The effect of cation size (H+, Li+, Na+, and K+) on McLafferty-type rearrangement of even-electron ions in mass spectrometry

Lithiophilic Dibenzamide Linkages to Impart Lithium Storage Capacity in Porous Polybenzamides

Polymer-based organic cathode materials have shown immense promise for lithium storage, owing to their structural diversity and functional group tunability. However, designing appropriate high-performance cathode materials with a high-rate capability and long cycle life remains a significant challenge. It is quintessential to design polymer-based electrodes with lithiophilic linkages. Herein, we design a bifurcated dibenzamide (DBA) linkage having lithiophilic functionalities. 1H NMR has been used as an experimental tool to understand the lithiophilic nature of the DBAs. Considering the strong Li+ affinity of DBAs, a series of polybenzamides have been designed as lithium storage systems. The design of porous polybenzamides consists of amides as only redox-active functionalities, and the rest are inactive phenyl units. Porous polybenzamides, when tested as cathodes against a Li-metal anode, displayed high capacity and rate performance, demonstrating their redox activity. The most efficient polybenzamide (TAm-TA) delivered a specific capacity of 248 mA h g-1 at 1C. TAm-TA retained 63% of its specific capacity at a very high rate of 10C (157 mA h g-1). Notably, polybenzamides displayed a capacity enhancement during long cycling, tending to achieve their theoretical capacity. Long cycling stability tests over 3000 cycles at a rate of 1.3C and over 6000 cycles at elevated rates (5C to 40C) demonstrate the electrochemical robustness of dibenzamide linkages. Finally, two full-cell experiments using TAm-TA as both cathode and anode were conducted, which delivered high capacity, demonstrating that TAm-TA is a promising candidate for Li+-ion batteries (LIBs). Furthermore, the ex situ Fourier transform infrared (FT-IR), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) studies revealed the stepwise lithiation/delithiation mechanism for polybenzamides.

Competitive McLafferty-type rearrangements of sodium adduct of anti-2,3-dihydroxy-1-phenylpentane-1,4-dione compounds in tandem mass spectrometry

Sodium adducts of anti-2,3-dihydroxy-1-phenylpentane-1,4-dione compounds with different substituents were studied by collision-induced dissociation. McLafferty-type rearrangements preceding fragmentation were found as their main fragmentation pathway. Coordination of sodium cation to the oxygen functions may either lead to formation of a five-membered or a six-membered ring. Two McLafferty-type rearrangement product ions exhibiting a mass difference of 2 u indicated that two competitive McLafferty-type rearrangements through a six-membered ring coordination occurred. Relative abundances of the corresponding product ions were studied by energy-resolved collision-induced dissociation experiments and density functional theory calculations. Furthermore, the influence of different substituents was probed.

Influence of Linkage Stereochemistry and Protecting Groups on Glycosidic Bond Stability of Sodium Cationized Glycosyl Phosphates

Energy-resolved collision-induced dissociation (ER-CID) experiments of sodium cationized glycosyl phosphate complexes, [GP _x +Na]⁺, are performed to elucidate the effects of linkage stereochemistry (α versus β), the geometry of the leaving groups (1,2-cis versus 1,2-trans), and protecting groups (cyclic versus non-cyclic) on the stability of the glycosyl phosphate linkage via survival yield analyses. A four parameter logistic dynamic fitting model is used to determine CID_50% values, which correspond to the level of rf excitation required to produce 50% dissociation of the precursor ion complexes. Present results suggest that dissociation of 1,2-trans [GP _x +Na]⁺ occurs via a McLafferty-type rearrangement that is facilitated by a syn orientation of the leaving groups, whereas dissociation of 1,2-cis [GP_x+Na]⁺ is more energetic as it involves the formation of an oxocarbenium ion intermediate. Thus, the C1-C2 configuration plays a major role in determining the stability/reactivity of glycosyl phosphate stereoisomers. For 1,2-cis anomers, the cyclic protecting groups at the C4 and C6 positions stabilize the glycosidic bond, whereas for 1,2-trans anomers, the cyclic protecting groups at the C4 and C6 positions tend to activate the glycosidic bond. The C3 O-benzyl (3 BnO) substituent is key to determining whether the sugar or phosphate moiety retains the sodium cation upon CID. For 1,2-cis anomers, the 3 BnO substituent weakens the glycosidic bond, whereas for 1,2-trans anomers, the 3 BnO substituent stabilizes the glycosidic bond. The C2 O-benzyl substituent does not significantly impact the glycosidic bond stability regardless of its orientation. Graphical abstract ᅟ.

Neutral losses of sodium benzoate and benzoic acid in the fragmentation of the [M + Na]+ ions of methoxyfenozide and tebufenozide via intramolecular rearrangement in electrospray ionization tandem mass spectrometry

RATIONALE

Electrospray ionization (ESI) tandem mass spectrometry can be applied to determine structural information about organic compounds. The [M + Na]⁺ ion is one of the major precursor ions in ESI mass spectrometry, but its fragmentation mechanism study is still insufficient. This study reveals the interesting fragmentation reactions of the [M + Na]⁺ ions of methoxyfenozide and tebufenozide.

METHODS

The fragmentations of the [M + Na]⁺ , [M + Li]⁺ , and [M + H]⁺ ions of methoxyfenozide and tebufenozide were studied using a hybrid quadrupole-orbitrap mass spectrometer and an ion trap mass spectrometer. A hydrogen/deuterium (H/D)-exchange experiment in the amide group of methoxyfenozide allowed for the confirmation of the fragmentation mechanism. Density functional theory (DFT) calculations were performed for a further understanding of the fragmentation mechanism of the [M + Na]⁺ ion of methoxyfenozide.

RESULTS

Neutral losses of sodium benzoate and benzoic acid in the fragmentation of the [M + Na]⁺ ions of methoxyfenozide and tebufenozide were observed as the major fragmentation pathways. In contrast, similar fragmentations were not observed or minor pathways in the fragmentation of the [M + Li]⁺ and [M + H]⁺ ions of methoxyfenozide and tebufenozide. In addition, a minor product ion resulting from loss of NaOH was identified, which was the first reported example in the fragmentation of sodiated compounds in mass spectrometry.

CONCLUSIONS

Losses of sodium benzoate and benzoic acid in the fragmentation of the [M + Na]⁺ ions of methoxyfenozide and tebufenozide are proposed to be formed through an intramolecular rearrangement reaction, which is supported by DFT calculations. An H/D-exchange experiment confirms that the carboxyl hydrogen of benzoic acid and the hydrogen of NaOH exclusively derive from the amide hydrogen of the precursor ion. This study enriches our knowledge on the Na⁺ -induced fragmentation reactions. Copyright © 2016 John Wiley & Sons, Ltd.

Investigation of c ions formed by N-terminally charged peptides upon collision-induced dissociation

Peptide fragments such as b and y sequence ions generated upon low-energy collision-induced dissociation have been routinely used for tandem mass spectrometry (MS/MS)-based peptide/protein identification. The underlying formation mechanisms have been studied extensively and described within the literature. As a result, the 'mobile proton model' and 'pathways in competition model' have been built to interpret a majority of peptide fragmentation behavior. However, unusual peptide fragments which involve unfamiliar fragmentation pathways or various rearrangement reactions occasionally appear in MS/MS spectra, resulting in confused MS/MS interpretations. In this work, a series of unfamiliar c ions are detected in MS/MS spectra of the model peptides having an N-terminal Arg or deuterohemin group upon low-energy collision-induced dissociation process. Both the protonated Arg and deuterohemin group play an important role in retention of a positive charge at the N-terminus that is remote from the cleavage sites. According to previous reports and our studies involving amino acid substitutions and hydrogen-deuterium exchange, we propose a McLafferty-type rearrangement via charge-remote fragmentation as the potential mechanism to explain the formation of c ions from precursor peptide ions or unconventional b ions. Density functional theory calculations are also employed in order to elucidate the proposed fragmentation mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.

Sensitive and fast beverage/fruit antioxidant evaluation by TiO2 -Au/graphene nanocomposites coupled with MALDI-MS

RATIONALE

Because the ratio of tripeptide glutathione (GSH) to glutathione disulphide (GSSG) is tightly associated with the oxidative stress and antioxidant level of an organism, it is often considered to be an indicator of the redox states of cells. Therefore, developing an integrated protocol for rapid, efficient, and low-cost detection of GSH to measure antioxidant ability is of significant importance for the diagnosis of oxidative stress-associated diseases.

METHODS

TiO2 -Au/graphene nanocomposites were synthesized to integrate the characteristics of graphene as matrix for MALDI-MS, gold NPs as selective probes for GSH and the photocatalytic property of TiO2 . Under UV-visible (UV-Vis) light irradiation, OH can be produced by the TiO2 -Au/G composites, which can oxidize GSH to form GSSG. When various antioxidants are introduced into the aforementioned system, OH can be scavenged, thereby leaving part of GSH in its reductive format. Based on the ratio of GSH/GSSG, the antioxidative capability of various beverages and fruits can be determined.

RESULTS

TiO2 -Au/G composites were employed to enrich GSH, where 0.01 mg/mL GSH can be efficiently extracted by the nanocomposites from aqueous solution and detected by MS with high signal-to-noise ratio. The proposed strategy was applied to evaluate three often-used antioxidants: Vitamin C, Vitamin E, and β-carotene; and demonstrated that the antioxidative ability of VC is the strongest. To further evaluate the feasibility of the proposed strategy for antioxidative ability evaluation of complex sample, commercial juices and fresh fruit were also studied.

CONCLUSIONS

A novel strategy for sensitive and fast characterization of the antioxidative ability of various beverages and fruits was developed based on TiO2 -Au/G nanocomposites coupled with MALDI-MS. Copyright © 2016 John Wiley & Sons, Ltd.

Structural Characterization of Anticancer Drug Paclitaxel and Its Metabolites Using Ion Mobility Mass Spectrometry and Tandem Mass Spectrometry

Paclitaxel (PTX) is a popular anticancer drug used in the treatment of various types of cancers. PTX is metabolized in the human liver by cytochrome P450 to two structural isomers, 3′-p-hydroxypaclitaxel (3p-OHP) and 6α-hydroxypaclitaxel (6α-OHP). Analyzing PTX and its two metabolites, 3p-OHP and 6α-OHP, is crucial for understanding general pharmacokinetics, drug activity, and drug resistance. In this study, electrospray ionization ion mobility mass spectrometry (ESI-IM-MS) and collision induced dissociation (CID) are utilized for the identification and characterization of PTX and its metabolites. Ion mobility distributions of 3p-OHP and 6α-OHP indicate that hydroxylation of PTX at different sites yields distinct gas phase structures. Addition of monovalent alkali metal and silver metal cations enhances the distinct dissociation patterns of these structural isomers. The differences observed in the CID patterns of metalated PTX and its two metabolites are investigated further by evaluating their gas-phase structures. Density functional theory calculations suggest that the observed structural changes and dissociation pathways are the result of the interactions between the metal cation and the hydroxyl substituents in PTX metabolites.

Characteristic neutral loss of CH3CHO from Thr-containing sodium-associated peptides

A characteristic neutral loss of 44 Da is observed in the MS/MS spectra of Thr-containing sodiated peptides. A combination of tandem mass spectrometry and quantum chemical calculations calculated at the B3LYP/6-311G (d, p) level of ab initio theory is used to elucidate this fragmentation pathway. The high resolution mass spectrometry data indicate this neutral loss is acetaldehyde lost from the side chain of Thr rather than CO2. The intensity of this neutral loss can be enhanced when Thr residue is far from the C-terminus and when the C-terminus is esterified as well. The mechanism of the acetaldehyde loss is proposed to adopt a McLafferty-type rearrangement reaction, which involves a proton transfer from the hydroxyl of Thr side chain to its C-terminal neighboring carbonyl oxygen inducing the cleavage of the Ca-Cβ bond. This mechanism is further supported by examining the fragmentation of a [GT(tBu)G + Na](+) peptide derivative and by comparing the product ion spectra of [M + Na-44](+) of [GTGA + Na](+) with [M + Na](+) of [GGGA + Na](+). A similar neutral loss of HCHO can also be detected in Ser-containing peptides. Our computational results reveal that the most stable [GTG + Na](+) ion is present as a tridentate charge-solvated structure and the dissociation leading to the 44 loss is dynamically and energetically favorable.