A novel mass spectrometry (MS)-based lipidomics strategy that exposes glycerophospholipids to an ethereal solution of diazomethane and acid, derivatizing them to contain a net fixed, permanent positive charge, is described. The sensitivity of modified lipids to MS detection is enhanced via improved ionization characteristics as well as consolidation of ion dissociation to form one or two strong, characteristic polar headgroup fragments. Our strategy has been optimized to enable a priori prediction of ion fragmentation patterns for four subclasses of modified glycerophospholipid species. Our method enables analyte ionization regardless of proton affinity, thereby decreasing ion suppression and permitting predictable precursor ion-based quantitation with improved sensitivity in comparison to MS-based methods that are currently used on unmodified lipid precursors.

Significant sensitivity enhancements in the tandem mass spectrometry-based analysis of complex mixtures of several phospholipid classes has been achieved via (13)C-TrEnDi. (13)C-TrEnDi-modified phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylcholine (PC) lipids extracted from HeLa cells demonstrated greater sensitivity via precursor ion scans (PISs) than their unmodified counterparts. Sphingomyelin (SM) species exhibited neither an increased nor decreased sensitivity following modification. The use of isotopically labeled diazomethane enabled the distinction of modified PE and modified PC species that would yield isobaric species with unlabeled diazomethane. (13)C-TrEnDi created a PE-exclusive PIS of m/z 202.1, two PS-exclusive PISs of m/z 148.1 and m/z 261.1, and a PIS of m/z 199.1 for PC species (observed at odd m/z values) and SM species (observed at even m/z values). The standardized average area increase after TrEnDi modification was 10.72-fold for PE species, 2.36-fold for PC, and 1.05-fold for SM species. The sensitivity increase of PS species was not quantifiable, as there were no unmodified PS species identified prior to derivatization. (13)C-TrEnDi allowed for the identification of 4 PE and 7 PS species as well as the identification and quantitation of an additional 4 PE and 4 PS species that were below the limit of detection (LoD) prior to modification. (13)C-TrEnDi also pushed 24 PE and 6 PC lipids over the limit of quantitation (LoQ) that prior to modification were above the LoD only.

Defining cellular processes relies heavily on elucidating the temporal dynamics of proteins. To this end, mass spectrometry (MS) is an extremely valuable tool; different MS-based quantitative proteomics strategies have emerged to map protein dynamics over the course of stimuli. Herein, we disclose our novel MS-based quantitative proteomics strategy with unique analytical characteristics. By passing ethereal diazomethane over peptides on strong cation exchange resin within a microfluidic device, peptides react to contain fixed, permanent positive charges. Modified peptides display improved ionization characteristics and dissociate via tandem mass spectrometry (MS(2)) to form strong a2 fragment ion peaks. Process optimization and determination of reactive functional groups enabled a priori prediction of MS(2) fragmentation patterns for modified peptides. The strategy was tested on digested bovine serum albumin (BSA) and successfully quantified a peptide that was not observable prior to modification. Our method ionizes peptides regardless of proton affinity, thus decreasing ion suppression and permitting predictable multiple reaction monitoring (MRM)-based quantitation with improved sensitivity.

Trimethylation enhancement using diazomethane (TrEnDi) is a derivatization technique that significantly enhances the signal intensity of glycerophospholipid species in mass spectrometry (MS) and tandem mass spectrometry (MS/MS) analyses. Here, we describe a novel apparatus that is able to conduct in situ TrEnDi (iTrEnDi) by generating and immediately reacting small amounts of gaseous diazoalkane with analyte molecules. iTrEnDi allows complete and rapid methylation of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), and sphingomyelin (SM) in a safe manner by removing any need for direct handling of dangerous diazoalkane solutions. iTrEnDi-modified PC ([PCTr]+) and PE ([PETr]+) showed similar sensitivity enhancements and fragmentation patterns compared to our previously reported methodology. iTrEnDi yielded dimethylated PA ([PATr]), which exhibited dramatically improved chromatographic behavior and a 14-fold increase in liquid chromatography MS (LCMS) sensitivity compared to unmodified PA. In comparison to in-solution-based TrEnDi, iTrEnDi demonstrated a modest decrease in sensitivity, likely due to analyte losses during handling. However, the enhanced safety benefits of iTrEnDi coupled with its ease of use and capacity for automation, as well as its accommodation of more-reactive diazoalkane species, vastly improve the accessibility and utility of this derivatization technique. Finally, as a proof of concept, iTrEnDi was used to produce diazoethane (DZE), a more-reactive diazoalkane than diazomethane. Reaction between DZE and PC yielded ethylated [PCTr]+, which fragmented via MS/MS to produce a high-intensity characteristic fragment ion, enabling a novel and highly sensitive precursor ion scan.

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The use of molecular editing in the elucidation of the mechanism of action of amphotericin B is presented. A modular strategy for the synthesis of amphotericin B and its designed analogues is developed, which relies on an efficient gram-scale synthesis of various subunits of amphotericin B. A novel method for the coupling of the mycosamine to the aglycone was identified. The implementation of the approach has enabled the preparation of 35-deoxy amphotericin B methyl ester. Investigation of the antifungal activity and efflux-inducing ability of this amphotericin B congener provided new clues to the role of the 35-hydroxy group and is consistent with the involvement of double barrel ion channels in causing electrolyte efflux.

[reaction: see text] Chiral bicyclic thioglycolate lactams may be prepared in three steps from inexpensive commercial materials. The resulting lactams may be alkylated three times, twice using basic enolization and once using reductive enolization, to form alpha-quaternary carboxylic acid derivatives in high yield and with high diastereoselectivity. The alkylation products may be cleaved under either acidic or reductive conditions to furnish either carboxylic acids or primary alcohols, respectively.

Methylation of phospholipids (PL) leads to increased uniformity in positive electrospray ionization (ESI) efficiencies across the various PL sub-classes. This effect is realized in the approach referred to as “trimethylation enhancement using 13C-diazomethane” (13C-TrEnDi), which results in the methyl esterification of all acidic sites and the conversion of amines to quaternary ammonium sites. Collision induced dissociation (CID) of these cationic, modified lipids enables class identification by forming distinctive head-group fragments based on the number of 13C atoms incorporated during derivatization. However, there are no distinctive fragment ions in positive mode that provide fatty acyl information for any of the modified lipids. Gas-phase ion/ion reactions of 13C-TrEnDi-modified PE, PS, PC, and SM cations with dicarboxylate anions are shown to charge-invert the positively charged phospholipids to the negative mode. An electrostatically-bound complex anion is shown to fragment predominantly via a novel head-group dication transfer to the reagent anion. Fragmentation of the resulting anionic product yields fatty acyl information, in the case of the glycerophospholipids (PE, PS, and PC), via ester bond cleavage. Analogous information is obtained from modified SM lipid anions via amide bond cleavage. Fragmentation of the anions generated from charge inversion of the 13C-TrEnDi modified phospholipids was also found to yield lipid class information without having to perform CID in positive mode. The combination of 13C-TrEnDi-modification of lipid mixtures with charge inversion to the negative ion mode retains the advantages of uniform ionization efficiency in the positive ion mode with the additional structural information available in the negative ion mode without requiring the lipids to be ionized directly in both ionization modes.