Mass spectrometric and theoretical studies on dissociation of the CS bond in the benzenesulfonic acid and benzenesulfinic acid anion series: Homolytic cleavage vs heterolytic cleavage

Real-Time Mass Spectrometric Detection of Reaction Intermediates Formed during Laser-Induced UV/H2O2 Advanced Oxidation of 2-Methylbenzoisothiazol-3-one

Knowledge on short-lived reaction intermediates is often essential for mechanistic investigations of organic reactions and for reaction optimization. Unfortunately, most conventional analytical methods are too slow to allow the detection of short-lived reaction intermediates. Herein, a direct laser desorption/ionization method coupled with linear quadrupole ion trap/orbitrap high-resolution tandem mass spectrometry was used for the detection and structural characterization of several previously proposed but undetected reaction intermediates formed during laser-induced UV/H2O2 advanced oxidation of 2-methylbenzoisothiazol-3-one. The elemental compositions of most detected (ionized) compounds were determined. Tandem mass spectrometry experiments based on gas-phase collision-activated dissociation (CAD) were conducted to gain information on the ion structures. The mechanisms of the CAD reactions were explored using high-level quantum chemical calculations to support the structures proposed for the neutral reaction intermediates formed during the laser-induced UV/H2O2 advanced oxidation of 2-methylbenzoisothiazol-3-one. In the negative-ion mode experiments, anions corresponding to three reaction intermediates were detected and structurally characterized: 1-hydroxy-2-methyl-1,2-dihydro-3H-1λ4-benzo[d]isothiazol-3-one, 2-(methylcarbamoyl)benzenesulfinic acid, and 2-(dihydroxy(oxo)-λ6-sulfaneyl)-N-methylbenzamide. One of the final products, 2-(methylcarbamoyl)benzenesulfonic acid, was also detected and characterized. In positive-ion mode experiments, cations corresponding to the reactant, 2-methylbenzoisothiazol-3-one, as well as an intermediate reaction product and the two final reaction products, 2-methylbenzo[d]isothiazol-3(2H)-one 1-oxide, N-methylsaccharine, and 2-(methylcarbamoyl)benzenesulfonic acid, respectively, were detected and identified. This research substantially improved the understanding on the reaction intermediates formed during laser-induced UV/H2O2 advanced oxidation of 2-methylbenzoisothiazol-3-one, which facilitates the delineation of the reaction mechanisms occurring in these processes.

Linking Electronic Relaxation Dynamics and Ionic Photofragmentation Patterns for the Deprotonated UV Filter Benzophenone-4

Understanding how deprotonation impacts the photophysics of UV filters is critical to better characterize how they behave in key alkaline environments including surface waters and coral reefs. Using anion photodissociation spectroscopy, we have measured the intrinsic absorption electronic spectroscopy (400-214 nm) and numerous accompanying ionic photofragmentation pathways of the benzophenone-4 anion ([BP4-H]^-). Relative ion yield plots reveal the locations of the bright S₁ and S₃ excited states. For the first time for an ionic UV filter, ab initio potential energy surfaces are presented to provide new insight into how the photofragment identity maps the relaxation pathways. These calculations reveal that [BP4-H]^- undergoes excited-state decay consistent with a statistical fragmentation process where the anion breaks down on the ground state after nonradiative relaxation. The broader relevance of the results in providing a basis for interpreting the relaxation dynamics of a wide range of gas-phase ionic systems is discussed.

Structural Characterization of Acidic Compounds in Pyrolysis Liquids Using Collision-Induced Dissociation and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

In this study, a novel approach to characterize and identify acidic oil compounds utilizing the fragmentational behavior of their corresponding precursor ions is presented. Precursor ions of seven analyzed pyrolysis oils that were generated from pyrolysis educts of different origins and degrees of coalification were produced by electrospray ionization in the negative ion mode (ESI(-)). Following a fragmentation of all ions in the ion cloud by collision-induced dissociation (CID), the precursor and product ions were subsequently detected by ultrahigh resolving Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The ESI(-)-CID data sets were evaluated by applying either a targeted classification or untargeted clustering approach. In the case of the targeted classification, 10% of the ionized precursor ions of the analyzed pyrolysis liquid samples could be classified into one of 11 compound classes utilizing theoretical fragmentation pathways of these classes. In contrast, theoretical fragmentation pathways were not necessary for the untargeted clustering approach, making it the more transmittable method. Results from both approaches were verified by analyzing standard compounds of known structure. The analysis and data evaluation methods presented in this work can be used to characterize complex organic mixtures, such as pyrolysis oils, and their compounds in-depth on a structural level.

Electrochemical versus Enzymatic in Vitro Oxidations of 6-propyl-2-thiouracil: Identification, Detection, and Characterization of Metabolites

6-Propylthiouracil, PTU, is a well-known antithyroid drug that has been the mainstay of treatment of Graves' disease. It is, however, also associated with liver toxicity and idiosyncratic toxicity. These toxicities are generally associated with metabolites derived from its bioactivation. In this manuscript, bioactivation of PTU was studied via two separate techniques: electrochemical oxidation and through the use of human liver microsomes. The aim of this work was to compare the bioactivation products of these two techniques. The electrochemical technique was studied online with a mass spectrometer, EC/ESI/MS. The microsomal oxidations were studied in tandem with liquid chromatography. The EC/ESI/MS technique was devoid of the normal reducing biological matrix prevalent in microsomal incubations. The predominant product at 400 mV was the dimeric PTU species with negligible formation of other metabolites. At higher potentials, complete desulfurization of PTU was observed with formation of sulfate. No sulfonic acid was observed, suggesting that the cleavage of the C-S bond was effected at the sulfinic acid stage, releasing a highly reducing sulfur species which is known to give rise to genotoxicity. The microsomal oxidations, surprisingly, showed formation of the unstable sulfenic acid, the S-oxide. Further incubation showed both the sulfinic and sulfonic acids. None of the systems showed any adducts with nucleophiles such as glutathione, showing that none of the reactive metabolites were stable enough to be adducted to nucleophiles in both the biological matrix and the electrochemical oxidizing environment.

Characterization of multiple fragmentation pathways initiated by collision-induced dissociation of multifunctional anions formed by deprotonation of 2-nitrobenzenesulfonylglycine

The correlation of anion structure with the fragmentation behavior of deprotonated nitrobenzenesulfonylamino acids was investigated using tandem mass spectrometry, isotopic labeling and computational methods. Four distinct fragmentation pathways resulting from the collision-induced dissociation (CID) of deprotonated 2-nitrobenzenesulfonylglycine (NsGly) were characterized. The unusual loss of the aryl nitro substituent as HONO was the lowest energy process. Subsequent successive losses of CO, HCN and SO2 indicated that an ortho cyclization reaction had accompanied loss of HONO. Other pathways involving rearrangement of the ionized sulfonamide group, dual bond cleavage and intramolecular nucleophilic displacement were proposed to account for the formation of phenoxide, arylsulfinate and arylsulfonamide product ions at higher collision energies. The four distinct fragmentation pathways were consistent with precursor-product relationships established by CID experiments, isotopic labeling results and the formation of analogous product ions from 2,4-dinitrobenzenesulfonylglycine and the Ns derivatives of alanine and 2-aminoisobutyric acid. The computations confirmed a low barrier for ortho cyclization with loss of HONO and feasible energetics for each reaction step in the four pathways. Computations also indicated that three of the fragmentation pathways started from NsGly ionized at the carboxyl group. Overall, the pathways identified for the fragmentation of the NsGly anion differed from processes reported for anions containing a single functional group, demonstrating the importance of functional group interactions in the fragmentation pathways of multifunctional anions.

Fragmentation of typical sulfonamide drugs via heterolytic bond cleavage and stepwise rearrangement

Although many experiments have been carried out to elucidate the fragmentation of typical sulfonamide drugs, little effort has been devoted to understanding the reaction process theoretically.