Gas-Phase Internal Proton-Transfer of Protonated para-Aminobenzoic Acid Catalyzed by One Methanol Molecule

Abstract

Electrospray ionization (ESI) is used to deliver analytes for mass analysis across a huge range of mass spectrometry applications. Despite its ubiquitous application and many mechanistic investigations, it remains that a fundamental understanding of ESI processes is not complete. In particular, all the factors that influence the populations of protonation isomers are elusive such that it remains a challenge to optimize experimental conditions to favor one isomer over another. The molecule para-aminobenzoic acid has emerged as an archetype for the study of protonation isomers, with both amino and carboxylic acid protonation site isomers (protomers) typically formed upon ESI, with the isomer ratio shown to be sensitive to several physical and chemical parameters. Here we report an ion-trap mass spectrometry study of the time-resolved methanol-catalyzed proton transfer between the amine and carboxylic acid moieties of para-aminobenzoic acid. The experimental and computational results presented are consistent with a bimolecular mechanism where isomerization is mediated by a single methanol rather than a multimolecular Grotthuss proton transfer process. Pseudo-first-order rate constants for protomer specific product ions are reported and confirm the depletion of the amino protomer is correlated to the growth of the carboxylic acid protomer. Under the controlled conditions of a low-pressure ion-trap mass spectrometer (2.5 mTorr, 300 K), the number of methanol molecules required to isomerize para-aminobenzoic acid is determined to be one, and the second-order rate constant for methanol-catalyzed isomerization is (1.9 ± 0.1) × 10–11 cm3 molecule–1 s–1. The para-aminobenzoic acid vehicle mechanism is explored computationally at the DSD-PBEP86-D3BJ/aug-cc-pVDZ level of theory and reveals that the transition state for proton transfer is submerged (−10 kJ mol–1) relative to the separated reactant energies. The findings from this paper show that single-solvent catalyzed intramolecular proton transfer reactions are possible and must be considered during the late stages of ESI to predict the site(s) of protonation and the ion’s stability in the presence of solvent molecules.

Highly Efficient Intramolecular Proton Transfer in p-Aminobenzoic Acid by a Single Ammonia Molecule as a Vehicle

“…With this experimental condition of the concentration and flow rate of the mixture gas, the reaction between the N-protomer and NH 3 was confirmed to be the pseudo-first order reaction. Thus, the bimolecular reaction, PABA·H + (N-protomer) + NH 3 , proceeds in the ion trap, and the proton transfer reaction is concluded to proceed by a vehicle mechanism, as in the PABA·H + + CH 3 OH reaction reported very recently . Furthermore, a complex of PABA·H + with NH 3 (PABA·H + ·NH 3 ) was detected in the present study by Cryo-IM-MS. From the arrival time of PABA·H + ·NH 3 , the collision cross section of this complex with He was determined to be 97.6 Å 2 , which was assigned to a complex of the O-protomer and NH 3 .…”

“…Only a small portion of the protons may have finally been transferred to form the O-protomer after the intermolecular proton transfer, BC·H + (N-protomer) + NH 3 → BC + NH 4 + . Very recently, intramolecular proton transfer of PABA·H + was observed in the reaction of PABA·H + + CH 3 OH by ion trap mass spectrometry . Interconversion between protomers with a vehicle mechanism was identified by CID mass spectrometry, reaction kinetics measurements, and theoretical calculations in that study …”

“…Mass spectrometry (collision-induced dissociation (CID) and H/D exchange reaction) of PABA·H + produced by electrospray ionization (ESI) showed that the predominant protomer observed in the gas phase was dependent on the solvent used in the sample solution for the spray: The O-protomer was formed in the gas phase for the methanol/water mixture solvent, while N-protomer was observed from the sample solution with a solvent of acetonitrile/water mixture . After this mass spectrometric study, the PABA·H + protomer was studied in the gas phase by infrared photodissociation spectroscopy, − ion mobility spectrometry (IMS), ,− ultraviolet photodissociation spectroscopy, , threshold collision-induced dissociation, ion–molecule reaction, ,,− and molecular dynamics (MD) calculations . As a result of these studies, the solvent dependence of the protomer abundance of PABA·H + in ESI mass spectrometry is related to the ease of proton transfer during the evaporation of droplets to form PABA·H + .…”

Dependence of Collision-Induced Mass Spectra of Protonated Michler’s Ketone on the Nature of LC-MS Mobile Phase

“…Evidently, the conversion of the N -protonated tautomer to the O -protonated form is assisted by the presence of methanol (or ammonia) vapor in the ion source. Analogous tautomerization to more energetically favored forms under mass spectrometric conditions have been noted previously for several bifunctional analytes. ,,,, …”

Mechanism of formation and ion mobility separation of protomers and deprotomers of diaminobenzoic acids and aminophthalic acids

“…, H 2 O and CH 3 OH) is attributed to a solvent-assisted intramolecular proton transfer from the NH 3 + group to the oxygen atom (Grotthuss mechanism), 23–26 there is no experimental evidence for the formation of these solvent bridges in the gas phase. 27,28…”