Structural Characterization of Methylenedianiline Regioisomers by Ion Mobility-Mass Spectrometry, Tandem Mass Spectrometry, and Computational Strategies. 3. MALDI Spectra of 2-Ring Isomers

Differentiation of regioisomers of sulfobenzoic acid by traveling-wave ion mobility mass spectrometry

An ion mobility mass spectrometry (IM-MS) investigation using a Synapt G2 mass spectrometer was conducted to separate anions generated from the three regioisomers of sulfobenzoic acid. The results revealed that the differences in arrival time distributions (ATDs) were inadequate to differentiate the isomers unambiguously. However, the ATD profiles of the product ions, generated by fragmenting the respective mass-selected m/z 201 precursor ions in the Trap region of the three-compartment traveling-wave ion guide of the Synapt G2 mass spectrometer, were distinctly different, enabling definitive differentiation of the isomers. An arrival-time peak for an ion of m/z 157 resulting from the loss of CO2 from the respective precursors was common to all three mobilograms. However, only the profile recorded from the para-isomer exhibited a unique arrival-time peak for an ion of m/z 137, originating from an SO2 loss. Such a peak corresponding to an SO2 loss was absent in the ATD profiles of the ortho- and meta-isomers. Additionally, the mobilogram of the meta-isomer displayed a unique peak at 3.42 ms. Based on its product ion spectrum, this peak was attributed to the bisulfite anion (m/z 81; HSO3-). Previously, this meta-isomer specific m/z 81 ion had been proposed to originate from a two-step process involving the intermediacy of an m/z 157 ion formed by CO2 loss. However, our detailed tandem mass spectrometric experiments suggest that the m/z 81 is not a secondary product but rather an ion that originated from a direct elimination of a benzyne derivative from the m/z 201 precursor ion.

Accurate Identification of Bioactive Meliaceae Limonoids by UHPLC-MS/MS Based Structure-Fragment Relationships (SFRs)

Limonoids are bioactive plant specialized metabolites found in the Meliaceae family. The basic limonoids, i.e., azadiradione, epoxyazadiradione, and gedunin have been exploited for various bioactivities and therefore are the potential drug leads for tomorrow. However, their low abundance, structural similarity, and lack of adequate mass fragmentation data have hampered their accurate identification and quantification from various sources. In the present study, basic limonoids such as azadirone, azadiradione, epoxyazadiradione, and gedunin isolated from Neem were utilized for the synthesis of their derivatives and isotopologs. A total of 30 one compounds were used in this study among which five were isolated, two were biotransformed, and 24 were synthesized. Among the synthesized compounds nine are novel compounds including six deuterated analogs/isotopologs which are (1,3-²H)-1,2-dihydro-3β-hydroxyazadiradione (9), (1,3,16-²H)-1,2-dihydro-3β-16β-dihydroxyazadiradione (10), 3β-hydroxyazadiradione (11), 3β-16β-dihydroxyazadiradione (12), (3-²H)-3β-hydroxyazadiradione (13), (3,16-²H)-3β-16β-dihydroxyazadiradione (14), (1,3,7-²H)-1,2-dihydro-3β-hydroxy-7-deacetylazadiradione (15), 1,2,20,21,22,23-hexahydroazadiradione (17), and (1,3-²H)-1,2-dihydro-3β-hydroxygedunin (29). These limonoids along with their semisynthesized derivatives were subjected to ultra high performance liquid chromatography mass spectrometry (UHPLC-MS/MS) and the fragmentation pathway was established based on structure-fragment relationships (SFRs), utilizing high resolution MS/MS data. We have developed a most reliable and easily reproducible protocol describing in depth analysis of SFRs based on the structural modifications and synthesis of isotopologs. Also, the MS/MS fragment library of these basic limonoids generated in this study acts as a fingerprint for accurate identification and quantification of limonoids by MS/MS analysis in various plant tissue extracts, phytopharmaceutical formulations and biological samples.

Graph theory-based reaction pathway searches and DFT calculations for the mechanism studies of free radical-initiated peptide sequencing mass spectrometry (FRIPS MS): a model gas-phase reaction of GGR tri-peptide

Graph theory-based reaction pathway searches (ACE-Reaction program) and density functional theory calculations were performed to shed light on the mechanisms for the production of [a_n + H]⁺, x_n⁺, y_n⁺, z_n⁺, and [y_n + 2H]⁺ fragments formed in free radical-initiated peptide sequencing (FRIPS) mass spectrometry measurements of a small model system of glycine-glycine-arginine (GGR). In particular, the graph theory-based searches, which are rarely applied to gas-phase reaction studies, allowed us to investigate reaction mechanisms in an exhaustive manner without resorting to chemical intuition. As expected, radical-driven reaction pathways were favorable over charge-driven reaction pathways in terms of kinetics and thermodynamics. Charge- and radical-driven pathways for the formation of [y_n + 2H]⁺ fragments were carefully compared, and it was revealed that the [y_n + 2H]⁺ fragments observed in our FRIPS MS spectra originated from the radical-driven pathway, which is in contrast to the general expectation. The acquired understanding of the FRIPS fragmentation mechanism is expected to aid in the interpretation of FRIPS MS spectra. It should be emphasized that graph theory-based searches are powerful and effective methods for studying reaction mechanisms, including gas-phase reactions in mass spectrometry.

Mass Spectrometry of Polyurethanes

This review covers the applications of mass spectrometry (MS) and its hyphenated techniques to characterize polyurethane (PU) synthetic polymers and their respective hard and soft segments. PUs are commonly composed of hard segments including methylene bisphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), and soft segments including polyester and polyether polyols. This literature review highlights MS techniques such as electrospray ionization (ESI), matrix assisted laser/desorption ionization (MALDI), ion mobility-mass spectrometry (IM-MS), and computational methods that have been used for the characterization of this polymer system. Here we review specific case studies where MS techniques have elucidated unique features pertaining to the makeup and structural integrity of complex PU materials and PU precursors.

Structural Characterization of Methylenedianiline Regioisomers by Ion Mobility-Mass Spectrometry and Tandem Mass Spectrometry. 4. 3-Ring and 4-Ring Isomers

Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) is used to characterize methylenedianiline (MDA) 3-ring and 4-ring species. Building on our previous MALDI-MS 2-ring MDA isomer study, here we compare 3-ring and 4-ring electrospray ionization (ESI) and MALDI results. In ESI, 3-ring and 4-ring MDAs each form a single [M + H]⁺ parent ion. However, in MALDI, each MDA multimer forms three unique precursor ions: [M + H]⁺, [M^•]⁺, and [M - H]⁺. In this study, 3-ring and 4-ring MDA precursors are characterized to identify the unique fragment ions formed and their respective fragmentation pathways. In addition to the three possible precursors, the 3-ring and 4-ring species are higher-order oligomer precursors in polyurethane (PUR) production and thus provide additional insight into the polymeric behavior of these PUR hard block precursors. The combination of ion mobility-mass spectrometry (IM - MS) and tandem mass spectrometry (MS/MS) allow the structural characterization of these larger MDA multimers.