Evaluation of the ALiPHAT method for PC-IDMS and correlation of limits-of-detection with nonpolar surface area

Electrohydrodynamics of Gas-Assisted Electrospray Ionization Mass Spectrometry

Gas-flow assistance is commonly used in ESI-MS for improved transport and desolvation, and fundamental understanding of the underlying phenomena is essential for improvement of aerodynamic interfaces that couple ESI sources and MS. For this purpose, an electrohydrodynamic model is developed for simulation of charged droplet dynamics under the combined effects of gas flow and electric fields with consideration of space charge interactions within the charged aerosol plume. The model is implemented in COMSOL by exploiting a formalism for establishing the droplet trajectories as a sequence of successive droplets ejected at a frequency defined by the electrospray current. The model is used to assess the effect of two distinct flow configurations and compared to the baseline care of electrospray without assist gas. The simulated flows are jet flows oriented coaxially with the ESI spray, with and without imposed vorticity (swirling). Droplet trajectory simulations of a bimodal droplet population consisting of large primary droplets and small progeny droplets reveal a unique capability for vortical assist jet flow to selectively transmit smaller droplets into the MS due to inertial separation. ESI-MS analysis of fluorinated phosphazines subjected to the different gas flow conditions supports the model predictions. The electrohydrodynamic model developed in this work provides a versatile tool to analyze and design aerodynamic ESI interfaces with rigorous incorporation of drag, inertia, and space-charge repulsion and can be used as a powerful simulation methodology for optimizing charged droplet transmission and ultimately improved analytical performance of gas-assisted ESI-MS workflows.

New ionization tags based on the structure of the 5-azoniaspiro[4.4]nonyl tag for a sensitive peptide sequencing by mass spectrometry

Quaternary ammonium salts (QAS), both linear and bicyclic, are often utilized to improve the mass spectrometry (MS) analysis of peptides by fixing a permanent positive charge on the analyzed molecule. However, during collision-induced dissociation (CID) experiments, QAS undergo unwanted side reactions-Hofmann elimination as well as a tertiary amine loss- rendering the data interpretation complicated. In this work, we present 2-thia- and 2-oxa-5-azoniaspiro[4.4]nonyl groups as heterocyclic derivatives of the highly stable ionization group, 5-azoniaspiro[4.4]nonyl, for a sensitive peptide analysis by MS. Due to the permanent positive charge, labeled peptides are characterized by enhanced ionization efficiency during electrospray mass spectrometry (ESI-MS) conditions. Moreover, interpretation of the CID fragmentation of labeled peptides is facilitated since a series of generated fragmentation ions enable a complete sequence coverage. Introduction of a heteroatom into the 5-azoniaspiro[4.4]nonyl scaffold allows for liberation of a stable reporter ion which could be used in selected reaction monitoring (SRM)-targeted quantification experiments. Additionally, we synthesized a deuterated analog of the tag for LC-SRM-targeted quantitative analysis. The obtained results indicate the general usefulness of the proposed heterocyclic quaternary ammonium ionization tag for sequencing and quantification of peptides. Graphical abstract New reagents based on the structure of the 5-azoniaspiro[4.4]nonyl tag for peptide analysis by tandem mass spectrometry.

Improving the analytical performance and versatility of paper spray mass spectrometry via paper microfluidics

Paper-based microfluidic techniques were explored to increase paper spray mass spectrometry's performance and versatility.

Evaluating nonpolar surface area and liquid chromatography/mass spectrometry response: an application for site occupancy measurements for enzyme intermediates in polyketide biosynthesis

RATIONALE

Site occupancy measurements using liquid chromatography/mass spectrometry (LC/MS) are reported throughout the literature. However, site occupancy quantification suffers from ionization bias between modified and unmodified peptides containing the active site. In this study, we explore the MS signal as a function of nonpolar surface area (NPSA) in order to better understand this bias in electrospray response. The correlation between hydrophobicity and LC/MS response was evaluated and applied to study enzyme intermediates in polyketide synthases.

METHODS

Site occupancy methods were developed to study acyltransferase activity. To further evaluate these methods, several standard peptides containing one cysteine residue were modified with alkylation reagents of increasing hydrophobicity to study the MS signal as a function of NPSA.

RESULTS

A consistent trend in MS response was observed which is dependent on the NPSA of the analyte. An optimal NPSA zone was observed for the peptides studied.

CONCLUSIONS

Nonpolar surface area can be used as metric to determine relative LC/MS response for peptides and evaluate site occupancy measurements.

High sensitivity mass spectrometric quantification of serum growth hormone by amphiphilic peptide conjugation

Amphiphilic peptide conjugation affords a significant increase in sensitivity with protein quantification by electrospray-ionization mass spectrometry. This has been demonstrated for human growth hormone (GH) in serum using N-(3-iodopropyl)-N,N,N-dimethyloctylammonium iodide as derivatizing reagent. The signal enhancement achieved is up to a factor of 5-6 and enables extension of the applicable concentration range down to the very low concentrations (≤ 1.0 µg/L) as encountered with clinical glucose suppression tests for patients with acromegaly. The method has been validated using a set of serum samples spiked with known amounts of recombinant 22 kDa GH in the range of 0.48 to 7.65 µg/L. The coefficient of variation (CV) calculated based on the deviation of results from the expected concentrations was 3.5%. The limit of detection (LoD) was determined as 0.1 µg/L and the limit of quantification (LoQ) as 0.4 µg/L. The potential of the method as a tool in clinical practice has been demonstrated with patient samples of about 1 µg/L.

Absolute quantification of free glutathione and cysteine in aquatic insects using isotope dilution and selected reaction monitoring

A simple and robust isotope dilution mass spectrometry-based assay was developed for the determination of free cysteine and glutathione (GSH) in aquatic insects. Several experimental parameters were evaluated and optimized to provide specific and sensitive detection of both compounds by in situ derivatization with N-ethylmaleimide followed by acid alkylation quenching and reverse-phased liquid chromatography coupled with selected reaction monitoring. For both targets, the assay was evaluated over a concentration range of 0.313 to 320 μM and was demonstrated to have a quantitative dynamic range spanning nearly three orders of magnitude, with lower limits of quantification being 0.330 μM for GSH and 0.370 μM for cysteine. Additionally, measurements were observed to be highly reproducible over the course of several days. When applied to the analysis of four different species of insects, large biological variation between and within species was observed. Different feeding regimens were also tested within two species of insects but statistical comparisons revealed no significant difference in the levels of either compound.

Hydrophobic derivatization of N-linked glycans for increased ion abundance in electrospray ionization mass spectrometry

A library of neutral, hydrophobic reagents was synthesized for use as derivatizing agents in order to increase the ion abundance of N-linked glycans in electrospray ionization mass spectrometry (ESI MS). The glycans are derivatized via hydrazone formation and are shown to increase the ion abundance of a glycan standard more than 4-fold. Additionally, the data show that the systematic addition of hydrophobic surface area to the reagent increases the glycan ion abundance, a property that can be further exploited in the analysis of glycans. The results of this study will direct the future synthesis of hydrophobic reagents for glycan analysis using the correlation between hydrophobicity and theoretical non-polar surface area calculation to facilitate the development of an optimum tag for glycan derivatization. The compatibility and advantages of this method are demonstrated by cleaving and derivatizing N-linked glycans from human plasma proteins. The ESI-MS signal for the tagged glycans are shown to be significantly more abundant, and the detection of negatively charged sialylated glycans is enhanced.

Alkylating tryptic peptides to enhance electrospray ionization mass spectrometry analysis

A major limitation of mass spectrometry-based proteomics is inefficient and differential ionization during electrospray ionization (ESI). This leads to problems such as increased limits of detection and incomplete sequence coverage of proteins. Incomplete sequence coverage is especially problematic for analyses that require the detection and identification of specific peptides from a protein, such as the analysis of post-translational modifications. We describe here the development and use of aldehyde-based chemistry for the alkylation of peptide primary amines to increase peptide hydrophobicity, providing increased ionization efficiency and concomitant signal enhancement. When employed to modify the peptide products of protein tryptic digests, increased sequence coverage is obtained from combined modified and unmodified digests. To evaluate the utility of alkylation of peptides for selected reaction monitoring (SRM) assays, we alkylated a peptide from the protein Oct4, known to play a role in regulating stem cell differentiation. Increased chromatographic retention and ionization efficiency is observed for the alkylated Oct4 peptide compared to its unmodified form.

Interplay of permanent charge and hydrophobicity in the electrospray ionization of glycans

The analysis of N-linked glycans by mass spectrometry (MS) has been characterized by low signal-to-noise ratios and high limits of detection due to their hydrophilicity and lack of basic sites able to be protonated. As a result, every step in glycan sample preparation must be thoroughly optimized in order to minimize sample loss, contamination, and analytical variability. Importantly, properties of glycans and their derivatized counterparts must be thoroughly studied in order to exploit certain characteristics for enhancing MS analysis. Herein, the effectiveness of the incorporation of a permanent charge is studied and determined to hamper glycan analysis. Also, a procedure for glycan hydrazone formation is optimized and outlined where a large number of variables were simultaneously analyzed using a fractional factorial design (FFD) in order to determine which conditions affected the reaction efficiency of the hydrazone formation reaction. Finally, the hydrophobic tagging of glycans is shown to be a viable opportunity to further increase the ion abundance of glycans in MS.

Improving limits of detection for B-type natriuretic peptide using PC-IDMS: an application of the ALiPHAT strategy

Hydrophobic tagging of biomolecules has been reported by our group and others to increase their ionization efficiency during electrospray ionization and facilitate their detection by mass spectrometry. As such, hydrophobic tagging should provide a viable method for augmenting MS-based quantification of low abundance proteins by decreasing their detection limits. Herein we have evaluated two commercial alkylation reagents and several newly synthesized hydrophobic alkylation reagents for their utility in quantifying B-type Natriuretic Peptide, a low abundance cardiac biomarker, by protein cleavage isotope dilution mass spectrometry. For the cysteine containing tryptic peptide evaluated, a approximately 3.5-fold decrease in the detection limit was observed for the best performing hydrophobic reagent, 2-iodo-N-octylacetamide, relative to the commonly used alkylation reagent, iodoacetamide. Additionally, we have evaluated the use of nonpolar surface areas as a metric for assessing the effectiveness of the alkylation reagents in improving ESI response.