Tracking the Structural Evolution of 4-Aminobenzoic Acid in the Transition from Solution to the Gas Phase

Dependency of fentanyl analogue protomer ratios on solvent conditions as measured by ion mobility-mass spectrometry

Recently, our group has shown that fentanyl and many of its analogues form prototropic isomers ("protomers") during electrospray ionization. These different protomers can be resolved using ion mobility spectrometry and annotated using mobility-aligned tandem mass spectrometry fragmentation. However, their formation and the extent to which experimental variables contribute to their relative ratio remain poorly understood. In the present study, we systematically investigated the effects of mixtures of common chromatographic solvents (water, methanol, and acetonitrile) and pH on the ratio of previously observed protomers for 23 fentanyl analogues. Interestingly, these ratios (N-piperidine protonation vs. secondary amine/O = protonation) decreased significantly for many analogues (e.g., despropionyl ortho-, meta-, and para-methyl fentanyl), increased significantly for others (e.g., cis-isofentanyl), and remained relatively constant for the others as solvent conditions changed from 100% organic solvent (methanol or acetonitrile) to 100% water. Interestingly, pH also had significant effects on this ratio, causing the change in ratio to switch in many cases. Lastly, increasing conditions to pH ≥ 4.0 also prompted the appearance of new mobility peaks for ortho- and para-methyl acetyl fentanyl, where all previous studies had only showed one single distribution. Because these ratios have promise to be used qualitatively for identification of these (and emerging) fentanyl analogues, understanding how various conditions (i.e., mobile phase selection and/or chromatographic gradient) affect their ratios is critically important to the development of advanced ion mobility and mass spectrometry methodologies to identify fentanyl analogues.

Manipulating Non-Dissociative Transformations of Gaseous Ion Ensembles Prior to Ion-Mobility Separation

Molecules with multiple sites capable of accepting protons form ensembles of protomers. The manifested protomer ratios in such ensembles are influenced by many experimental conditions. In a Synapt G2 ion mobility (IM)-enabled mass spectrometry system, there are several physical locations where ion population changes can be manifested. Using APCI-generated protomers of aminonaphthalenes, we investigated its intramolecular proton transfers from the N-protomer to the C-protomer. This lossless transformation of the N-protomer to the thermodynamically favored C-protomer can take place in the ion source itself. Initially, we learned that the cone gas slows down the transformation to the C-protomer. Gaseous ions are then accelerated in the first vacuum region, where ions undergo collisional activation (heating), which facilitates the transformation to the C-protomer. Afterward, the ions are mass selected and transferred to the pre-IM (Trap)-collision cell, where ions can also be transformed to the thermodynamically favored protomers. Trap accumulated ions are then released to the IM separator via a helium-filled entry cell. The role of helium is to minimize ion activation and scattering taking place upon entry to the high-pressure T-Wave IM separator (TWIMS). The helium cell is known to increase the IM peak resolution. However, we found that significant changes occur depending on the presence or absence of helium. Without helium, source-generated protomers rapidly changed to a predominantly thermodynamically favorable ensemble protomers. Apparently, the introduction of helium into the precell induced a dramatic decrease in collisional "heating" effect, which effectively slowed down the conversion rate of the amino-protomer into the more favorable ring-protomer. The final message is that mobilograms should not be considered as direct real-time, or intrinsic, representations of the protomer ratios in the ion source.

Observation of Solvent-Dependence in the Mechanism of Neutral-Catalyzed Isomerization of para-Aminobenzoic Acid Protomers

Proton-transfer reactions are commonplace during electrospray ionization (ESI) mass spectrometry experiments and are often responsible for imparting charge to analyte molecules. Multiple protonation-site isomers (protomers) can arise for polyfunctional molecules and these isomers can interconvert via solvent-mediated proton transfer reactions during various stages of the ESI process. Studying the populations and interconversion of protonation isomers provides key insight into the ESI process, ion-molecule interactions, and ion dissociation mechanisms. An archetype molecule to study protomer interconversion fundamentals in this context is para-aminobenzoic acid (pABA), where both the amino and carboxylic acid protomers are typically formed under ESI and the mechanisms for interconversion are still under refinement. Using ion-trap mass spectrometry reaction kinetics (2.5 mTorr, 300 K), this study examines gas-phase interconversion catalysis of pABA protomers by seven neutral species, which are commen solvents and additives used for ESI: water, formic acid, methanol, ethanol, propanol, ammonia, and acetonitrile. Three distinct reaction cases are reported: (i) formic acid, methanol, ethanol, propanol, and ammonia each catalyze the interconversion between the amino and carboxylic acid protomers via a n = 1 solvent-molecule vehicle mechanism; (ii) for water, however, a n = 6 adduct complex is detected and this suggests that the observed protomer interconversion occurs through a Grotthuss mechanism, in accord with literature reports; (iii) acetonitrile inhibits proton transfer by the formation of particularly stable n = 1 and 2 adduct complexes. The second-order rate constants for the protomer interconversion are observed to increase in the following order: H2O < HCO2H < MeOH < EtOH < PrOH < NH3. Potential energy schemes are reported for all neutral-catalyzed proton transfer reactions using the DSD-PBEP86-D3(BJ)/aug-cc-pVDZ level of theory. A central transition state, which connects the protonation site adducts, is shown to be the key rate-limiting step. The energy of this transition state is sensitive to the proton affinity of the neutral solvent, and this is supported by the correlation between the reaction rate and the solvent proton affinity.

Transition from vehicle to Grotthuss proton transfer in a nanosized flask: cryogenic ion spectroscopy of protonated p-aminobenzoic acid solvated with D2O

Proton transfer (PT) is one of the most ubiquitous reactions in chemistry and life science. The unique nature of PT has been rationalized not by the transport of a solvated proton (vehicle mechanism) but by the Grotthuss mechanism in which a proton is transported to the nearest proton acceptor along a hydrogen-bonded network. However, clear experimental evidence of the Grotthuss mechanism has not been reported yet. Herein we show by infrared spectroscopy that a vehicle-type PT occurs in the penta- and hexahydrated clusters of protonated p-aminobenzoic acid, while Grotthuss-type PT is observed in heptahydrated clusters, indicating a change in the PT mechanism depending on the degree of hydration. These findings emphasize the importance of the usually ignored vehicle mechanism as well as the degree of hydration. It highlights the possibility of controlling the PT mechanism by the number of water molecules in chemical and biological environments.

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

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.

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

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.

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

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 (CCS_N2) 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.

Hydration-induced protomer switching in p-aminobenzoic acid studied by cold double ion trap infrared spectroscopy

Para-Aminobenzoic acid (PABA) is a benchmark molecule to study solvent-induced proton site switching. Protonation of the carboxy and amino groups of PABA generates O- and N-protomers of PABAH⁺, respectively. Ion mobility mass spectrometry (IMS) and infrared photodissociation (IRPD) studies have claimed that the O-protomer most stable in the gas phase is converted to the N-protomer most stable in solution upon hydration with six water molecules in the gas-phase cluster. However, the threshold size has remained ambiguous because the arrival time distributions in the IMS experiments exhibit multiple peaks. On the other hand, IRPD spectroscopy could not detect the N-protomer for smaller hydrated clusters because of broad background due to annealing required to reduce kinetic trapping. Herein, we report the threshold size for O → N protomer switching without ambiguity using IR spectroscopy in a double ion trap spectrometer from 1300 to 1800 cm^-1. The pure O-protomer is prepared by electrospray, and size-specific hydrated clusters are formed in a reaction ion trap. The resulting clusters are transferred into a second cryogenic ion trap and the distribution of O- and N-protomers is determined by mid-IR spectroscopy without broadening. The threshold to promote O → N protomer switching is indeed five water molecules. It is smaller than the value reported previously, and as a result, its pentahydrated structure does not support the Grotthuss mechanism proposed previously. The extent of O → N proton transfer is evaluated by collision-assisted stripping IR spectroscopy, and the N-protomer population increases with the number of water molecules. This result is consistent with the dominant population of the N-protomer in aqueous solution.

Understanding of protomers/deprotomers by combining mass spectrometry and computation

Multifunctional compounds may form different prototropic isomers under different conditions, which are known as protomers/deprotomers. In biological systems, these protomer/deprotomer isomers affect the interaction modes and conformational landscape between compounds and enzymes and thus present different biological activities. Study on protomers/deprotomers is essentially the study on the acidity/basicity of each intramolecular functional group and its effect on molecular structure. In recent years, the combination of mass spectrometry (MS) and computational chemistry has been proven to be a powerful and effective means to study prototropic isomers. MS-based technologies are developed to discriminate and characterize protomers/deprotomers to provide structural information and monitor transformations, showing great superiority than other experimental methods. Computational chemistry is used to predict the thermodynamic stability of protomers/deprotomers, provide the simulated MS/MS spectra, infrared spectra, and calculate collision cross-section values. By comparing the theoretical data with the corresponding experimental results, the researchers can not only determine the protomer/deprotomer structure, but also investigate the structure-activity relationship in a given system. This review covers various MS methods and theoretical calculations and their devotion to isomer discrimination, structure identification, conformational transformation, and phase transition investigation of protomers/deprotomers.

The hitchhiker's guide to dynamic ion-solvent clustering: applications in differential ion mobility spectrometry

This article highlights the fundamentals of ion-solvent clustering processes that are pertinent to understanding an ion's behaviour during differential mobility spectrometry (DMS) experiments. We contrast DMS with static-field ion mobility, where separation is affected by mobility differences under the high-field and low-field conditions of an asymmetric oscillating electric field. Although commonly used in mass spectrometric (MS) workflows to enhance signal-to-noise ratios and remove isobaric contaminants, the chemistry and physics that underpins the phenomenon of differential mobility has yet to be fully fleshed out. Moreover, we are just now making progress towards understanding how the DMS separation waveform creates a dynamic clustering environment when the carrier gas is seeded with the vapour of a volatile solvent molecule (e.g., methanol). Interestingly, one can correlate the dynamic clustering behaviour observed in DMS experiments with gas-phase and solution-phase molecular properties such as hydrophobicity, acidity, and solubility. However, to create a generalized, global model for property determination using DMS data one must employ machine learning. In this article, we provide a first-principles description of differential ion mobility in a dynamic clustering environment. We then discuss the correlation between dynamic clustering propensity and analyte physicochemical properties and demonstrate that analytes exhibiting similar ion-solvent interactions (e.g., charge-dipole) follow well-defined trends with respect to DMS clustering behaviour. Finally, we describe how supervised machine learning can be used to create predictive models of molecular properties using DMS data. We additionally highlight open questions in the field and provide our perspective on future directions that can be explored.

Grotthuss Molecular Dynamics Simulations for Modeling Proton Hopping in Electrosprayed Water Droplets

Excess protons in water exhibit unique transport properties because they can rapidly hop along H-bonded water wires. Considerable progress has been made in unraveling this Grotthuss diffusion mechanism using quantum mechanical-based computational techniques. Unfortunately, high computational cost tends to restrict those techniques to small systems and short times. Molecular dynamics (MD) simulations can be applied to much larger systems and longer time windows. However, standard MD methods do not permit the dissociation/formation of covalent bonds, such that Grotthuss diffusion cannot be captured. Here, we bridge this gap by combining atomistic MD simulations (using Gromacs and TIP4P/2005 water) with proton hopping. Excess protons are modeled as hydronium ions that undergo H₃O⁺ + H₂O → H₂O + H₃O⁺ transitions. In accordance with ab initio MD data, these Grotthuss hopping events are executed in "bursts" with quasi-instantaneous hopping across one or more waters. The bursts are separated by regular MD periods during which H₃O⁺ ions undergo Brownian diffusion. The resulting proton diffusion coefficient agrees with the literature value. We apply this Grotthuss MD technique to highly charged water droplets that are in a size regime encountered during electrospray ionization (5 nm radius, ∼17,000 H₂O). The droplets undergo rapid solvent evaporation and occasional H₃O⁺ ejection, keeping them at ca. 81% of the Rayleigh limit. The simulated behavior is consistent with phase Doppler anemometry data. The Grotthuss MD technique developed here should be useful for modeling the behavior of various proton-containing systems that are too large for high-level computational approaches. In particular, we envision future applications related to electrospray processes, where earlier simulations used metal cations while in reality excess protons dominate.

Collision-assisted stripping for determination of microsolvation-dependent protonation sites in hydrated clusters by cryogenic ion trap infrared spectroscopy: the case of benzocaineH+(H2O)n

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.

Solvent-Mediated Proton-Transfer Catalysis of the Gas-Phase Isomerization of Ciprofloxacin Protomers

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.

Electronic spectroscopy of differential mobility-selected prototropic isomers of protonated para-aminobenzoic acid

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.

p-Aminobenzoic acid protonation dynamics in an evaporating droplet by ab initio molecular dynamics

Protonation equilibria are known to vary from the bulk to microdroplet conditions, which could induce many chemical and physical phenomena. Protonated p-aminobenzoic acid (PABA + H⁺) can be considered a model for probing the protonation dynamics in an evaporating droplet, as its protonation equilibrium is highly dependent on the formation conditions from solution via atmospheric pressure ionization sources. Experiments using diverse experimental techniques have shown that protic solvents allow formation of the O-protomer (PABA protonated in the carboxylic acid group) stable in the gas phase, while aprotic solvents yield the N-protomer (protonated in the amino group) that is the most stable protomer in solution. In this work, we explore the protonation equilibrium of PABA solvated by different numbers of water molecules (n = 0 to 32) using ab initio molecular dynamics. For n = 8-32, the protonation is either at the NH₂ group or in the solvent network. The solvent network interacts with the carboxylic acid group, but there is no complete proton transfer to form the O-protomer. For smaller clusters, however, solvent-mediated proton transfers to the carboxylic acid were observed, both via the Grotthuss mechanism and the vehicle or shuttle mechanism (for n = 1 and 2). Thermodynamic considerations allowed a description of the origins of the kinetic trapping effect, which explains the observation of the solution structure in the gas phase. This effect likely occurs in the final evaporation steps, which are outside the droplet size range covered by previous classical molecular dynamics simulations of charged droplets. These results may be considered relevant in determining the nature of the species observed in the ubiquitous ESI based mass spectrometry analysis, and in general for droplet chemistry, explaining how protonation equilibria are drastically changed from bulk to microdroplet conditions.

Protons Are Fast and Smart; Proteins Are Slow and Dumb: On the Relationship of Electrospray Ionization Charge States and Conformations

We present simple considerations of how differences in time scales of motions of protons, the lightest and fastest chemical moiety, and the much longer time scales associated with the dynamics of proteins, among the heaviest and slowest analytes, may allow many protein conformations from solution to be kinetically trapped during the process of electrospraying protein solutions into the gas phase. In solution, the quantum nature of protons leads them to change locations by tunneling, an instantaneous process; moreover, the Grotthuss mechanism suggests that these small particles can respond nearly instantaneously to the dynamic motions of proteins that occur on much longer time scales. A conformational change is accompanied by favorable or unfavorable variations in the free energy of the system, providing the impetus for solvent ↔ protein proton exchange. Thus, as thermal distributions of protein conformations interconvert, protonation states rapidly respond, as specific acidic and basic sites are exposed or protected. In the vacuum of the mass spectrometer, protons become immobilized in locations that are specific to the protein conformations from which they were incorporated. In this way, conformational states from solution are preserved upon electrospraying them into the gas phase. These ideas are consistent with the exquisite sensitivity of electrospray mass spectra to small changes of the local environment that alter protein structure in solution. We might remember this approximation for the protonation of proteins in solution with the colloquial expression-protons are fast and smart; proteins are slow and dumb.

THE IMS PARADOX: A PERSPECTIVE ON STRUCTURAL ION MOBILITY-MASS SPECTROMETRY

Studies of large proteins, protein complexes, and membrane protein complexes pose new challenges, most notably the need for increased ion mobility (IM) and mass spectrometry (MS) resolution. This review covers evolutionary developments in IM-MS in the authors' and key collaborators' laboratories with specific focus on developments that enhance the utility of IM-MS for structural analysis. IM-MS measurements are performed on gas phase ions, thus "structural IM-MS" appears paradoxical-do gas phase ions retain their solution phase structure? There is growing evidence to support the notion that solution phase structure(s) can be retained by the gas phase ions. It should not go unnoticed that we use "structures" in this statement because an important feature of IM-MS is the ability to deal with conformationally heterogeneous systems, thus providing a direct measure of conformational entropy. The extension of this work to large proteins and protein complexes has motivated our development of Fourier-transform IM-MS instruments, a strategy first described by Hill and coworkers in 1985 (Anal Chem, 1985, 57, pp. 402-406) that has proved to be a game-changer in our quest to merge drift tube (DT) and ion mobility and the high mass resolution orbitrap MS instruments. DT-IMS is the only method that allows first-principles determinations of rotationally averaged collision cross sections (CSS), which is essential for studies of biomolecules where the conformational diversities of the molecule precludes the use of CCS calibration approaches. The Fourier transform-IM-orbitrap instrument described here also incorporates the full suite of native MS/IM-MS capabilities that are currently employed in the most advanced native MS/IM-MS instruments. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Impact of Ambient Vapors Present in an Electrospray Ionization Source on Gas-Phase Ion Structures

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.

Microhydration of Biomolecules: Revealing the Native Structures by Cold Ion IR Spectroscopy

The native-like structures of protonated glycine and peptide Gly₃H⁺ were elucidated using cold ion IR spectroscopy of these biomolecules hydrated by a controlled number of water molecules. The complexes were generated directly from an aqueous solution using gentle electrospray ionization. Already with a single retained water molecule, GlyH⁺ exhibits the native-like structure characterized by a lack of intramolecular hydrogen bonds. We use our spectra to calibrate the available data for the same complexes, which are produced by cryogenic condensation of water onto the gas-phase glycine. In some conformers of these complexes, GlyH⁺ adopts the native-like structure, while in the others, it remains "kinetically" trapped in the intrinsic state. Upon condensation of 4-5 water molecules, the embedded amino acid fully adopts its native-like structure. Similarly, condensation of one water molecule onto the tripeptide is insufficient to fully eliminate its kinetically trapped intrinsic states.

Vibrational spectroscopy of the cryogenically cooled O- and N-protomers of 4-Aminobenzoic acid: Tag effects, isotopic labels, and identification of the E,Z isomer of the O-protomer

4-Aminobenzoic acid (4ABA) is a biologically relevant, small organic molecule with two protonation sites: the amino group (N-protomer) and the carboxyl group (O-protomer). The O-protomer is energetically preferred in the gas-phase, while the higher energy N-protomer can be trapped using aprotic solvents such as acetonitrile during electrospray ionization. Here, we focus on the structure of the O-protomer, which can occur in three low-lying isomeric forms that result from different orientations of the OH groups relative to the benzene ring. We report the vibrational spectra of both N- and O-protomers of the cryogenically cooled ions in the gas phase over the spectral range 800-4000 cm-1. The bands arising from the OH stretches are isolated from the nearby NH stretching fundamentals using isotopic labeling as well as by analysis of the shifts in these fundamentals upon attachment of D2 and N2 molecules to the OH groups of the O-protomer. The spectra of isomers derived from the different locations of the adducts were isolated using two-color, IR-IR photofragmentation spectroscopy. The docking motifs by which the O-protomer binds to another 4ABA molecule is also explored and found to feature a bifurcated arrangement involving attachment of both OH groups of the protonated head group to the carbonyl group of the neutral partner.

Long-distance proton transfer induced by a single ammonia molecule: ion mobility mass spectrometry of protonated benzocaine reacted with NH3

A long-distance proton transfer via the vehicle mechanism in the absence of a hydrogen-bonded solvent-bridge in molecules.