On-Chip Spyhole Mass Spectrometry for Droplet-Based Microfluidics

Quantification of Biocatalytic Transformations by Single Microbial Cells Enabled by Tailored Integration of Droplet Microfluidics and Mass Spectrometry

Improving the performance of chemical transformations catalysed by microbial biocatalysts requires a deep understanding of cellular processes. While the cellular heterogeneity of cellular characteristics, such as the concentration of high abundant cellular content, is well studied, little is known about the reactivity of individual cells and its impact on the chemical identity, quantity, and purity of excreted products. Biocatalytic transformations were monitored chemically specific and quantifiable at the single-cell level by integrating droplet microfluidics, cell imaging, and mass spectrometry. Product formation rates for individual Saccharomyces cerevisiae cells were obtained by i) incubating nanolitre-sized droplets for product accumulation in microfluidic devices, ii) an imaging setup to determine the number of cells in the droplets, and iii) electrospray ionisation mass spectrometry for reading the chemical contents of individual droplets. These findings now enable the study of whole-cell biocatalysis at single-cell resolution.

Microdroplet Mass Spectrometry Enables Extremely Accelerated Pepsin Digestion of Proteins

In microdroplets, rates of chemical or biomolecular reactions can exceed those in the bulk phase by more than a million times. As electrospray ionization-based mass spectrometry (MS) involves the formation of charged microdroplets, reaction acceleration and online MS monitoring of reaction products can readily be performed at the same time. We investigated accelerated enzymatic reactions in microdroplets and focused on the proteolytic enzyme pepsin. Electrosonic spray ionization (ESSI) was utilized for developing the ultrarapid pepsin in-spray digestion of two different proteins, cytochrome c and RocC, at low pH values. The optimization of the protein digestion aimed at achieving maximum sequence coverage for the analyzed proteins. Furthermore, carefully designed control experiments allowed us to unambiguously prove that enzymatic protein cleavage almost exclusively occurs within the spray at a millisecond time scale and not prior to microdroplet generation.

Opening up the Single-Cell Toolbox for Microbial Natural Products Research

The diverse microbes that produce natural products represent an important source of novel therapeutics, drug leads, and scientific tools. However, the vast majority have not been grown in axenic culture and are members of complex communities. While meta-'omic methods such as metagenomics, -transcriptomics, and -proteomics reveal collective molecular features of this "microbial dark matter", the study of individual microbiome members can be challenging. To address these limits, a number of techniques with single-bacterial resolution have been developed in the last decade and a half. While several of these are embraced by microbial ecologists, there has been less use by researchers interested in mining microbes for natural products. In this review, we discuss the available and emerging techniques for targeted single-cell analysis with a particular focus on applications to the discovery and study of natural products.

High-Throughput Bioassays using "Dip-and-Go" Multiplexed Electrospray Mass Spectrometry

A multiplexed system based on inductive nanoelectrospray mass spectrometry (nESI-MS) has been developed for high-throughput screening (HTS) bioassays. This system combines inductive nESI and field amplification micro-electrophoresis to achieve a "dip-and-go" sample loading and purification strategy that enables nESI-MS based HTS assays in 96-well microtiter plates. The combination of inductive nESI and micro-electrophoresis makes it possible to perform efficient in situ separations and clean-up of biological samples. The sensitivity of the system is such that quantitative analysis of peptides from 1-10 000 nm can be performed in a biological matrix. A prototype of the automation system has been developed to handle 12 samples (one row of a microtiter plate) at a time. The sample loading and electrophoretic clean-up of biosamples can be done in parallel within 20 s followed by MS analysis at a rate of 1.3 to 3.5 s per sample. The system was used successfully for the quantitative analysis of BACE1-catalyzed peptide hydrolysis, a prototypical HTS assay of relevance to drug discovery. IC₅₀ values for this system were in agreement with LC-MS but recorded in times more than an order of magnitude shorter.

Achieving Stable Electrospray Ionization Mass Spectrometry Detection from Microfluidic Chips

The past two decades have witnessed remarkable advances in the development of microfluidic devices as bioanalytical platforms for the analysis of biological molecules. The implementation of mass spectrometry (MS) detection systems on these devices has become inevitable, and various chip-MS ionization interfaces have been developed. As electrospray ionization (ESI) is particularly relevant for the analysis of large biological molecules such as proteins or peptides, efforts have focused on advancing interfaces that meet the demands of nano-separation techniques that are typically used prior to MS detection. Achieving stable ESI conditions that enable sensitive MS detection is, however, not trivial, especially when the spray is generated from a microfabricated platform. This chapter is aimed at providing a step-by-step protocol for producing stable and efficient electrospray sample ionization from microfluidic chips that are used for capillary electrophoresis (CE) separations.

Alleviation of electrochemical oxidation for peptides and proteins in electrospray ionization: obtaining more accurate mass spectra with induced high voltage

Accurate mass spectrometry (MS) signal for peptide/protein analysis, which could be affected by various MS conditions, plays an essential role in identification and quantification of biological samples. Herein, we tried to alleviate the possible interferences from electrochemical oxidations during electrospray ionization (ESI). Three most common electrochemical oxidation reactions in ESI include oxidation of analyte, solvent, and electrode. With introduction of induced electrospray ionization (IESI) (a variant form of ESI), these interferences were significantly alleviated for peptides/proteins. That effect was also tested with flow injection experiments with different solution flow rates, electrolyte concentrations and solvent compositions, which was to simulate various chromatography conditions in conventional liquid chromatography (LC) separations. For all chromatography conditions tested, electrochemical oxidation was significantly alleviated for the absence of physical contact between spray solution and electrode.