Enzyme-Catalyzed Hydrolysis of Lipids in Immiscible Microdroplets Studied by Contained-Electrospray Ionization

Real-time bottom-up characterization of protein mixtures enabled by online microdroplet-assisted enzymatic digestion (MAED)

Enzymatic digestion remains one of the "rate-determining steps" in the bottom-up analysis of proteins. However, by performing digestion in microdroplets generated from electrosonic spray, the reaction could be accelerated to a timescale lower than milliseconds. Here, we describe a simple and rapid online digestion platform named online microdroplet-assisted enzymatic digestion (MAED). It involves the integration of intact protein separation with enzymatic digestion in microdroplets. Via online MAED, various protein standards, including an antibody standard, were characterized in a bottom-up manner without prior digestion, and high sequence coverages were obtained. We further extended the application of online MAED to a more complex sample, mouse brain extract, where protein identifications were successfully yielded. Compared with the conventional bottom-up approach, a more comprehensive characterization could be obtained particularly for low molecular weight proteins. In short, we provide a rapid and alternative bottom-up analysis in a top-down fashion as well as a new possibility for microdroplet chemistry.

Two-dimensional isomer differentiation using liquid chromatography-tandem mass spectrometry with in-source, droplet-based derivatization

Saccharides are increasingly used as biomarkers and for therapeutic purposes. Their characterization is challenging due to their low ionization efficiencies and inherent structural heterogeneity. Here, we illustrate how the coupling of online droplet-based reaction, in a form of contained electrospray (ES) ion source, with liquid chromatography (LC) tandem mass spectrometry (MS/MS) allows the comprehensive characterization of sucrose isomers. We used the reaction between phenylboronic acid and cis-diols for on-the-fly derivatization of saccharides eluting from the LC column followed by in situ MS/MS analysis, which afforded diagnostic fragment ions that enabled differentiation of species indistinguishable by chromatography or mass spectrometry alone. For example, chromatograms differing only by 2% in retention times were flagged to be different based on incompatible MS/MS fragmentation patterns. This orthogonal LC-contained-ES-MS/MS method was applied to confirm the presence of turanose, palatinose, maltulose, and maltose, which are structural isomers of sucrose, in three different honey samples. The reported workflow does not require modification to existing mass spectrometers, and the contained-ES platform itself acts both as the ion source and the reactor, all promising widespread application.

Dual Tunability for Uncatalyzed N-Alkylation of Primary Amines Enabled by Plasma-Microdroplet Fusion

The fusion of non-thermal plasma with charged microdroplets facilitates catalyst-free N-alkylation for a variety of primary amines, without halide salt biproduct generation. Significant reaction enhancement (up to >200×) is observed over microdroplet reactions generated from electrospray. This enhancement for the plasma-microdroplet system is attributed to the combined effects of energetic collisions and the presence of reactive oxygen species (ROS). The ROS (e.g., O2 ⋅- ) act as a proton sink to increase abundance of free neutral amines in the charged microdroplet environment. The effect of ROS on N-alkylation is confirmed through three unique experiments: (i) utilization of radical scavenging reagent, (ii) characterization of internal energy distribution, and (iii) controls performed without plasma, which lacked reaction acceleration. Establishing plasma discharge in the wake of charged microdroplets as a green synthetic methodology overcomes two major challenges within conventional gas-phase plasma chemistry, including the lack of selectivity and product scale-up. Both limitations are overcome here, where dual tunability is achieved by controlling reagent concentration and residence time in the microdroplet environment, affording single or double N-alkylated products. Products are readily collected yielding milligram quantities in eight hours. These results showcase a novel synthetic strategy that represents a straightforward and sustainable C-N bond-forming process.

Revealing the Impacts of Chemical Complexity on Submicrometer Sea Spray Aerosol Morphology

Sea spray aerosol (SSA) ejected through bursting bubbles at the ocean surface is a complex mixture of salts and organic species. Submicrometer SSA particles have long atmospheric lifetimes and play a critical role in the climate system. Composition impacts their ability to form marine clouds, yet their cloud-forming potential is difficult to study due to their small size. Here, we use large-scale molecular dynamics (MD) simulations as a "computational microscope" to provide never-before-seen views of 40 nm model aerosol particles and their molecular morphologies. We investigate how increasing chemical complexity impacts the distribution of organic material throughout individual particles for a range of organic constituents with varying chemical properties. Our simulations show that common organic marine surfactants readily partition between both the surface and interior of the aerosol, indicating that nascent SSA may be more heterogeneous than traditional morphological models suggest. We support our computational observations of SSA surface heterogeneity with Brewster angle microscopy on model interfaces. These observations indicate that increased chemical complexity in submicrometer SSA leads to a reduced surface coverage by marine organics, which may facilitate water uptake in the atmosphere. Our work thus establishes large-scale MD simulations as a novel technique for interrogating aerosols at the single-particle level.

Plasma-Droplet Reaction Systems: A Direct Mass Spectrometry Approach for Enhanced Characterization of Lipids at Multiple Isomer Levels

Neutral triacylglyceride (TG) lipids are critical in cellular function, signaling, and energy storage. Multiple molecular pathways control TG structure via nonselective routes making them structurally complex and analytically challenging to characterize. The presence of C=C bond positional isomers exacerbates this challenge as complete structural elucidation is not possible by conventional tandem mass spectrometric methods such as collision-induced dissociation (CID), alone. Herein, we report a custom-made coaxial contained-electrospray ionization (ESI) emitter that allows the fusion of plasma discharge with charged microdroplets during electrospray (ES). Etched capillaries were incorporated into this contained-ES emitter, facilitating the generation of reactive oxygen species (ROS) at low (3 kV) ESI voltages and allowing stable ESI ion signal to be achieved at an unprecedented high (7 kV) spray voltage. The analytical utility of inducing plasma discharge during electrospray was investigated using online ionization of neutral TGs, in situ epoxidation of unsaturation sites, and C=C bond localization via conventional CID mass spectrometry. Collisional activation of the lipid epoxide generated during the online plasma-droplet fusion experiment resulted in a novel fragmentation pattern that showed a quadruplet of diagnostic ions for confident assignment of C=C bond positions and subsequent isomer differentiation. This phenomenon enabled the identification of a novel TG lipid, composed of conjugated linoleic acid, that is isomeric with two other TG lipids naturally found in extra virgin olive oil. To validate our findings, we analyzed various standards of TG lipids, including triolein, trilinolein, and trilinolenin, and isomeric mixtures in the positive-ion mode, each of which produced the expected quadruplet diagnostic fragment ions. Further validation was obtained by analyzing standards of free fatty acids expected from the hydrolysis of the TG lipids in the negative-ion mode, together with isomeric mixtures. The chemistry governing the gas-phase fragmentation of the lipid epoxides was carefully elucidated for each TG lipid analyzed. This comprehensive shotgun lipidomic approach has the potential to impact biomedical research since it can be accomplished on readily available mass spectrometers without the need for instrument modification.

Rapid Characterization of Antibodies via Automated Flow Injection Coupled with Online Microdroplet Reactions and Native-pH Mass Spectrometry

Microdroplet reactions have aroused much interest due to significant reaction acceleration (e.g., ultrafast protein digestion in microdroplets could occur in less than 1 ms). This study integrated a microdroplet protein digestion technique with automated sample flow injection and online mass spectrometry (MS) analysis, to develop a rapid and robust method for structural characterization of monoclonal antibodies (mAbs) that is essential to assess the antibody drug's safety and quality. Automated sequential aspiration and mixing of an antibody and an enzyme (IdeS or IgdE) enabled rapid analysis with high reproducibility (total analysis time: 2 min per sample; reproducibility: ∼2% coefficient of variation). Spraying the sample in ammonium acetate buffer (pH 7) using a jet stream source allowed efficient digestion of antibodies and efficient ionization of resulting antibody subunits under native-pH conditions. Importantly, it also provided a platform to directly study specific binding of an antibody and an antigen (e.g., detecting the complexes mAb/RSFV antigen and F(ab')2/RSVF in this study). Furthermore, subsequent tandem MS analysis of a resulting subunit from microdroplet digestion enabled localizing post-translational modifications on particular domains of a mAb in a rapid fashion. In combination with IdeS digestion of an antibody, additional tris(2-carboxyethyl)phosphine (TCEP) reduction and N-glycosidase F (PNGase F) deglycosylation reactions that facilitate antibody analysis could be realized in "one-pot" spraying. Interestingly, increased deglycosylation yield in microdroplets was found, simply by raising the sample temperature. We expect that our method would have a high impact for rapid characterization of monoclonal antibodies.

Online Cross-Linking of Peptides and Proteins during Contained-Electrospray Ionization Mass Spectrometry

Recent advancements in mass spectrometry (MS) now enable all levels of protein structures to be characterized, including primary protein sequence, post-translational modifications, and three-dimensional protein conformations. However, protein conformational studies by MS require the use of many separate techniques that are performed independently of each other. Herein, we described a contained-electrospray (ES) experiment that has potential to integrate peptide/protein cross-linking with the general MS workflow. In our experiment, cross-linking of protein/peptide occurs simultaneously with ionization after analytes, and cross-linkers are sprayed from two separate ES emitters. The online cross-linking process occurring in the charged microdroplet environment was optimized using trilysine peptide and bis(sulfosuccinimidyl)suberate cross-linker. We detected the electrostatic complex between analyte and cross-linker, the mono-linked intermediate, and the fully cross-linked product, allowing us to correctly predict the sequence of reaction events in the cross-linking process. Importantly, we observed that the terminal fully cross-linked product is composed of two distinct conformations. In one form, the product involved cross-linking between two ε-NH₂ amines in lysine residues, while the other conformer was formed by a reaction between one ε-NH₂ amine and the N-terminus. The experimental conditions for selecting one cross-linked species over others during the online ES ionization-MS analysis have been detailed. Appropriate parameters enabled the reaction between α-lactalbumin proteins and cross-linkers using a non-denaturing spray condition. These results establish a framework for a future development in high-throughput structural MS method, where all levels of protein information can be gathered in a single experiment.

Spatial reorganization of analytes in charged aqueous microdroplets

Imprinted charged aqueous droplets of micrometer dimensions containing spherical gold and silver nanoparticles, gold nanorods, proteins and simple molecules were visualized using dark-field and transmission electron microscopies. With such studies, we hoped to understand the unusual chemistry exhibited by microdroplets. These droplets with sizes in the range of 1-100 μm were formed using a home-built electrospray source with nitrogen as the nebulization gas. Several remarkable features such as mass/size-selective segregation and spatial localization of solutes in nanometer-thin regions of microdroplets were visualized, along with the formation of micro-nano vacuoles. Electrospray parameters such as distance between the spray tip and surface, voltage and nebulization gas pressure influenced particle distribution within the droplets. We relate these features to unusual phenomena such as the enhancement of rates of chemical reactions in microdroplets.

Liquid Chromatography-Tandem Mass Spectrometry with Online, In-Source Droplet-Based Phenylboronic Acid Derivatization for Sensitive Analysis of Saccharides

The ability to identify abnormalities in the body's saccharide profile is a promising means for early disease detection but requires analytical tools capable of detecting saccharides at low concentrations and/or for volume-limited samples. The preferred analysis approach for these compounds, liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS), often lacks sensitivity due to poor ionization efficiency. In this work, we employ a modified electrospray interface-termed contained-electrospray (contained-ESI) to couple accelerated droplet chemistry to conventional LC-MS for the online and automated separation, derivatization, and detection of saccharides. The chromatographic component enables complex sample and mixtures analysis with low sample volume requirements, while the enhanced reaction kinetics afforded by electrosprayed microdroplets facilitates rapid, on-the-fly derivatization to boost sensitivity. Derivatization occurs during ion formation as analytes elute from the column, eliminating the need for superfluous post-column derivatization hardware or complicated benchtop protocols. A grounded coupler was incorporated to shield the LC from the high-voltage ion source, and method conditions were optimized to accommodate the low flow rates preferred for microdroplet reactions. The new LC-contained-ESI-MS/MS platform was demonstrated for the analysis of several mono-, di-, and oligosaccharides using in-source droplet-based phenylboronic acid derivatization. Femtomole limits of detection were achieved for a 1 μL injection, representing sensitivity enhancement of 1-2 orders of magnitude over conventional LC-ESI-MS/MS without derivatization. In addition, isobaric saccharides that are difficult to differentiate by MS alone were easily distinguished. Method precision, accuracy, and linearity were established, and the ability to detect oligosaccharides at trace levels in human urine and plasma was demonstrated.

Investigation of Tryptic Protein Digestion in Microdroplets and in Bulk Solution

Recent studies have shown that ultrafast enzymatic digestion of proteins can be achieved in microdroplet within 250 μs. Further investigation of peptides resulting from microdroplet digestion (MD) would be necessary to evaluate it as an alternative to the conventional bulk digestion for bottom-up and biotherapeutic protein characterization. Herein we examined and compared protein tryptic digestion in both MD and bulk solution. In the case of MD of β-lactoglobulin B, the preservation of long peptides was observed due to the short digestion time. In addition, MD is applicable to digest both high- and low-abundance proteins in mixture. In the case of digesting NIST 8671 mAb antibody containing a low level of commonly encountered host cell protein (HCP) PLBL2 (mAb:PLBL2 = 100:1 by weight), MD produced lower levels of digestion-induced chemical modifications of asparagine/glutamine deamidation, compared with overnight digestion. No significant difference between MD and bulk digestion was observed in terms of trypsin digestion specificity based on examination of semi- and unspecific-cleaved peptides. Our study suggests that MD, a fast digestion approach, could be adopted for bottom-up proteomics research and for peptide mapping of mAbs to characterize site-specific deamidation and glycosylation, for the purpose of development of biopharmaceuticals.

Growth Rates of Hydrogen Microbubbles in Reacting Femtoliter Droplets

Chemical reactions in small droplets are extensively explored to accelerate the discovery of new materials and increase the efficiency and specificity in catalytic biphasic conversion and high-throughput analytics. In this work, we investigate the local rate of the gas-evolution reaction within femtoliter droplets immobilized on a solid surface. The growth rate of hydrogen microbubbles (≥500 nm in radius) produced from the reaction was measured online with high-resolution confocal microscopic images. The growth rate of bubbles was faster in smaller droplets and near the droplet rim in the same droplet. The results were consistent for both pure and binary reacting droplets and on substrates of different wettability. Our theoretical analysis based on diffusion, chemical reaction, and bubble growth predicted that the concentration of the reactant depended on the droplet size and the bubble location inside the droplet, in good agreement with experimental results. Our results reveal that the reaction rate may be spatially nonuniform in the reacting microdroplets. The findings may have implications for formulating the chemical properties and uses of these droplets.