted via immunostaining. Here we report the use of an automated high-throughput system based on desorption electrospray ionization (DESI) mass spectrometry (MS) for the rapid generation and online analysis of high-density (6144 samples/array) TMAs, at rates better than 1 sample/second. Direct open-air analysis of tissue samples (hundreds of nanograms) not subjected to prior preparation, plus the ability to provide molecular characterization by tandem mass spectrometry (MS/MS), make this experiment versatile and applicable to both targeted and untargeted analysis in a label-free manner. These capabilities are demonstrated in a proof-of-concept study of frozen brain tissue biopsies where we showcase (i) a targeted MS/MS application aimed at identification of isocitrate dehydrogenase mutation in glioma samples and (ii) an untargeted MS tissue type classification using lipid profiles and correlation with tumor cell percentage estimates from histopathology. The small sample sizes and large sample numbers accessible with this methodology make for a powerful analytical system that facilitates the identification of molecular markers for later use in intraoperative applications to guide precision surgeries and ultimately improve patient outcomes.

Simultaneous and Spontaneous Oxidation and Reduction in Microdroplets by the Water Radical Cation/Anion Pair

Microdroplets show unique chemistry, especially in their intrinsic redox properties, and to this we here add a case of simultaneous and spontaneous oxidation and reduction. We report the concurrent conversions of several phosphonates to phosphonic acids by reduction (R-P → H-P) and to pentavalent phosphoric acids by oxidation. The experimental results suggest that the active reagent is the water radical cation/anion pair. The water radical cation is observed directly as the ionized water dimer while the water radical anion is only seen indirectly though the spontaneous reduction of carbon dioxide to formate. The coexistence of oxidative and reductive species in turn supports the proposal of a double-layer structure at the microdroplet surface, where the water radical cation and radical anion are separated and accumulated.

Spontaneous Oxidation of Aromatic Sulfones to Sulfonic Acids in Microdroplets

Reactions in microdroplets can be accelerated and can present unique chemistry compared to reactions in bulk solution. Here, we report the accelerated oxidation of aromatic sulfones to sulfonic acids in microdroplets under ambient conditions without the addition of acid, base, or catalyst. The experimental data suggest that the water radical cation, (H₂O)^+•, derived from traces of water in the solvent, is the oxidant. The substrate scope of the reaction indicates the need for a strong electron-donating group (e.g., p-hydroxyl) in the aromatic ring. An analogous oxidation is observed in an aromatic ketone with benzoic acid production. The shared mechanism is suggested to involve field-assisted ionization of water at the droplet/air interface, its reaction with the sulfone (M) to form the radical cation adduct, (M + H₂O)^+•, followed by 1,2-aryl migration and C-O cleavage. A remarkably high reaction rate acceleration (∼10³) and regioselectivity (∼100-fold) characterize the reaction.

Desorption Electrospray Ionization Mass Spectrometry Assay for Label-Free Characterization of SULT2B1b Enzyme Kinetics

The sulfotransferase (SULT) 2B1b, which catalyzes the sulfonation of 3β-hydroxysteroids, has been identified as a potential target for prostate cancer treatment. However, a major limitation for SULT2B1b-targeted drug discovery is the lack of robust assays compatible with high-throughput screening and inconsistency in reported kinetic data. For this reason, we developed a novel label-free assay based on high-throughput (>1 Hz) desorption electrospray ionization mass spectrometry (DESI-MS) for the direct quantitation of the sulfoconjugated product (CV<10 %; <1 ng analyte). The performance of this DESI-based assay was compared against a new fluorometric coupled-enzyme method that we also developed. Both methodologies provided consistent kinetic data for the reaction of SULT2B1b with its major substrates, indicating the affinity trend pregnenolone>DHEA>cholesterol, for both the phospho-mimetic and wild-type SULT2B1b forms. The novel DESI-MS assay developed here is likely generalizable to other drug discovery efforts and is particularly promising for identification of SULT2B1b inhibitors with potential as prostate cancer therapeutics.

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

Spontaneous oxidation of compounds containing diverse X=Y moieties (e.g., sulfonamides, ketones, esters, sulfones) occurs readily in organic-solvent microdroplets. This surprising phenomenon is proposed to be driven by the generation of an intermediate species [M+H₂O]^+·: a covalent adduct of water radical cation (H₂O^+·) with the reactant molecule (M). The adduct is observed in the positive ion mass spectrum while its formation in the interfacial region of the microdroplet (i.e., at the air-droplet interface) is indicated by the strong dependence of the oxidation product formation on the spray distance (which reflects the droplet size and consequently the surface-to-volume ratio) and the solvent composition. Importantly, based on the screening of a ca. 21,000-compound library and the detailed consideration of six functional groups, the formation of a molecular adduct with the water radical cation is a significant route to ionization in positive ion mode electrospray, where it is favored in those compounds with X=Y moieties which lack basic groups. A set of model monofunctional systems was studied and in one case, benzyl benzoate, evidence was found for oxidation driven by hydroxyl radical adduct formation followed by protonation in addition to the dominant water radical cation addition process. Significant implications of molecular ionization by water radical cations for oxidation processes in atmospheric aerosols, analytical mass spectrometry and small-scale synthesis are noted.

Bacterial growth monitored by two-dimensional tandem mass spectrometry

The growth of the bacterium E. coli was monitored by targeting the phospholipid constituents through desorption electrospray ionization and characterizing individual sets of isomers by recording the full 2D MS/MS data domain in a single scan of a modified quadrupole ion trap mass spectrometer. The experiments tested the applicability of the new instrumental capabilities which include sample interrogation at the molecular level for multiple components at speeds of <10 seconds/sample. The major lipids observed were phosphatidylethanolamines and phosphatidylglycerols and the growth experiment showed fatty acid chain modification from alkene to cyclopropyl groups over time. Notably, these novel MS scans were also performed using desorption electrospray ionization (DESI) to quickly sample complex mixtures without pre-separation. This demonstration experiment has implications for the value of ambient ionization mass spectrometry for monitoring biological systems on physiologically relevant timescales.

Pd Reaction Intermediates in Suzuki-Miyaura Cross-Coupling Characterized by Mass Spectrometry

Palladium-catalyzed Suzuki-Miyaura (SM) coupling is widely utilized in the construction of carbon-carbon bonds. In this study, nanoelectrospray ionization mass spectrometry (nanoESI-MS) is applied to simultaneously monitor precatalysts, catalytic intermediates, reagents, and products of the SM cross-coupling reaction of 3-Br-5-Ph-pyridine and phenylboronic acid. A set of Pd cluster ions related to the monoligated Pd (0) active catalyst is detected, and its deconvoluted isotopic distribution reveals contributions from two neutral molecules. One is assigned to the generally accepted Pd(0) active catalyst, seen in MS as the protonated molecule, while the other is tentatively assigned to an oxidized catalyst which was found to increase as the reaction proceeds. Oxidative stress testing of a synthetic model catalyst 1,5-cyclooctadiene Pd XPhos (COD-Pd-XPhos) performed using FeCl3 supported this assignment. The formation and conversion of the oxidative addition intermediate during the catalytic cycle was monitored to provide information on the progress of the transmetalation step.

Late-Stage Functionalization and Characterization of Drugs by High-Throughput Desorption Electrospray Ionization Mass Spectrometry

Late-stage functionalization (LSF) of drug molecules is an approach to generate modified molecules that retain functional groups present in the active drugs. Here, we report a study that seeks to characterize the potential value of high-throughput desorption electrospray ionization mass spectrometry (HT DESI-MS) for small-scale rapid LSF. In conventional route screening, HT-based DESI-MS provides contactless, rapid analysis, reliable and reproducible data, minimal sample requirement, and exceptional tolerance to high salt concentrations. Ezetimibe (E), an established hypertension drug, is targeted for modification by LSF. C-H alkenylation and azo-click reactions are utilized to explore this approach to synthesis and analytical characterization. The effect of choice of reactant, stoichiometry, catalyst, and solvent are studied for both reactions using high throughput DESI-MS experiments. Optimum conditions for the formation of LSF products are established with identification by tandem mass spectrometry (MS/MS).

Immediate and sensitive detection of sporulated Bacillus subtilis by microwave release and tandem mass spectrometry of dipicolinic acid

Spore lysis of Bacillus species is achieved by brief (1 min) microwave irradiation while tandem mass spectrometry (MS/MS) allows identification of the characteristic spore marker, dipicolinic acid. This rapid measurement, made on 10⁵-10⁸ spores, has significant implications for biothreat recognition.

PATH-13. INTRAOPERATIVE EVALUATION OF IDH MUTATION STATUS AND TUMOR INVASION IN GLIOMA

BACKGROUND

Intraoperative detection of residual tumor and isocitrate dehydrogenase (IDH) mutations can assist in maximizing surgical resection beyond contrast enhancing margins and guide intraoperative surgical decision making for glioma patients. We aimed to evaluate the use of desorption electrospray ionization-mass spectrometry (DESI-MS) for intraoperative assessment of IDH mutations and estimation of tumor cell percentage (TCP).

METHODS

This is a prospective study using intraoperative DESI-MS analysis of freshly obtained tissue samples to evaluate IDH mutations via 2-hydroxyglutarate (2-HG) intensity and TCP via measurement of N-acetylaspartic acid (NAA) intensity and characteristic lipid profiles. These IDH mutation and TCP estimates were subsequently validated by a senior neuropathologist.

RESULTS

A total of 247 biopsies from a 49-patient study were previously collected and analyzed at Indiana University. Assessment of TCP in 203 margin and core biopsies based on NAA intensity yielded sensitivity, specificity, and accuracy values of 91, 76, and 83%, whereas TCP assessment based on characteristic lipid profiles yielded 76, 85, and 81%, respectively. Assessment of IDH mutation status of 71 core biopsies yielded sensitivity, specificity, and accuracy values of 89, 100, and 94%. Further validation of the methodology is being performed in an ongoing collaboration with Mayo Clinic-Jacksonville, where we have collected 178 biopsies from 24 patients. Preliminary results of IDH mutation assessments indicate 100% sensitivity, specificity, and accuracy.

DISCUSSION/CONCLUSION

We present a novel system to allow intraoperative evaluation of IDH status and to guide surgical resection by TCP measurement from tissue biopsies. Prospectively, we propose to modify our DESI-MS system by placing a surgical material (e.g. cottonoid) along the surgical margin and transferring material from the blot to a microscope slide prior to DESI-MS analysis. This will allow the retention of the spatial distribution of diagnostic molecules while analyzing a wall of the surgical cavity without the need for biopsy.

Automated High-Throughput System Combining Small-Scale Synthesis with Bioassays and Reaction Screening

The Purdue Make It system is a unique automated platform capable of small-scale in situ synthesis, screening small-molecule reactions, and performing direct label-free bioassays. The platform is based on desorption electrospray ionization (DESI), an ambient ionization method that allows for minimal sample workup and is capable of accelerating reactions in secondary droplets, thus conferring unique advantages compared with other high-throughput screening technologies. By combining DESI with liquid handling robotics, the system achieves throughputs of more than 1 sample/s, handling up to 6144 samples in a single run. As little as 100 fmol/spot of analyte is required to perform both initial analysis by mass spectrometry (MS) and further MSⁿ structural characterization. The data obtained are processed using custom software so that results are easily visualized as interactive heatmaps of reaction plates based on the peak intensities of m/z values of interest. In this paper, we review the system's capabilities as described in previous publications and demonstrate its utilization in two new high-throughput campaigns: (1) the screening of 188 unique combinatorial reactions (24 reaction types, 188 unique reaction mixtures) to determine reactivity trends and (2) label-free studies of the nicotinamide N-methyltransferase enzyme directly from the bioassay buffer. The system's versatility holds promise for several future directions, including the collection of secondary droplets containing the products from successful reaction screening measurements, the development of machine learning algorithms using data collected from compound library screening, and the adaption of a variety of relevant bioassays to high-throughput MS.

Novel Ion Trap Scan Modes to Develop Criteria for On-Site Detection of Sulfonamide Antibiotics

Advances in ambient ionization techniques have facilitated the direct analysis of complex mixtures without sample preparation. Significant attention has been given to innovating ionization methods so that multiple options are now available, allowing for ready selection of the best methods for particular analyte classes. These ambient techniques are commonly implemented on benchtop systems, but their potential application with miniature mass spectrometers for in situ measurements is even more powerful. These applications require that attention be paid to tailoring the mass spectrometric methodology for the on-site operation. In this study, combinations of scan modes are employed to efficiently determine what tandem mass spectrometry (MS/MS) operations are most useful for detecting sulfonamides using a miniature ion trap after ionization. First, mixtures of representative sulfonamide antibiotics were interrogated using a 2D MS/MS scan on a benchtop ion trap in order to determine which class-specific fragments (ionic or neutral) are shared between the sulfonamides and thus have diagnostic value. Then, three less-used combination scans were recorded: (i) a simultaneous precursor ion scan was used to detect both analytes and an internal standard in a single ion injection event to optimize quantitative performance; (ii) a simultaneous precursor/neutral loss scan was used to improve detection limits; and finally, (iii) the simultaneous precursor/neutral loss scan was implemented in a miniature mass spectrometer and representative sulfonamides were detected at concentrations as low as 100 ng/mL by nano-electrospray and 0.5 ng absolute by paper spray ionization, although improvements in the stability of the home-built instrumentation are needed to further optimize performance.

Metabolic profiles of human brain parenchyma and glioma for rapid tissue diagnosis by targeted desorption electrospray ionization mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) is well suited for intraoperative tissue analysis since it requires little sample preparation and offers rapid and sensitive molecular diagnostics. Currently, intraoperative assessment of the tumor cell percentage of glioma biopsies can be made by measuring a single metabolite, N-acetylaspartate (NAA). The inclusion of additional biomarkers will likely improve the accuracy when distinguishing brain parenchyma from glioma by DESI-MS. To explore this possibility, mass spectra were recorded for extracts from 32 unmodified human brain samples with known pathology. Statistical analysis of data obtained from full-scan and multiple reaction monitoring (MRM) profiles identified discriminatory metabolites, namely gamma-aminobutyric acid (GABA), creatine, glutamic acid, carnitine, and hexane-1,2,3,4,5,6-hexol (abbreviated as hexol), as well as the established biomarker NAA. Brain parenchyma was readily differentiated from glioma based on these metabolites as measured both in full-scan mass spectra and by the intensities of their characteristic MRM transitions. New DESI-MS methods (5 min acquisition using full scans and MS/MS), developed to measure ion abundance ratios among these metabolites, were tested using smears of 29 brain samples. Ion abundance ratios based on signals for GABA, creatine, carnitine, and hexol all had sensitivities > 90%, specificities > 80%, and accuracies > 85%. Prospectively, the implementation of diagnostic ion abundance ratios should strengthen the discriminatory power of individual biomarkers and enhance method robustness against signal fluctuations, resulting in an improved DESI-MS method of glioma diagnosis.

Reaction Acceleration Promoted by Partial Solvation at the Gas/Solution Interface

The kinetics of organic reactions of different types in microvolumes (droplets, thin films, and sealed tubes) show effects of gas/solution interfacial area, reaction molecularity and solvent polarity. Partial solvation at the gas/solution interface is a major contributor to the 10⁴ -fold reaction acceleration seen in bimolecular but not unimolecular reactions in microdroplets. Reaction acceleration can be used to manipulate selectivity by solvent choice.

Atmospheric Pressure Drift Tube Ion Mobility Spectrometry Coupled with Two-Dimensional Tandem Mass Spectrometry

Atmospheric pressure drift tube ion mobility was coupled with two-dimensional tandem mass spectrometry (2D MS/MS) in a linear ion trap to simultaneously collect ion mobility and the entire MS/MS data domain. Utilizing ion intensities from precursor ion and neutral loss scan lines, ion mobility spectra of multiple compounds with particular functional groups were acquired in a single experiment. Functional group-specific ion mobility spectra were demonstrated for a standard mixture of lipids. Additionally, ion mobility was used to separate isobaric ions prior to 2D MS/MS. The combination of these two methods offers improvements for the analysis of complex mixtures.

Fragmentation of Polyfunctional Compounds Recorded Using Automated High-Throughput Desorption Electrospray Ionization

Using desorption electrospray ionization (DESI) as part of an automated high-throughput system, tandem mass spectra of the compounds in a pharmaceutical library were recorded in the positive mode under standardized conditions. Quality control filtering yielded an MS/MS library of 16 662 spectra. Fragmentation of subsets of the compounds in the library chosen to contain a single instance of a particular functional group (amide, piperazine, sulfonamide) was predicted by experts, and the results were compared with the experimental data. Expert performance was good to excellent for all the cases evaluated. Substituents on the functional groups were found to exert important secondary control over the fragmentation, with the main effect observed being product ion stabilization by aromatic substitution, which was consistent across the different groups evaluated. These substituent effects are generally explicable in terms of standard physical organic chemistry considerations of product ion stability as controlling fragmentation. A somewhat unexpected feature was the incidence of homolytic cleavages, driven by the stability of substituted amine radical cations. The findings of this study are intended to lay the groundwork for machine learning approaches to performing MS/MS spectrum → structure and structure → MS/MS spectrum operations on the same experimental data set. The effort involved and the success achieved in computer-aided interpretation, now underway, will be compared with the expert performance as described here.

Glass surface as strong base, 'green' heterogeneous catalyst and degradation reagent

Systematic screening of accelerated chemical reactions at solid/solution interfaces has been carried out in high-throughput fashion using desorption electrospray ionization mass spectrometry and it provides evidence that glass surfaces accelerate various base-catalyzed chemical reactions. The reaction types include elimination, solvolysis, condensation and oxidation, whether or not the substrates are pre-charged. In a detailed mechanistic study, we provide evidence using nanoESI showing that glass surfaces can act as strong bases and convert protic solvents into their conjugate bases which then act as bases/nucleophiles when participating in chemical reactions. In aprotic solvents such as acetonitrile, glass surfaces act as ‘green’ heterogeneous catalysts that can be recovered and reused after simple rinsing. Besides their use in organic reaction catalysis, glass surfaces are also found to act as degradation reagents for phospholipids with increasing extents of degradation occurring at low concentrations. This finding suggests that the storage of base/nucleophile-labile compounds or lipids in glass containers should be avoided.

Glass surfaces are found to be strong bases, ‘green’ heterogeneous catalysts and degradation reagents: glass microspheres act as strong bases to accelerate multiple base-catalyzed reaction types by a factor of 26–2021.

Intraoperative Mass Spectrometry Platform for IDH Mutation Status Prediction, Glioma Diagnosis, and Estimation of Tumor Cell Infiltration

BACKGROUND

Surgical tumor resection is the primary treatment option for diffuse glioma, the most common malignant brain cancer. The intraoperative diagnosis of gliomas from tumor core samples can be improved by use of molecular diagnostics. Further, residual tumor at surgical margins is a primary cause of tumor recurrence and malignant progression. This study evaluates a desorption electrospray ionization mass spectrometry (DESI-MS) system for intraoperative isocitrate dehydrogenase (IDH) mutation assessment, estimation of tumor cell infiltration as tumor cell percentage (TCP), and disease status. This information could be used to enhance the extent of safe resection and so potentially improve patient outcomes.

METHODS

A mobile DESI-MS instrument was modified and used in neurosurgical operating rooms (ORs) on a cohort of 49 human subjects undergoing craniotomy with tumor resection for suspected diffuse glioma. Small tissue biopsies (ntotal = 203) from the tumor core and surgical margins were analyzed by DESI-MS in the OR and classified using univariate and multivariate statistical methods.

RESULTS

Assessment of IDH mutation status using DESI-MS/MS to measure 2-hydroxyglutarate (2-HG) ion intensities from tumor cores yielded a sensitivity, specificity, and overall diagnostic accuracy of 89, 100, and 94%, respectively (ncore = 71). Assessment of TCP (categorized as low or high) in tumor margin and core biopsies using N-acetyl-aspartic acid (NAA) intensity provided a sensitivity, specificity, and accuracy of 91, 76, and 83%, respectively (ntotal = 203). TCP assessment using lipid profile deconvolution provided sensitivity, specificity, and accuracy of 76, 85, and 81%, respectively (ntotal = 203). Combining the experimental data and using PCA-LDA predictions of disease status, the sensitivity, specificity, and accuracy in predicting disease status are 63%, 83%, and 74%, respectively (ntotal = 203).

CONCLUSIONS

The DESI-MS system allowed for identification of IDH mutation status, glioma diagnosis, and estimation of tumor cell infiltration intraoperatively in a large human glioma cohort. This methodology should be further refined for clinical diagnostic applications.

Reaction acceleration at air-solution interfaces: Anisotropic rate constants for Katritzky transamination

To disentangle the factors controlling the rates of accelerated reactions in droplets, we used mass spectrometry to study the Katritzky transamination in levitated Leidenfrost droplets of different yet constant volumes over a range of concentrations while holding concentration constant by adding back the evaporated solvent. The set of concentration and droplet volume data indicates that the reaction rate in the surface region is much higher than that in the interior. These same effects of concentration and volume were also seen in bulk solutions. Three pyrylium reagents with different surface activity showed differences in transamination reactivity. The conclusion is drawn that reactions with surface-active reactants are subject to greater acceleration, as seen particularly at lower concentrations in systems of higher surface-to-volume ratios. These results highlight the key role that air-solution interfaces play in Katritzky reaction acceleration. They are also consistent with the view that reaction-increased rate constant is at least in part due to limited solvation of reagents at the interface.

Reaction Acceleration at Solid/Solution Interfaces: Katritzky Reaction Catalyzed by Glass Particles

The Katritzky reaction in bulk solution at room temperature is accelerated significantly by the surface of a glass container compared to a plastic container. Remarkably, the reaction rate is increased by more than two orders of magnitude upon the addition of glass particles with the rate increasing linearly with increasing amounts of glass. A similar phenomenon is observed when glass particles are added to levitated droplets, where large acceleration factors are seen. Evidence shows that glass acts as a "green" heterogeneous catalyst: it participates as a base in the deprotonation step and is recovered unchanged from the reaction mixture. Reaction acceleration at two separate interfaces is recognized in this study: i) air/solution phase acceleration, as is well known in microdroplets; ii) solid/solution phase, where such acceleration appears to be a new phenomenon.

Multiple reaction monitoring profiling (MRM profiling): Small molecule exploratory analysis guided by chemical functionality

Small molecules, including metabolites and lipids, provide information on metabolic pathways and active biological processes in living organisms. They are often diagnostic of disease. Current exploratory methods for metabolomics and lipidomics mostly rely on separation using liquid or gas chromatography (LC or GC) coupled with mass spectrometers capable of acquiring high resolution data to generate an enormous data, but at the cost of lengthy processing and data acquisition. Even though many molecules can be identified and quantified by these methods, the laborious protocols for purification, identification, and validation limit the accessible sample chemical information. To improve the speed and efficiency of exploratory metabolomics and lipidomics, multiple reaction monitoring profiling (MRM profiling) has been developed. This strategy involves a three-stage workflow which starts by considering the metabolome as a collection of functional groups. The Discovery Stage interrogates a representative sample mixture for functional groups using the functional group specific precursor ion (Prec) scans and neutral loss (NL) scans. This experiment usually uses a triple quadrupole mass spectrometer without chromatography, i.e. by direct sample infusion. In the second Screening Stage, the main features seen in the Prec and NL scans are organized into lists of precursor ion/product ion transitions (MRMs) which are then used for the fast, specific, and sensitive interrogation of each individual sample. Data analysis by univariate and multivariate statistical methods is used to identify the most informative MRMs and so classify the individual samples. The compounds (biomarkers) which are responsible for the most informative MRMs in particular sample classes can be investigated in an optional third Identification Stage i.e. in a structural identification study. MRM profiling benefits from the much smaller number of functional groups compared to the number of individual metabolites existing in biological samples (where most metabolites are still unknown), resulting in acquisition of a much smaller data set and a shorter analysis time. The application of MRM Profiling to several biological and clinical problems is used to illustrate its features.

Collection and Characterization by Mass Spectrometry of the Neutral Serine Octamer Generated upon Sublimation

Serine forms neutral octameric clusters during sublimation, as demonstrated by electrostatically deflecting thermally ionized serine species from the sublimate, then gently ionizing the remaining neutrals for examination by mass spectrometry (MS). The MS results demonstrate a strong homochiral preference in the neutral octamer (measured after its gentle ionization), while the smaller serine clusters are achiral. In the initial stages of its sublimation, nonracemic solid serine generates a neutral serine monomer as the principal species in the vapor phase, with a significant enantiomeric enrichment relative to the solid. The serine monomer, when the flux is sufficient, assembles into the octamer, which displays a much higher chiral purity than the monomer. The serine octamer is separated from other neutral clusters in the sublimate by a new method based on the different distances that the clusters travel in an inert gas stream before they condense in a cooled collector. The deposited octamer is subsequently dissolved, and the solution is investigated by MS. The spectrum confirms that the collected serine octamer has undergone chiral enrichment relative to the starting solid used in the sublimation. The chiral enrichment observed in going from the serine monomer to octamer can be accommodated using a chemical model, grounded on the homochiral preference of the neutral serine octamer. Using the enantiomeric excess (ee %) of the vapor-phase monomer as the input, the model output matches the experimental octamer ee % when subliming solid serine with various initial ee % values.

Accelerated reactions of amines with carbon dioxide driven by superacid at the microdroplet interface

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk. Carbon dioxide undergoes C–N bond formation reactions with amines at the interface of droplets to form carbamic acids. Electrospray ionization mass spectrometry displays the reaction products in the form of the protonated and deprotonated carbamic acid. Electrosonic spray ionization (ESSI) utilizing carbon dioxide as nebulization gas, confines reaction to the gas–liquid interface where it proceeds much faster than in the bulk. Intriguingly, trace amounts of water accelerate the reaction, presumably by formation of superacid or superbase at the water interface. The suggested mechanism of protonation of CO₂ followed by nucleophilic attack by the amine is analogous to that previously advanced for imidazole formation from carboxylic acids and diamines.

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk.

Aldol Reactions of Biorenewable Triacetic Acid Lactone Precursor Evaluated Using Desorption Electrospray Ionization Mass Spectrometry High-Throughput Experimentation and Validated by Continuous Flow Synthesis

Desorption electrospray ionization-mass spectrometry (DESI-MS) was used as a high-throughput experimentation (HTE) tool to rapidly identify derivatives of the biobased platform molecule triacetic acid lactone (TAL). TAL is a platform molecule capable of conversion to a wide range of useful commodity chemicals, agrochemicals, and advanced pharmaceutical intermediates. In the present study, a diverse family of aldol reaction mixtures were prepared in high-density microtiter plates with a liquid handling robot, then printed with a pin tool onto a PTFE surface for analysis by DESI-MS. Our DESI-MS results indicate that aldol products of TAL were obtained for each substrate tested, in good agreement with previously reported TAL reactivity. These HTE experiments also revealed solvent-dependent reactivity trends that facilitated reaction scale up. Our findings suggest that DESI-MS analysis can rapidly inform the selection of optimal reaction conditions from a wide variety of conditions for scale up using continuous synthesis conditions.

Accelerated Reaction Kinetics in Microdroplets: Overview and Recent Developments

Various organic reactions, including important synthetic reactions involving C-C, C-N, and C-O bond formation as well as reactions of biomolecules, are accelerated when the reagents are present in sprayed or levitated microdroplets or in thin films. The reaction rates increase by orders of magnitude with decreasing droplet size or film thickness. The effect is associated with reactions at the solution-air interface. A key factor is partial solvation of the reagents at the interface, which reduces the critical energy for reaction. This phenomenon is of intrinsic interest and potentially of practical value as a simple, rapid method of performing small-scale synthesis.

High-throughput screening of organic reactions in microdroplets using desorption electrospray ionization mass spectrometry (DESI-MS): hardware and software implementation

This study describes an automated system used for high throughput screening of reaction conditions based on accelerated reactions occurring in small volumes of reagents. Reaction mixtures are prepared in array format using a fluid handling robot and spotted on a flat polytetrafluoroethylene plate at densities up to 6144 per plate. The reaction and analysis steps are performed simultaneously using desorption electrospray ionization (DESI) to release microdroplets containing the reaction mixture from the plate for reaction prior to arrival at a mass spectrometer. Analysis rates are up to 1 reaction mixture per second and data are recorded in real time using an ion trap mass spectrometer. Beacon compounds are used to triangulate position on the plate and this allows tandem mass spectrometry (MS/MS) to be performed on confirm products of interest. Custom software allows the user to control the system. It is also used to receive data from the DESI mass spectrometer to screen the spectra for compounds of interest, to perform MS/MS and to save data. This custom software also communicates with the software controlling the fluid handling robot (Biomek i7) as well as the Beckman software used to prepare reaction mixtures and also the software that controls the solvent used as the DESI spray. Data were recorded for N-alkylation, N-acylation and N-sulfonylation reactions in three 8 hour experiments on successive days to establish the ruggedness and repeatability of the system. Repeatability is high (94-97%) over this period with false negative 6% (depending on noise threshold chosen). Plates containing 384 reaction mixtures are analyzed in 7 min by moving the DESI sprayer in steps under the sprayer instead of continuously.

Accelerated microdroplet synthesis of benzimidazoles by nucleophilic addition to protonated carboxylic acids

We report a metal-free novel route for the accelerated synthesis of benzimidazole and its derivatives in the ambient atmosphere. The synthetic procedure involves 1,2-aromatic diamines and alkyl or aryl carboxylic acids reacting in electrostatically charged microdroplets generated using a nano-electrospray (nESI) ion source. The reactions are accelerated by orders of magnitude in comparison to the bulk. No other acid, base or catalyst is used. Online analysis of the microdroplet accelerated reaction products is performed by mass spectrometry. We provide evidence for an acid catalyzed reaction mechanism based on identification of the intermediate arylamides. Their dehydration to give benzimidazoles occurs in a subsequent thermally enhanced step. It is suggested that the extraordinary acidity at the droplet surface allows the carboxylic acid to function as a C-centered electrophile. Comparisons of this methodology with data from thin film and bulk synthesis lead to the proposal of three key steps in the reaction: (i) formation of an unusual reagent (protonated carboxylic acid) because of the extraordinary conditions at the droplet interface, (ii) accelerated bimolecular reaction because of limited solvation at the interface and (iii) thermally assisted elimination of water. Eleven examples are shown as evidence of the scope of this chemistry. The accelerated synthesis has been scaled-up to establish the substituent-dependence and to isolate products for NMR characterization.