Rapid differentiation of ortho-, meta-, and para-isomers of halogenated phenylmethylidene hydrazinecarbodithioates by metal complexation and electrospray ionization mass spectrometry

Characteristics of ionization behaviors of aminonitrotoluene isomers produced by atmospheric pressure chemical ionization

RATIONALE

Six of the isomers of aminonitrotoluene (ANT) are 2-amino-3-nitrotoluene (2A3NT), 2A4NT, 2A5NT, 2A6NT, 4A2NT, and 4A3NT. Some of them can be identified by chromatography and spectroscopy. Biochemical transformation of 2,4,6-trinitrotoluene (TNT) and dinitrotoluenes (DNTs) is very complex and ANTs are decomposition products of TNT and DNTs.

METHODS

Each isomer in acetone was ionized using atmospheric pressure chemical ionization in positive and negative ion modes, and kinds and abundances of the product ions were analyzed. Energy-minimized structures of the product ions and their energies were calculated to explain the analysis results.

RESULTS

The [M + H]⁺ , [M + H + Acetone - H₂ O]⁺ , and [M + H + Acetone]⁺ ions as positive product ions were detected, while [M - H]^- , M^•- , and [M + O₂ ]^•- ions as negative ones were observed. The order of the ionization efficiencies for the positive product ions was 4A3NT > 4A2NT > 2A4NT > 2A6NT > 2A5NT > 2A3NT, while that of the negative ones was 2A5NT > 2A3NT > 4A3NT > 2A4NT > 2A6NT > 4A2NT. Ion abundance ratios for 2A3NT and 2A5NT showed very similar trends, while those of 2A6NT and 4A2NT also showed similar trends. Differences in the ionization behaviors were explained using the heats of reaction.

CONCLUSIONS

The product ions were produced by ion-molecule reactions with the reactant ions of [2Acetone + H]⁺ and [Acetone + O₂ ]^•- . The [M + H + Acetone]⁺ ion was fragmented to produce [M + H]⁺ and [M + H + Acetone - H₂ O]⁺ , while the [M + O₂ ]^•- ion was fragmented to generate the [M - H]^- and M^•- ions. Differences in the ionization behaviors of the ANTs can be used for their differentiation.

Mass spectrometric-based investigation of differentially protected azatryptophan derivatives using Orbitrap mass spectrometry: Differentiation of positional isomers under protonation and alkali-cationization conditions

RATIONALE

Differentiation and structural characterization of positional isomers of differentially protected azatryptophan derivatives using electrospray ionization high-resolution tandem mass spectrometry (ESI-HRMS/MS) is important from the perspective of drug discovery research. Also, these derivatives can be used as building blocks for the synthesis of various biologically active compounds and have attracted significant attention in the field of modern drug discovery, especially peptide-based drugs, protein folding and protein-protein interactions because of their interesting spectral properties.

METHODS

ESI-HRMS/MS in positive ionization mode was used to differentiate and characterize positional isomers of protected azatryptophan derivatives.

RESULTS

ESI-HRMS/MS of [M + H]⁺ and [M + Na]⁺ ions of positional isomers of differentially protected azatryptophan derivatives display distinct fragmentation patterns. The MS/MS of [M + H]⁺ ion of isomer 1 showed an additional ion at m/z 358.0846 ([M + H-Boc-C₁₄ H₁₀ -HF]⁺ ) which was not present for 4. The fragment ion at m/z 332.0857 was observed for 1 and not for 4 which would be formed by the expulsion of butyloxycarbonyl (Boc) and fluorenylmethyloxycarbonyl (Fmoc) groups. Moreover, the ions 422.0812 and 378.0912 are found to be relatively more abundant for isomer 4 which could be probably attributed to the formation of stable ions. Similarly, other positional isomers exhibited distinct fragmentation from one another.

CONCLUSIONS

The present study demonstrates that ESI-HRMS/MS can be used for differentiation and structural characterization of positional isomers of protected azatryptophan derivatives. The MS/MS of [M + H]⁺ and [M + Na]⁺ ions of these positional isomers displayed differences in their fragmentation behaviour. The impact of different substitutions at different positions (1 and 6) of protected azatryptophan derivatives (1-6) on their fragmentation behaviour was also investigated in detail. Also, the nitrogen atom at different positions in the pyrrolopyridine ring led to different fragmentation patterns.

Synthesis and characterization of a series of N,N'-substituted urea derivatives by using electrospray ionization tandem mass spectrometry: Differentiation of positional isomers

RATIONALE

Characterization of N,N'-substituted ureas was found to be challenging by nuclear magnetic resonance (NMR) spectroscopy, particularly N-di- and tri-alkylated ureas because of the absence of adjacent protons. In the present study, electrospray ionization tandem mass spectrometry has been used to differentiate positional isomeric pairs and to characterize a series of N,N'-substituted ureas, as these compounds have significant importance for drug discovery. Additionally, urea is an essential functionality in several bioactive compounds as well as a variety of clinically approved therapies.

METHODS

High-resolution electrospray ionization tandem mass spectrometry (ESI-HR-MS/MS) has been used to characterize a series of N,N'-substituted urea derivatives and differentiate two pairs of positional isomers. The data was acquired by Xcaliber application in positive ionization mode.

RESULTS

ESI-HR-MS/MS spectra of [M + H]⁺ ions of the positional isomeric urea derivatives 8a and 8b show distinct fragmentation patterns. For example, the MS/MS spectrum of the [M + H]⁺ ion of isomer 8a displays the abundant fragment ion at m/z 285.1595, which was totally absent in isomer 8b. This would be plausibly formed by the cleavage of the C-N bond of the urea group with the elimination of the isocyanate moiety. In contrast, the MS/MS spectrum of the [M + H]⁺ ion of isomer 8b shows an intense ion at m/z 311.1389 which is completely absent in isomer 8a which would be formed by the cleavage of the C-N bond attached to the ring nitrogen. Similarly, another pair of positional isomers, 8c and 8d, have been clearly distinguished by their fragmentation behaviour. In addition, a series of N,N'-substituted urea derivatives were studied to investigate the impact of different substitution on the fragmentation behaviour.

CONCLUSIONS

The present study demonstrates that ESI-HR-MS/MS can be used to differentiate pairs of N,N'-substituted urea positional isomers and characterize a series of derivatives. It was observed that a characteristic fragment ion was formed by the C-N bond cleavage with the elimination of an isocyanate moiety. The proposed mechanism of fragmentation was supported by the change in the fragmentation pathway upon alkylation of the NH. In order to generalize this fragmentation pattern, a series of N-alkylated ureas was synthesized and studied by MS/MS.

Investigating the exceptional adducts of alkoxides with Ru(II)-arene cations in alkyl alcohol solution using electrospray ionization mass spectrometry

RATIONALE

Exploring the formation mechanism of the exceptional adducts of alkoxides with Ru(II)-arene cations in alkyl alcohol solution using electrospray ionization mass spectrometry (ESI-MS) is crucial for further understanding the physicochemical properties of Ru(II)-arene complexes in solution.

METHODS

All mass spectra were collected with an AB SCIEX TripleTOF 5600⁺ mass spectrometer in positive mode. Theoretical calculations were carried out using the density functional theory method at the B3LYP level with a hybrid basis set consisting of 6-31G(d,p) and LanL2DZ in the Gaussian 03 program.

RESULTS

When ruthenated [1₅ ]paracyclophanes (Ru-[1₅ ]PCPs) and Ru(II)-arene dimers were dissolved in alkyl alcohol solvents, the adducts of alkoxides with Ru(II)-arene cations were observed under positive ion mode ESI-MS, which resulted from the coordination of alkyl alcohol molecules with the Ru(II)-arene cations followed by the deprotonation of O-H bonds of the coordinated alcohols. Furthermore, the number of alkoxides binding to Ru-[1₅ ]PCPs was regulated by the number of ruthenium atoms. Attributed to good solubility and small steric hindrance, the signal intensity of the adducts of methoxides with Ru(II)-arene cations was the strongest among the three alkyl alcohols used in this study.

CONCLUSIONS

The characteristic adducts of alkoxides with Ru(II)-arene cations were pervasively present in positive ion mode ESI-MS of nine Ru(II)-arene complexes dissolved in alkyl alcohol solvents. Taking into consideration the solubility and signal response, methanol is the most suitable solvent for the ESI-MS experiments with Ru(II)-arene complexes among the solvents studied, where almost only the diagnostic adducts of methoxides with Ru(II)-arene cations are present.

Formation of the exceptional [M - H]+ cation in atmospheric pressure ionization mass spectrometry analysis of 2-(diphenylsilyl) cyclopropanecarboxylate esters

RATIONALE

In general, ionization of analytes in atmospheric pressure ionization mass spectrometry (API-MS) in positive ion mode results in the formation of protonated molecules ([M + H]⁺ ) and/or cationized molecules (e.g., [M + Na]⁺ ). The formation of specific [M - H]⁺ cations in the API process is of significant interest for further investigation.

METHODS

The ionization processes of 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate esters were investigated using electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization-MS in positive ion mode. Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level.

RESULTS

The anomalous [M - H]⁺ ion and the regular [M + Na]⁺ ion were both observed using ESI-MS. Interestingly, no [M + H]⁺ ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent accelerated the formation of [M - H]⁺ rather than [M + H]⁺ ion. DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate (1) indicated that the [1 + H]⁺ ion can thermodynamically and kinetically undergo facile H₂ elimination to generate [1 - H]⁺ .

CONCLUSIONS

The favorable formation of [M - H]⁺ ions in these compounds is attributed to the unique diphenylhydrosilyl group in their structure. The [M + H]⁺ ion formed easily underwent H₂ elimination to produce the [1 - H]⁺ ion in the API source, and thus, acidification of the ESI solvent apparently accelerates the formation of the [1 - H]⁺ ion.

Fragmentation pathways of protonated coumarin by ESI-QE-Orbitrap-MS/MS coupled with DFT calculations

Coumarin is one of the basic structures of naturally oxygen heterocyclic compound, which was investigated in this paper for its gas-phase fragmentation behaviors using electrospray quadrupole extractive orbitrap mass spectrometry in the positive mode. The possible fragmentation pathways were proposed based on electrospray ionization (ESI)- mass spectrometry (MS)/MS data and theory calculation. The elimination of two CO and CO₂ was observed for protonated coumarin, which was followed by the formation of a stabilized seven-, six-, and five-membered ring carbocation by loss of C2H2. The possible protonation sites occurred at Oxygen 11 atom of coumarin were the main fragmentation pathways. The relative abundance of characteristic fragment ions and the energy-resolved breakdown curves were used to confirm the cleavage mechanism of protonated coumarin. The methodology and results of present work would contribute to the chemical structure identification of other coumarins.

Fragmentation pathways and differentiation of positional isomers of sorafenib and structural analogues by ESI-IT-MSn and ESI-Q-TOF-MS/MS coupled with DFT calculations

Sorafenib is an orally active multikinase inhibitor for the treatment of renal cell carcinoma. A series of sorafenib structural analogues were investigated in this work for their gas-phase fragmentation behaviors using electrospray ionization ion trap mass spectrometry and quadrupole time-of-flight mass spectrometry in the positive mode. The possible fragmentation pathways were proposed based on ESI-MS/MS data and theoretical calculation. Different from the typical α-cleavage of amide, consecutive reactions that involved elimination of H₂ O and CH₃ NC were observed for 2-pyridinecarboxamide derivatives, which were followed by the formation of a stabilized 7-membered ring carbocation by loss of CO. Two possible protonation sites occurred at carbonyl oxygen atoms for aryl-urea derivatives and the α-cleavage of urea was the main fragmentation pathways, which was followed by the formation of stable benzo [d] oxazole ring characteristic to aryl-urea derivatives. The relative abundance of characteristic fragment ions and the energy-resolved breakdown curves were used to distinguish the 4 sets of positional isomers of sorafenib and analogues. The methodology and results of the present work would contribute to the chemical structure identification of other structural analogues and the potential impurities presented in active pharmaceutical ingredients and drug formulations.

Metal ions induced secondary structure rearrangements: mechanically interlocked lassovs.unthreaded branched-cyclic topoisomers

Metal ions can play a significant role in a variety of important functions in protein systems including cofactor for catalysis, protein folding, assembly, structural stability and conformational change.