Ion/ion reactions of MALDI-derived peptide ions: increased sequence coverage via covalent and electrostatic modification upon charge inversion

Imaging Mass Spectrometry of Sulfatide Isomers from Rat Brain Tissue Using Gas-Phase Charge Inversion Ion/Ion Reactions

Sulfatides are abundant components of the brain, and dysregulation of these molecules has been linked to several diseases. In sulfatide structures, a sugar is linked to a sphingoid backbone via an α-glycosidic or β-glycosidic linkage. While sulfatides are readily generated in negative ion mode imaging mass spectrometry experiments, resolving sulfatide diastereomers is challenging; therefore, identifications are usually reported as a single sulfatide. Herein, a gas-phase charge inversion ion/ion reaction between sulfatides and a strontium tris-phenanthroline [Sr(Phen)3]2+ reagent is performed to separate the diastereomers, as they form complexes containing different numbers of phenanthroline ligands. The ability to separate these diastereomers using the reaction alone, without the need for any further dissociation, allows for the workflow to be readily implemented in an imaging mass spectrometry experiment. Imaging mass spectrometry was performed on sulfatides generated directly from rat brain tissue, and both the α- and β-linked sulfatide images were obtained.

Structural Elucidation and Relative Quantification of Fatty Acid Double Bond Positional Isomers in Biological Tissues Enabled by Gas-Phase Charge Inversion Ion/Ion Reactions

Fatty acids (FAs) contain a vast amount of structural diversity, and differences in fatty acid structure have been associated with various disease states. Accurate identification and characterization of fatty acids is critical to fully understand the biochemical roles these compounds play in disease progression. Conventional tandem mass spectrometry (MS/MS) workflows do not provide sufficient structural information, necessitating alternative dissociation methods. Gas-phase charge inversion ion/ion reactions can be used to alter the ion type subjected to activation to provide improved or complementary structural information. Herein, we have used an ion/ion reaction between fatty acid (FA) anions and magnesium tris-phenanthroline [Mg(Phen)3] dications to promote charge remote fragmentation of carbon-carbon bonds along the fatty acid chain, allowing for localization of carbon-carbon double bond (C=C) positions to successfully differentiate monounsaturated fatty acid isomers. Relative quantification was also performed to obtain the relative abundance of fatty acid isomers in different biological tissues. For example, the relative abundance of FA 18:1 (9) was determined to vary across regions of rat brain, rat kidney, and mouse pancreas, and FA 16:1 (9) was found to have a higher relative abundance in the dermis layer compared to the sebaceous glands in human skin tissue.

Relative Quantification of Lipid Isomers in Imaging Mass Spectrometry Using Gas-Phase Charge Inversion Ion/Ion Reactions and Infrared Multiphoton Dissociation

Accurate structural identification of lipids in imaging mass spectrometry is critical to properly contextualizing spatial distributions with tissue biochemistry. Gas-phase charge inversion ion/ion reactions alter the ion type prior to dissociation to allow for more structurally informative fragmentation and improve lipid identification at the isomeric level. In this work, infrared multiphoton dissociation (IRMPD) was interfaced with a commercial hybrid Qh-FT-ICR mass spectrometer to enable the rapid fragmentation of gas-phase charge inversion ion/ion reaction products at every pixel in imaging mass spectrometry experiments. An ion/ion reaction between phosphatidylcholine (PC) monocations generated from rat brain tissue via matrix-assisted laser desorption/ionization (MALDI) and 1,4-phenylenediproprionic acid reagent dianions generated via electrospray ionization (ESI) followed by IRMPD of the resulting product ion complex produces selective fatty acyl chain cleavages indicative of fatty acyl carbon compositions in the lipid. Ion/ion reaction images using this workflow allow for mapping of the relative spatial distribution of multiple PC isomers under a single sum composition lipid identification. Lipid isomers display significantly different relative spatial distributions within rat brain tissue, highlighting the importance of resolving isomers in imaging mass spectrometry experiments.

Nanoparticle-based applications by atmospheric pressure matrix assisted desorption/ionization mass spectrometry

Recently, the development of atmospheric pressure matrix assisted desorption/ionization mass spectrometry (AP MALDI MS) has made contributions not only to biomolecule analysis but also to spatial distribution. This has positioned AP MALDI as a powerful tool in multiple domains, thanks to its comprehensive advantages compared to conventional MALDI MS. These developments have addressed challenges associated with previous AP MALDI analysis systems, such as optimization of apparatus settings, synthesis of novel matrices, preconcentration and isolation strategies before analysis. Herein, applications in different fields using AP MALDI MS were described, including peptide and protein analysis, metabolite analysis, pharmaceutical analysis, and mass spectrometry imaging.

Identification of Phosphatidylcholine Isomers in Imaging Mass Spectrometry Using Gas-Phase Charge Inversion Ion/Ion Reactions

Gas-phase ion/ion reactions have been enabled on a commercial dual source, hybrid QhFT-ICR mass spectrometer for use during imaging mass spectrometry experiments. These reactions allow for the transformation of the ion type most readily generated from the tissue surface to an ion type that gives improved chemical structural information upon tandem mass spectrometry (MS/MS) without manipulating the tissue sample. This process is demonstrated via the charge inversion reaction of phosphatidylcholine (PC) lipid cations generated from rat brain tissue via matrix-assisted laser desorption/ionization (MALDI) with 1,4-phenylenedipropionic acid (PDPA) reagent dianions generated via electrospray ionization (ESI). Collision-induced dissociation (CID) of the resulting demethylated PC product anions allows for the determination of the lipid fatty acyl tail identities and positions, which is not possible via CID of the precursor lipid cations. The abundance of lipid isomers revealed by this workflow is found to vary significantly in different regions of the brain. As each isoform may have a unique cellular function, these results underscore the importance of accurately separating and identifying the many isobaric and isomeric lipids and metabolites that can complicate image interpretation and spectral analysis.

Gas-Phase Oxidation of Neutral Basic Residues in Polypeptide Cations by Periodate

The gas-phase oxidation of doubly protonated peptides containing neutral basic residues to various products, including [M + H + O]⁺, [M - H]⁺, and [M - H - NH₃]⁺, is demonstrated here via ion/ion reactions with periodate. It was previously demonstrated that periodate anions are capable of oxidizing disulfide bonds and methionine, tryptophan, and S-alkyl cysteine residues. However, in the absence of these easily oxidized sites, we show here that systems containing neutral basic residues can undergo oxidation. Furthermore, we show that these neutral basic residues primarily undergo different types of oxidation (e.g., hydrogen abstraction) reactions than those observed previously (i.e., oxygen transfer to yield the [M + H + O]⁺ species) upon gas-phase ion/ion reactions with periodate anions. This chemistry is illustrated with a variety of systems, including a series of model peptides, a cell-penetrating peptide containing a large number of unprotonated basic sites, and ubiquitin, a roughly 8.6 kDa protein. Graphical Abstract ᅟ.

Modulation of Phosphopeptide Fragmentation via Dual Spray Ion/Ion Reactions Using a Sulfonate-Incorporating Reagent

The labile nature of phosphoryl groups has presented a long-standing challenge for the characterization of protein phosphorylation via conventional mass spectrometry-based bottom-up proteomics methods. Collision-induced dissociation (CID) causes preferential cleavage of the phospho-ester bond of peptides, particularly under conditions of low proton mobility, and results in the suppression of sequence-informative fragmentation that often prohibits phosphosite determination. In the present study, the fragmentation patterns of phosphopeptides are improved through ion/ion-mediated peptide derivatization with 4-formyl-1,3-benezenedisulfonic acid (FBDSA) anions using a dual spray reactor. This approach exploits the strong electrostatic interactions between the sulfonate moieties of FBDSA and basic sites to facilitate gas-phase bioconjugation and to reduce charge sequestration and increase the yield of phosphate-retaining sequence ions upon CID. Moreover, comparative CID fragmentation analysis between unmodified phosphopeptides and those modified online with FBDSA or in solution via carbamylation and 4-sulfophenyl isothiocyanate (SPITC) provided evidence for sulfonate interference with charge-directed mechanisms that result in preferential phosphate elimination. Our results indicate the prominence of charge-directed neighboring group participation reactions involved in phosphate neutral loss, and the implementation of ion/ion reactions in a dual spray reactor setup provides a means to disrupt the interactions by competing hydrogen-bonding interactions between sulfonate groups and the side chains of basic residues.

Gas-Phase Amidation of Carboxylic Acids with Woodward's Reagent K Ions

Gas-phase amidation of carboxylic acids in multiply-charged peptides is demonstrated via ion/ion reactions with Woodward's reagent K (wrk) in both positive and negative mode. Woodward's reagent K, N-ethyl-3-phenylisoxazolium-3'-sulfonate, is a commonly used reagent that activates carboxylates to form amide bonds with amines in solution. Here, we demonstrate that the analogous gas-phase chemistry occurs upon reaction of the wrk ions and doubly protonated (or doubly deprotonated) peptide ions containing the carboxylic acid functionality. The reaction involves the formation of the enol ester intermediate in the electrostatic complex. Upon collisional activation, the ethyl amine on the reagent is transferred to the activated carbonyl carbon on the peptide, resulting in the formation of an ethyl amide (addition of 27 Da to the peptide) with loss of a neutral ketene derivative. Further collision-induced dissociation (CID) of the products and comparison with solution-phase amidation product confirms the structure of the ethyl amide.

Gas-Phase Amidation of Carboxylic Acids with Woodward's Reagent K Ions

Gas-phase amidation of carboxylic acids in multiply-charged peptides is demonstrated via ion/ion reactions with Woodward's reagent K (wrk) in both positive and negative mode. Woodward's reagent K, N-ethyl-3-phenylisoxazolium-3'-sulfonate, is a commonly used reagent that activates carboxylates to form amide bonds with amines in solution. Here, we demonstrate that the analogous gas-phase chemistry occurs upon reaction of the wrk ions and doubly protonated (or doubly deprotonated) peptide ions containing the carboxylic acid functionality. The reaction involves the formation of the enol ester intermediate in the electrostatic complex. Upon collisional activation, the ethyl amine on the reagent is transferred to the activated carbonyl carbon on the peptide, resulting in the formation of an ethyl amide (addition of 27 Da to the peptide) with loss of a neutral ketene derivative. Further collision-induced dissociation (CID) of the products and comparison with solution-phase amidation product confirms the structure of the ethyl amide.

High-throughput bioconjugation for enhanced 193 nm photodissociation via droplet phase initiated ion/ion chemistry using a front-end dual spray reactor

Fast online chemical derivatization of peptides with an aromatic label for enhanced 193 nm ultraviolet photodissociation (UVPD) is demonstrated using a dual electrospray reactor implemented on the front-end of a linear ion trap (LIT) mass spectrometer. The reactor facilitates the intersection of protonated peptides with a second population of chromogenic 4-formyl-1,3-benzenedisulfonic acid (FBDSA) anions to promote real-time formation of ion/ion complexes at atmospheric pressure. Subsequent collisional activation of the ion/ion intermediate results in Schiff base formation generated via reaction between a primary amine in the peptide cation and the aldehyde moiety of the FBDSA anion. Utilizing 193 nm UVPD as the subsequent activation step in the MS(3) workflow results in acquisition of greater primary sequence information relative to conventional collision induced dissociation (CID). Furthermore, Schiff-base-modified peptides exhibit on average a 20% increase in UVPD efficiency compared to their unmodified counterparts. Due to the efficiency of covalent labeling achieved with the dual spray reactor, we demonstrate that this strategy can be integrated into a high-throughput LC-MS(n) workflow for rapid derivatization of peptide mixtures.

Transformation of [M + 2H](2+) Peptide Cations to [M - H](+), [M + H + O](+), and M(+•) Cations via Ion/Ion Reactions: Reagent Anions Derived from Persulfate

The gas-phase oxidation of doubly protonated peptides is demonstrated here using ion/ion reactions with a suite of reagents derived from persulfate. Intact persulfate anion (HS2O8(-)), peroxymonosulfate anion (HSO5(-)), and sulfate radical anion (SO4(-•)) are all either observed directly upon negative nanoelectrospray ionization (nESI) or easily obtained via beam-type collisional activation of persulfate into the mass spectrometer. Ion/ion reactions between each of these reagents and doubly protonated peptides result in the formation of a long-lived complex. Collisional activation of the complex containing a peroxymonosulfate anion results in oxygen transfer from the reagent to the peptide to generate the [M + H + O](+) species. Activation of the complex containing intact persulfate anion either results in oxygen transfer to generate the [M + H + O](+) species or abstraction of two hydrogen atoms and a proton to generate the [M - H](+) species. Activation of the complex containing sulfate radical anion results in abstraction of one hydrogen atom and a proton to form the peptide radical cation, [M](+•). This suite of reagents allows for the facile transformation of the multiply protonated peptides obtained via nESI into a variety of oxidized species capable of providing complementary information about the sequence and structure of the peptide.

Synthesis, X-ray structure, spectroscopic properties and DFT studies of a novel Schiff base

A series of Schiff bases, salicylideneaniline derivatives 1-4, was synthesized under mild conditions and characterized by 1H NMR, HRMS, UV-Vis and fluorescence spectra, and single-crystal X-ray diffraction. In solid and aprotic solvents 1-4 exist mainly as E conformers that possess an intramolecular six-membered-ring hydrogen bond. A weak intramolecular C-H···F hydrogen bond is also observed in fluoro-functionalized Schiff base 4, which generates another S(6) ring motif. The C-H···F hydrogen bond further stabilizes its structure and leads it to form a planar configuration. Compounds 1-3 exhibit solely a long-wavelength proton-transfer tautomer emission, while dipole-functionalized Schiff base 4 shows remarkable dual emission originated from the excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) states. Furthermore, the geometric structures, frontier molecular orbitals (MOs) and the potential energy curves for 1-4 in the ground and the first singlet excited state were fully rationalized by density functional theory (DFT) and time-dependent DFT calculations.

Oxidation of methionine residues in polypeptide ions via gas-phase ion/ion chemistry

The gas-phase oxidation of methionine residues is demonstrated here using ion/ion reactions with periodate anions. Periodate anions are observed to attach in varying degrees to all polypeptide ions irrespective of amino acid composition. Direct proton transfer yielding a charge-reduced peptide ion is also observed. In the case of methionine and, to a much lesser degree, tryptophan-containing peptide ions, collisional activation of the complex ion generated by periodate attachment yields an oxidized peptide product (i.e., [M + H + O](+)), in addition to periodic acid detachment. Detachment of periodic acid takes place exclusively for peptides that do not contain either a methionine or tryptophan side chain. In the case of methionine-containing peptides, the [M + H + O](+) product is observed at a much greater abundance than the proton transfer product (viz., [M + H](+)). Collisional activation of oxidized Met-containing peptides yields a signature loss of 64 Da from the precursor and/or product ions. This unique loss corresponds to the ejection of methanesulfenic acid from the oxidized methionine side chain and is commonly used in solution-phase proteomics studies to determine the presence of oxidized methionine residues. The present work shows that periodate anions can be used to 'label' methionine residues in polypeptides in the gas phase. The selectivity of the periodate anion for the methionine side chain suggests several applications including identification and location of methionine residues in sequencing applications.

A new splitting method for both analytical and preparative LC/MS

This paper presents a novel splitting method for liquid chromatography/mass spectrometry (LC/MS) application, which allows fast MS detection of LC-separated analytes and subsequent online analyte collection. In this approach, a PEEK capillary tube with a micro-orifice drilled on the tube side wall is used to connect with LC column. A small portion of LC eluent emerging from the orifice can be directly ionized by desorption electrospray ionization (DESI) with negligible time delay (6~10 ms) while the remaining analytes exiting the tube outlet can be collected. The DESI-MS analysis of eluted compounds shows narrow peaks and high sensitivity because of the extremely small dead volume of the orifice used for LC eluent splitting (as low as 4 nL) and the freedom to choose favorable DESI spray solvent. In addition, online derivatization using reactive DESI is possible for supercharging proteins and for enhancing their signals without introducing extra dead volume. Unlike UV detector used in traditional preparative LC experiments, this method is applicable to compounds without chromophores (e.g., saccharides) due to the use of MS detector. Furthermore, this splitting method well suits monolithic column-based ultra-fast LC separation at a high elution flow rate of 4 mL/min. Figure ᅟ

Gas Phase Dissociation Behavior of Acyl-Arginine Peptides

The gas phase dissociation behavior of peptides containing acyl-arginine residues is investigated. These acylations are generated via a combination of ion/ion reactions between arginine-containing peptides and N-hydroxysuccinimide (NHS) esters and subsequent tandem mass spectrometry (MS/MS). Three main dissociation pathways of acylated arginine, labeled Paths 1-3, have been identified and are dependent on the acyl groups. Path 1 involves the acyl-arginine undergoing deguanidination, resulting in the loss of the acyl group and dissociation of the guanidine to generate an ornithine residue. This pathway generates selective cleavage sites based on the recently discussed "ornithine effect". Path 2 involves the coordinated losses of H₂O and NH₃ from the acyl-arginine side chain while maintaining the acylation. We propose that Path 2 is initiated via cyclization of the δ-nitrogen of arginine and the C-terminal carbonyl carbon, resulting in rapid rearrangement from the acyl-arginine side chain and the neutral losses. Path 3 occurs when the acyl group contains α-hydrogens and is observed as a rearrangement to regenerate unmodified arginine while the acylation is lost as a ketene.

Strategies for the Gas Phase Modification of Cationized Arginine via Ion/ion Reactions

The gas phase acetylation of cationized arginine residues is demonstrated here using ion/ion reactions with sulfosuccinimidyl acetate (sulfo-NHS acetate) anions. Previous reports have demonstrated the gas phase modification of uncharged primary amine (the N-terminus and ε-amino side chain of lysine) and uncharged guanidine (the arginine side chain) functionalities via sulfo-NHS ester chemistry. Herein, charge-saturated arginine-containing peptides that contain sodium ions as the charge carriers, such as [ac-ARAAARA+2Na]²⁺, are shown to exhibit strong reactivity towards sulfo-NHS acetate whereas the protonated peptide analogues exhibit no such reactivity. This difference in reactivity is attributed to the lower sodium ion (as compared to proton) affinity of the arginine, which results in increased nucleophilicity of the cationized arginine guanidinium functionality. This increased nucleophilicity improves the arginine residue's reactivity towards sulfo-NHS esters and enhances the gas phase covalent modification pathway. No such dramatic increase in reactivity towards sulfo-NHS acetate has been observed upon sodium cationization of lysine amino acid residues, indicating that this behavior appears to be unique to arginine. The sodium cationization process is demonstrated in the condensed phase by simply spiking sodium chloride into the peptide sample solution and in the gas phase by a peptide-sodium cation exchange process with a sulfo-NHS acetate sodium-bound dimer cluster reagent. This methodology demonstrates several ways by which arginine can be covalently modified in the gas phase even when it is charged. Collisional activation of an acetylated arginine product can result in deguanidination of the residue, generating an ornithine. This gas phase ornithination exhibits similar site-specific fragmentation behavior to that observed with peptides ornithinated in solution and may represent a useful approach for inducing selective peptide cleavages.

Reagent cluster anions for multiple gas-phase covalent modifications of peptide and protein cations

Multiple gas phase ion/ion covalent modifications of peptide and protein ions are demonstrated using cluster-type reagent anions of N-hydroxysulfosuccinimide acetate (sulfo-NHS acetate) and 2-formyl-benzenesulfonic acid (FBMSA). These reagents are used to selectively modify unprotonated primary amine functionalities of peptides and proteins. Multiple reactive reagent molecules can be present in a single cluster ion, which allows for multiple covalent modifications to be achieved in a single ion/ion encounter and at the 'cost' of only a single analyte charge. Multiple derivatizations are demonstrated when the number of available reactive sites on the analyte cation exceeds the number of reagent molecules in the anionic cluster (e.g., data shown here for reactions between the polypeptide [K10 + 3H](3+) and the reagent cluster [5R(5Na) - Na](-)). This type of gas-phase ion chemistry is also applicable to whole protein ions. Here, ubiquitin was successfully modified using an FBMSA cluster anion which, upon collisional activation, produced fragment ions with various numbers of modifications. Data for the pentamer cluster are included as illustrative of the results obtained for the clusters comprised of two to six reagent molecules.

Gas-phase transformation of phosphatidylcholine cations to structurally informative anions via ion/ion chemistry

Gas-phase transformation of synthetic phosphatidylcholine (PC) monocations to structurally informative anions is demonstrated via ion/ion reactions with doubly deprotonated 1,4-phenylenedipropionic acid (PDPA). Two synthetic PC isomers, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC(16:0/18:1)) and 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (PC(18:1/16:0)), were subjected to this ion/ion chemistry. The product of the ion/ion reaction is a negatively charged complex, [PC + PDPA - H](-). Collisional activation of the long-lived complex causes transfer of a proton and methyl cation to PDPA, generating [PC - CH3](-). Subsequent collisional activation of the demethylated PC anions produces abundant fatty acid carboxylate anions and low-abundance acyl neutral losses as free acids and ketenes. Product ion spectra of [PC - CH3](-) suggest favorable cleavage at the sn-2 position over the sn-1 due to distinct differences in the relative abundances. In contrast, collisional activation of PC cations is absent of abundant fatty acid chain-related product ions and typically indicates only the lipid class via formation of the phosphocholine cation. A solution phase method to produce the gas-phase adducted PC anion is also demonstrated. Product ion spectra derived from the solution phase method are similar to the results generated via ion/ion chemistry. This work demonstrates a gas-phase means to increase structural characterization of phosphatidylcholines via ion/ion chemistry.