1
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Limbach MN, Do TD. Solvent-Free Nuclear Magnetic Resonance Spectroscopy of Charged Molecules. J Phys Chem A 2023; 127:9149-9157. [PMID: 37861438 DOI: 10.1021/acs.jpca.3c05241] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2023]
Abstract
Nuclear magnetic resonance (NMR) spectroscopy of small molecules protonated in a solvent-free environment was successfully demonstrated. The method is referred to as solvent-free protonation NMR (SoF-NMR). Leveraging matrix-assisted ionization (MAI), we generated protonated species of aniline, 4-chloroaniline, 4-aminobiphenyl, and benzocaine for NMR analysis under mild pressure and temperature conditions. The SoF-NMR spectra were compared to traditional solution NMR spectra, and the shift changes in nuclear spin resonance frequencies verify that these small molecules are protonated by 3-nitrobenzonitrile (3-NBN). As the sample pressure decreased, new spectral features appeared, indicating the presence of differently charged species. Several advantages of SoF-NMR are highlighted, such as the elimination of H/D exchange in labile protons, resulting in the precise observation of protons that are otherwise transient in solution. Notably, the data on benzocaine show evidence of neutral, N-protonated, and O-protonated species all in the same spectrum. SoF-NMR eliminates the solvent effects and interactions that can hinder important spectral features. Optimizing SoF-NMR will result in more cost-effective and efficient NMR experimentation to monitor high-temperature, solvent-free reactions. SoF-NMR has a viable future application for studying exchangeable protons, intermediates, and products in gas-phase chemistry.
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Affiliation(s)
- Miranda N Limbach
- Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
| | - Thanh D Do
- Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
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2
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Ohshimo K, Sato R, Takasaki Y, Tsunoda K, Ito R, Kanno M, Misaizu F. Highly Efficient Intramolecular Proton Transfer in p-Aminobenzoic Acid by a Single Ammonia Molecule as a Vehicle. J Phys Chem Lett 2023; 14:8281-8288. [PMID: 37677142 DOI: 10.1021/acs.jpclett.3c01996] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/09/2023]
Abstract
Proton transfer is classified into two mechanisms: the Grotthuss (proton-relay) and vehicle mechanisms. It has been well studied on gas-phase proton transfer by a proton relay involving multiple molecules. However, a vehicle mechanism in which a single molecule transports a proton has rarely been reported. Here, we have obtained clear evidence that the proton transfers efficiently between the two protonation sites in protonated p-aminobenzoic acid (PABA·H+) by a single ammonia molecule as a vehicle. The gaseous PABA·H+ ions were reacted with NH3 or ND3 under single-collision conditions in a cold ion trap, and the proton-transferred ions were identified by cryogenic ion mobility-mass spectrometry. A reaction intermediate PABA·H+·NH3 was also detected for the first time. The reaction pathway search calculations and ab initio molecular dynamics simulations supported the present experimental finding that intramolecular proton transfer occurs very efficiently by the vehicle mechanism.
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Affiliation(s)
- Keijiro Ohshimo
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Ryosuke Sato
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Yuya Takasaki
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Kengo Tsunoda
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Ryosuke Ito
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Manabu Kanno
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Fuminori Misaizu
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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3
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Salvi M, Uma NN, Dinesan H, Roy A, Kumar SS. A versatile 16-pole ion trap setup for investigating photophysics of biomolecular ions. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2023; 94:093203. [PMID: 37721505 DOI: 10.1063/5.0160407] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/01/2023] [Accepted: 08/26/2023] [Indexed: 09/19/2023]
Abstract
A linear 16-pole ion trap-based experimental setup has been designed, implemented, and characterized to investigate the photophysics of biomolecules in the gas phase. Electrospray ionization is employed to generate the ions in the gas phase at atmospheric pressure. The voltage configuration on the ion funnel, the ion optic device in the first vacuum interface, is used to control the energy of the ions. A home-built quadrupole mass-filter is utilized for the mass-selection of the ions of interest. A 16-pole ion trap designed and built in-house is implemented for ion trapping. The instrument's versatility and capability are showcased by demonstrating the fragmentation patterns of protonated and deprotonated tryptophan, as well as describing the photodetachment decay of deprotonated indole.
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Affiliation(s)
- M Salvi
- Department of Physics and the Center for Atomic, Molecular, and Optical Sciences & Technologies (CAMOST), Indian Institute of Science Education and Research Tirupati, AP, Tirupati 517507, India
| | - N N Uma
- Department of Physics and the Center for Atomic, Molecular, and Optical Sciences & Technologies (CAMOST), Indian Institute of Science Education and Research Tirupati, AP, Tirupati 517507, India
| | - Hemanth Dinesan
- CNRS Laboratoire de Physique des Lasers (LPL), Université Sorbonne Paris Nord Villetaneuse, Villetaneuse 93430, France
| | - Abheek Roy
- Department of Physics and the Center for Atomic, Molecular, and Optical Sciences & Technologies (CAMOST), Indian Institute of Science Education and Research Tirupati, AP, Tirupati 517507, India
| | - S Sunil Kumar
- Department of Physics and the Center for Atomic, Molecular, and Optical Sciences & Technologies (CAMOST), Indian Institute of Science Education and Research Tirupati, AP, Tirupati 517507, India
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4
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Kumar M, Samarasinghe I, Attygalle AB. Dependence of Collision-Induced Mass Spectra of Protonated Michler's Ketone on the Nature of LC-MS Mobile Phase. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2023; 34:1663-1674. [PMID: 37459424 DOI: 10.1021/jasms.3c00115] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/03/2023]
Abstract
Michler's ketone (MK) is a dimethylamino ketone that undergoes facile protonation under electrospray-ionization conditions to produce an ion of m/z 269. Initial LC-MS results showed that the collision-induced dissociation (CID) spectra of the m/z 269 ion depend heavily on the composition of the chromatographic mobile phase. Subsequent ion-mobility separation of the mass-selected m/z 269 ion revealed that protonated MK exists as two tautomeric forms. Moreover, the relative population of the two protomeric forms in the ion ensemble depends on the nature of the ambient molecules present in the atmospheric pressure ion source. For example, the ion-mobility arrival-time profile acquired from the mass-selected m/z 269 ion generated from an acetonitrile solution showed two peaks of near equal intensity. The peak with the shorter arrival time represented the O-protomer and that with the longer arrival time represented the N-protomer. However, when methanol or ammonia vapors were introduced to the ambient-pressure ion source, the intensity of the N-protomer peak decreased rapidly and that of the O-protomer signal soared until it became the dominant peak. When the introduction of methanol (or ammonia) vapors was stopped, the mobilogram signals gradually reverted back to their initial intensities. To rationalize this observation, we propose that the N-protomer of MK in the presence of methanol vapor undergoes transformation to the O-protomer by a Grotthuss-type mechanism via a methanol-based solvent bridge.
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Affiliation(s)
- Meenu Kumar
- Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States
| | - Ishira Samarasinghe
- Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States
| | - Athula B Attygalle
- Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States
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5
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Valadbeigi Y, Causon T. Mechanism of formation and ion mobility separation of protomers and deprotomers of diaminobenzoic acids and aminophthalic acids. Phys Chem Chem Phys 2023. [PMID: 37490344 DOI: 10.1039/d3cp01968c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/27/2023]
Abstract
Aminobenzoic acids are well-established candidates for understanding the formation of isomeric ions in positive mode electrospray ionization as they yield both N- and O-protomers (prototropic isomers) at the amine and carbonyl sites, respectively. In the present work, a combination of ion mobility-mass spectrometry and density functional theory calculations to determine the protonation and deprotonation behaviour of four diamino benzoic acid and four aminophthalic acid isomers is presented. The additional COOH group on the ring of aminophthalic acids provides experimental evidence regarding the mechanism of intramolecular NH3+ → O proton transfer, which has been the subject of debate in recent years. To determine the proton acceptor O atom, ion mobility spectra of the fragments of protomers were used as a new method for the confidential assignment of the O-protomer structure, confirming only short-distance intramolecular NH3+ → O proton transfer. Additionally, the substitution pattern both influences the basicity of the protonation sites and enables these molecules to form internal hydrogen bonds with the protonated or deprotonated sites. The formation of the hydrogen bonds in the deprotonated aminophthalic acids changed the charge distribution and subsequently their ion mobility-derived collision cross sections in nitrogen (CCSN2) leading to separation of the four isomers studied. Finally, an interesting effect of the substitution pattern was observed as a synergistic electron-donating effect of the amine groups of 3,5-diaminobenzoic acid on enhancing the basicity of the carbon atom C2 of the ring and previously unreported formation of a C-protomer within aminobenzoic acid systems.
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Affiliation(s)
- Younes Valadbeigi
- Department of Chemistry, Faculty of Science, Imam Khomeini International University, Qazvin, Iran.
| | - Tim Causon
- University of Natural Resources and Life Sciences, Vienna, Department of Chemistry, Institute of Analytical Chemistry, Muthgasse 18, 1190 Vienna, Austria.
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6
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Samarasinghe I, Attygalle AB. Impact of Ambient Vapors on Spectra of 4-Nitroaniline Recorded under Atmospheric Solids Analysis Probe (ASAP) Mass Spectrometric Conditions. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2023; 34:205-217. [PMID: 36689202 DOI: 10.1021/jasms.2c00259] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
Thermally desorbed 4-nitroaniline (4-NA), upon atmospheric pressure chemical ionization (APCI), generates gaseous ions for its protonated species. The APCI mass spectrum recorded under mild in-source ion-activating conditions from 4-NA showed a peak at m/z 139, whereas that acquired under high ion-activating conditions showed two additional peaks at m/z 122 (•OH loss) and 92 (•NO loss). The spectrum changed instantaneously when acetonitrile vapor was introduced to the source. In the new spectrum, both m/z 122 and 92 peaks were absent, while a new peak appeared at m/z 93. Ion-mobility separation carried out with the m/z 139 ion revealed that the initial ion represented the thermodynamically favored nitro-protonated tautomer. The ion population changed to an ensemble dominated by the less-favored amino-protomer when acetonitrile vapor was introduced to the ion source. The amino-protomer, upon collisional activation, loses •NO2 to generate an m/z 93 ion, which was confirmed to be the 4-dehydroanilinium ion. Ion mobility provided a practical way to monitor the changes secured by acetonitrile vapor because the two protomers showed different arrival times. Under spray-ionization conditions, the formation of the thermodynamically less favored protomer has been attributed to kinetic trapping. Our study demonstrated that the less favored amino-protomer could be generated by introducing acetonitrile vapor under nonspray conditions. Apparently, under APCI conditions, protonated water vapor attaches to the nitro group to generate a proton-bound heterodimer, which upon activation dissociates to yield the nitro-protomer. In contrast, protonated acetonitrile makes a tighter complex preferentially with the amino group, which upon activation breaks to the amino-protomer.
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Affiliation(s)
- Ishira Samarasinghe
- Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, New Jersey07030, United States
| | - Athula B Attygalle
- Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, New Jersey07030, United States
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7
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Zhang Y, Li W, Lei H, Dong X, Kenttämaa H. Differentiation of Seven Isomeric n-Pentylquinoline Radical Cations Based on Energy-Resolved Medium-Energy Collision-Activated Dissociation. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2023; 34:48-63. [PMID: 36507850 DOI: 10.1021/jasms.2c00239] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
Asphaltenes, a major and undesirable component of heavy crude oil, contain many different types of large aromatic compounds. These compounds include nitrogen-containing heteroaromatic compounds that are thought to be the main culprit in the deactivation of catalysts in crude oil refinery processes. Unfortunately, prevention of this is challenging as the structures and properties of the nitrogen-containing heteroaromatic compounds are poorly understood. To facilitate their structural characterization, an approach based on ion-trap collision-activated dissociation (ITCAD) tandem mass spectrometry followed by energy-resolved medium-energy collision-activated dissociation (ER-MCAD) was developed for the differentiation of seven isomeric molecular radical cations of n-pentylquinoline. The fragmentation of each isomer was found to be distinctly different and depended largely on the site of the alkyl side chain in the quinoline ring. In order to better understand the observed fragmentation pathways, mechanisms for the formation of several fragment ions were delineated based on quantum chemical calculations. The fast benzylic α-bond cleavage that dominates the fragmentation of analogous nonheteroaromatic alkylbenzenes was only observed for the 3-isomer as the major pathway due to the lack of favorable low-energy rearrangement reactions. All the other isomeric ions underwent substantially lower-energy rearrangement reactions as their alkyl chains were found to interact mostly via 6-membered transition states either with the quinoline nitrogen (2- and 8-isomers) or the adjacent carbon atom in the quinoline core (4-, 5-, 6-, and 7-isomers), which lowered the activation energies of the fragmentation reactions. The presented analytical approach will facilitate the structural characterization of nitrogen-containing heteroaromatic compounds in asphaltenes.
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Affiliation(s)
- Yuyang Zhang
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Wanru Li
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Haoran Lei
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Xueming Dong
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Hilkka Kenttämaa
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
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8
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Tammiku-Taul J, Burk P. Nonempirical Prediction of the Relative Electrospray Ionization Efficiencies of Nitroanilines by Combined CBS-QB3 and SCC-DFTB Calculations. J Phys Chem A 2022; 126:8939-8944. [DOI: 10.1021/acs.jpca.2c05420] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Affiliation(s)
- Jaana Tammiku-Taul
- Institute of Chemistry, University of Tartu, Ravila 14A, Tartu 50411, Estonia
| | - Peeter Burk
- Institute of Chemistry, University of Tartu, Ravila 14A, Tartu 50411, Estonia
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9
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Paciotti R, Chiavarino B, Coletti C, Scuderi D, Re N, Corinti D, Rotari L, Fornarini S, Crestoni ME. IRMPD Spectroscopy of Bare Monodeprotonated Genistein, an Antioxidant Flavonoid. ACS OMEGA 2022; 7:19535-19544. [PMID: 35721943 PMCID: PMC9202291 DOI: 10.1021/acsomega.2c01236] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2022] [Accepted: 05/17/2022] [Indexed: 05/11/2023]
Abstract
Genistein is a naturally occurring polyphenol belonging to the family of flavonoids with estrogenic properties and proven antioxidant, anti-inflammatory, and hormonal effects. Genistein and its derivatives are involved in radical scavenging activity by way of mechanisms based on sequential proton-loss electron transfer. In view of this role, a detailed structural characterization of its bare deprotonated form, [geni-H]-, generated by electrospray ionization, has been performed by tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy in the 800-1800 cm-1 spectral range. Quantum chemical calculations at the B3LYP/6-311+G(d,p) level of theory were carried out to determine geometries, thermochemical data, and anharmonic vibrational properties of low-lying isomers, enabling to interpret the experimental spectrum. Evidence is gathered that the conjugate base of genistein exists as a single isomeric form, which is deprotonated at the most acidic site (7-OH) and benefits from a strong intramolecular H-bond interaction between 5-OH and the adjacent carbonyl oxygen in the most stable arrangement.
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Affiliation(s)
- Roberto Paciotti
- Dipartimento
di Farmacia, Università G. D’Annunzio
Chieti-Pescara, Via dei Vestini 31, Chieti I-66100, Italy
| | - Barbara Chiavarino
- Dipartimento
di Chimica e Tecnologie del Farmaco, Università
di Roma “La Sapienza”, Piazzale Aldo Moro, 5, I-00185 Roma, Italy
| | - Cecilia Coletti
- Dipartimento
di Farmacia, Università G. D’Annunzio
Chieti-Pescara, Via dei Vestini 31, Chieti I-66100, Italy
| | - Debora Scuderi
- Institut
de Chimie Physique (UMR8000), CNRS, Université Paris-Saclay, 91405 Orsay, France
| | - Nazzareno Re
- Dipartimento
di Farmacia, Università G. D’Annunzio
Chieti-Pescara, Via dei Vestini 31, Chieti I-66100, Italy
| | - Davide Corinti
- Dipartimento
di Chimica e Tecnologie del Farmaco, Università
di Roma “La Sapienza”, Piazzale Aldo Moro, 5, I-00185 Roma, Italy
| | - Lucretia Rotari
- Dipartimento
di Chimica e Tecnologie del Farmaco, Università
di Roma “La Sapienza”, Piazzale Aldo Moro, 5, I-00185 Roma, Italy
| | - Simonetta Fornarini
- Dipartimento
di Chimica e Tecnologie del Farmaco, Università
di Roma “La Sapienza”, Piazzale Aldo Moro, 5, I-00185 Roma, Italy
| | - Maria Elisa Crestoni
- Dipartimento
di Chimica e Tecnologie del Farmaco, Università
di Roma “La Sapienza”, Piazzale Aldo Moro, 5, I-00185 Roma, Italy
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10
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Hirata K, Haddad F, Dopfer O, Ishiuchi SI, Fujii M. Collision-assisted stripping for determination of microsolvation-dependent protonation sites in hydrated clusters by cryogenic ion trap infrared spectroscopy: the case of benzocaineH +(H 2O) n. Phys Chem Chem Phys 2022; 24:5774-5779. [PMID: 35199812 DOI: 10.1039/d1cp05762f] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
The protonation site of molecules can be varied by their surrounding environment. Gas-phase studies, including the popular techniques of infrared spectroscopy and ion mobility spectrometry, are a powerful tool for the determination of protonation sites in solvated clusters but often suffer from inherent limits for larger hydrated clusters. Here, we present collision-assisted stripping infrared (CAS-IR) spectroscopy as a new technique to overcome these problems and apply it in a proof-of-principle experiment to hydrated clusters of protonated benzocaine (H+BC), which shows protonation-site switching depending on the degree of hydration. The most stable protomer of H+BC in the gas phase (O-protonated) is interconverted into its most stable protomer in aqueous solution (N-protonated) upon hydration with three water molecules. CAS-IR spectroscopy enables us to unambiguously assign protonation sites and quantitatively determine the relative abundance of various protomers.
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Affiliation(s)
- Keisuke Hirata
- Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan. .,Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.,Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovation Research, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, 4259, Yokohama, 226-8503, Japan.
| | - Fuad Haddad
- Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, Berlin 10623, Germany
| | - Otto Dopfer
- Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovation Research, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, 4259, Yokohama, 226-8503, Japan. .,Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, Berlin 10623, Germany
| | - Shun-Ichi Ishiuchi
- Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan. .,Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.,Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovation Research, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, 4259, Yokohama, 226-8503, Japan.
| | - Masaaki Fujii
- Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan. .,Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovation Research, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, 4259, Yokohama, 226-8503, Japan. .,School of Life Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa, 226-8503, Japan
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11
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Ucur B, Maccarone AT, Ellis SR, Blanksby SJ, Trevitt AJ. Solvent-Mediated Proton-Transfer Catalysis of the Gas-Phase Isomerization of Ciprofloxacin Protomers. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2022; 33:347-354. [PMID: 35014802 DOI: 10.1021/jasms.1c00331] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Understanding how neutral molecules become protonated during positive-ion electrospray ionization (ESI) mass spectrometry is critically important to ensure analytes can be efficiently ionized, detected, and unambiguously identified. The ESI solvent is one of several parameters that can alter the dominant site of protonation in polyfunctional molecules and thus, in turn, can significantly change the collision-induced dissociation (CID) mass spectra relied upon for compound identification. Ciprofloxacin─a common fluoroquinolone antibiotic─is one such example whereby positive-ion ESI can result in gas-phase [M + H]+ ions protonated at either the keto-oxygen or the piperazine-nitrogen. Here, we demonstrate that these protonation isomers (or protomers) of ciprofloxacin can be resolved by differential ion mobility spectrometry and give rise to distinctive CID mass spectra following both charge-directed and charge-remote mechanisms. Interaction of mobility-selected protomers with methanol vapor (added via the throttle gas supply) was found to irreversibly convert the piperazine N-protomer to the keto-O-protomer. This methanol-mediated proton-transport catalysis is driven by the overall exothermicity of the reaction, which is computed to favor the O-protomer by 93 kJ mol-1 (in the gas phase). Conversely, gas phase interactions of mobility-selected ions with acetonitrile vapor selectively depletes the N-protomer ion signal as formation of stable [M + H + CH3CN]+ cluster ions skews the apparent protomer population ratio, as the O-protomer is unaffected. These findings provide a mechanistic basis for tuning protomer populations to ensure faithful characterization of multifunctional molecules by tandem mass spectrometry.
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Affiliation(s)
- Boris Ucur
- Molecular Horizons and School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, New South Wales 2522, Australia
| | - Alan T Maccarone
- Molecular Horizons and School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, New South Wales 2522, Australia
| | - Shane R Ellis
- Molecular Horizons and School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, New South Wales 2522, Australia
- Illawarra Health and Medical Research Institute, Wollongong, New South Wales 2522, Australia
| | - Stephen J Blanksby
- Central Analytical Research Facility and School of Chemistry and Physics, Queensland University of Technology, Brisbane 4001, Australia
| | - Adam J Trevitt
- Molecular Horizons and School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, New South Wales 2522, Australia
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12
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Coughlan NJA, Fu W, Guna M, Schneider BB, Le Blanc JCY, Campbell JL, Hopkins WS. Electronic spectroscopy of differential mobility-selected prototropic isomers of protonated para-aminobenzoic acid. Phys Chem Chem Phys 2021; 23:20607-20614. [PMID: 34505849 DOI: 10.1039/d1cp02120f] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
para-Aminobenzoic acid (PABA) was electrosprayed from mixtures of protic and aprotic solvents, leading to formation of two prototropic isomers in the gas phase whose relative populations depended on the composition of the electrospray solvent. The two ion populations were separated in the gas phase using differential mobility spectrometry (DMS) within a nitrogen-only environment at atmospheric pressure. Under high-field conditions, the two prototropic isomers eluted with baseline signal separation with the N-protonated isomer having a more negative CV shift than the O-protonated isomer, in accord with previous DMS studies. The conditions most favorable for formation and separation of each tautomer were used to trap each prototropic isomer in a quadrupole ion trap for photodissociation action spectroscopy experiments. Spectral interrogation of each prototropic isomer in the UV region (3-6 eV) showed good agreement with previously recorded spectra, although a previously reported band (4.8-5.4 eV) was less intense for the O-protonated isomer in our measured spectrum. Without DMS selection, the measured spectra contained features corresponding to both protonated isomers even when solvent conditions were optimised for formation of a single isomer. Interconversion between protonated isomers within the ion trap was observed when protic ESI solvents were employed, leading to spectral cross contamination even with mobility selection. CCSD vertical excitation energies and vertical gradient (VG) Franck-Condon simulations are presented and reproduce the measured spectral features with near-quantitative agreement, providing supporting evidence for spectral assignments.
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Affiliation(s)
- Neville J A Coughlan
- Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada. .,Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
| | - Weiqiang Fu
- Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada. .,Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
| | - Mircea Guna
- SCIEX, Four Valley Drive, Concord, Ontario, L4K 4V8, Canada
| | | | | | - J Larry Campbell
- Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada. .,Bedrock Scientific Inc., Milton, Ontario, Canada.,WaterMine Innovation, Waterloo, Ontario, Canada
| | - W Scott Hopkins
- Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada. .,Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada.,WaterMine Innovation, Waterloo, Ontario, Canada.,Centre for Eye and Vision Research, Hong Kong Science Park, New Territories, Hong Kong
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Zheng Z, Attygalle AB. Impact of Ambient Vapors Present in an Electrospray Ionization Source on Gas-Phase Ion Structures. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2021; 32:725-735. [PMID: 33606934 DOI: 10.1021/jasms.0c00430] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
According to current consensus, structures of protomeric (or deprotomeric) tautomers of gaseous ions generated by electrospray ionization depend primarily on the nature of the spray solvent. To probe the effect of the spray solvent on protonation, 4-aminobenzoic acid (PABA) has often been selected as the model compound. It is widely accepted that the protonation in the gas phase takes place primarily on the carbonyl oxygen atom when the sample is sprayed in methanol and on the nitrogen atom when acetonitrile is used as the spray solvent. Although this observation is valid, our current results indicate that the determination of the predominant protomer in the gas phase by the spray solvent is an indirect effect moderated by the solvent vapor molecules present in the ambient ion source. To investigate real-time changes in protomer distributions due to solvents, we used ion-mobility mass spectrometry (IM-MS). Initially, when a PABA solution in methanol was electrosprayed, the ion-mobility arrival-time profile recorded showed essentially one peak for the O-protomer. However, when acetonitrile or acetone vapors were introduced to the ambient-pressure ion source via the flowing desolvation gas, the intensity of the O-protomer peak diminished rapidly, and the N-protomer signal became dominant. The moment the acetonitrile (or acetone) vapors were removed from the ion source, the protomer-distribution signals began gradually reverting back to their original intensities. Furthermore, when PABA samples in methanol and acetonitrile were electrosprayed separately via a dual-sprayer setup, which allowed for the selective blocking of the gaseous ion-generation cascade of charged droplets from either sprayer, the predominant signal corresponded only to the N-protomer, irrespective of the position of the mechanical barrier. Because the mechanical barrier prevents only the gaseous ion formation, but not the physical access of solvent vapors to the ion source, it is evident that the solvent vapor that engulfs the ion source is the governing factor that decides the protomer distribution, not the nature of the spray solvent. Noticeably, acetonitrile wields a stronger effect on the manifested protomer distribution than many other solvents, including methanol, water, hexanes, and toluene. Apparently, the so-called "memory" of the solution-phase structures and the phenomenon described as "kinetic trapping" are both due to indirect effects caused by the solvent vapor engulfing the atmospheric-pressure ion source. Moreover, the so-called "memory" effect can either be "saved" or "erased" by exposing the initially formed gaseous ions to different solvent vapors from an alternative source.
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Affiliation(s)
- Zhaoyu Zheng
- Center for Mass Spectrometry, Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States
| | - Athula B Attygalle
- Center for Mass Spectrometry, Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States
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Kumar R, Kenttämaa HI. Effects of Analyte Concentration on the Protonation Sites of 4-Aminobenzoic Acid upon Atmospheric Pressure Chemical Ionization As Revealed by Gas-Phase Ion-Molecule Reactions. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2020; 31:2210-2217. [PMID: 32852952 DOI: 10.1021/jasms.0c00285] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
The most basic site of 4-aminobenzoic acid in aqueous solution is the amino nitrogen, while the carbonyl oxygen is calculated to be the most basic site in the gas phase. However, the preferred protonation site of 4-aminobenzoic acid upon electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) depends upon the ionization solvent and ion source parameters. The influence of the concentration of the analyte on the manifested protonation sites upon APCI has not been investigated and is reported here. Gas-phase ion-molecule reactions of trimethoxymethylsilane were used to identify the protonation sites of 4-aminobenzoic acid ionized using APCI with methanol or acetonitrile-water as the solvent. The nitrogen-protomer was found to be about twice as abundant as the oxygen-protomer at low analyte concentrations (10-9-10-6 M) in methanol solvent. This finding was rationalized on the basis of a previous finding that when the O-protomer is surrounded by more than eight methanol molecules in the gas phase it starts behaving as if it were in an aqueous solution and converts to the N-protomer. At greater analyte concentrations (≥10-4 M), the amino group was predominantly protonated, which was rationalized based on the formation of a particularly stable proton-bound dimer of 4-aminobenzoic acid that preferentially dissociates to form the N-protomer. The above findings suggest that solution processes are much more important in APCI than commonly assumed, in agreement with recent literature. Indeed, when 1:1 (v/v) acetonitrile-water was used as the solvent system for 4-aminobenzoic acid, the N-protomer was predominantly generated at all analyte concentrations.
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Affiliation(s)
- Rashmi Kumar
- Chemistry Department, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Hilkka I Kenttämaa
- Chemistry Department, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
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Chandran J, Zheng Z, Thomas VI, Rajalakshmi C, Attygalle AB. LC-MS analysis of p-aminosalicylic acid under electrospray ionization conditions manifests a profound solvent effect. Analyst 2020; 145:5333-5344. [DOI: 10.1039/d0an00680g] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Under identical mass spectrometric conditions, chromatographic peak intensities of p-aminosalicylic acid recorded by LC-MS, using methanol as the mobile phase are drastically different from those acquired using is it acetonitrile as the eluent.
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Affiliation(s)
- Jisha Chandran
- Inter University Instrumentation Centre (IUIC)
- School of Environmental Sciences
- Mahatma Gandhi University
- Kottayam
- 686560 India
| | - Zhaoyu Zheng
- Center for Mass Spectrometry
- Department of Chemistry and Chemical Biology
- Stevens Institute of Technology
- Hoboken
- USA
| | | | | | - Athula B. Attygalle
- Center for Mass Spectrometry
- Department of Chemistry and Chemical Biology
- Stevens Institute of Technology
- Hoboken
- USA
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