Overcoming the challenge of potent endogenous interferences in limaprost quantification: An innovative methodology combining differential mobility spectrometry with LC-MS/MS for ultra-high sensitivity, selectivity and significantly enhanced throughput

Limaprost, an orally administered analogue of prostaglandin E1, possesses potent vasodilatory, antiplatelet, and cytoprotective properties. Due to its extremely low therapeutic doses and exceedingly low plasma concentrations, the pharmacokinetic and bioequivalence studies of limaprost necessitate a highly sensitive quantitative method with a sub-pg/mL level of lower limit of quantification. Moreover, the intensity of endogenous interferences can even exceed the maximum concentration level of limaprost in human plasma, presenting further challenge to the quantification of limaprost. As a result, existing methods have not yet met the necessary level of sensitivity, selectivity, and throughput needed for the quantitative analysis of limaprost in pharmacokinetic and bioequivalence investigations. This study presents a new methodology that combines differential mobility spectrometry (DMS) with liquid chromatography-tandem mass spectrometry (LC-MS/MS) and utilizes a distinctive strategy to achieve more accurate DMS conditions. This integration yields a method that is currently the most sensitive and features the shortest analytical time, making it the sole technique capable of meeting the requirements for limaprost pharmacokinetic and bioequivalence investigations. This method demonstrates robustness and is successfully employed in a pharmacokinetic investigation of limaprost in human subjects, underscoring that the combination of DMS with LC-MS/MS serves as an efficacious strategy for overcoming the challenges inherent in analyzing biological samples afflicted by multiple interferences.

Computational and Experimental IM-MS Determination of the Protonated Structures of Antimalarial Drugs

A combination of ion mobility-mass spectrometry (IM-MS) measurements and computational methods were used to study structural and physicochemical properties of a range of quinoline-based drugs: amodiaquine (AQ), cinchonine (CIN), chloroquine (CQ), mefloquine (MQ), pamaquine (PQ), primaquine (PR), quinacrine (QR), quinine (QN), and sitamaquine (SQ). In experimental studies, ionization of these compounds using atmospheric pressure chemical ionization (APCI) yields monoprotonated species in the gas phase while electrospray ionization (ESI) also produces diprotonated forms of AQ, CQ, and QR and also for PQ, SQ, and QN in the presence of formic acid as an additive. Comparison of the trajectory-method-calculated and experimental IM-derived collisional cross sections (CCSN2) were used to assign both the protonation sites and conformer geometry of all drugs considered with biases of 0.7-2.8% between calculated and experimental values. It was found that, in solution, AQ and QR are protonated at the ring nitrogen of the quinoline group, whereas the other drugs are protonated at the amine group of the alkyl chain. Finally, the conformers of [M + H]+ and [M + 2H]2+ assigned according to the lowest energies and CCSN2 calculations were used to calculate the pKa values of the antimalarial drugs and the relative abundance of these ions at different pH values that provided validation of the computational and experimental IM-MS results.

Protomers of the green and cyan fluorescent protein chromophores investigated using action spectroscopy

The photophysics of biochromophore ions often depends on the isomeric or protomeric distribution, yet this distribution, and the individual isomer contributions to an action spectrum, can be difficult to quantify. Here, we use two separate photodissociation action spectroscopy instruments to record electronic spectra for protonated forms of the green (pHBDI⁺) and cyan (Cyan⁺) fluorescent protein chromophores. One instrument allows for cryogenic (T = 40 ± 10 K) cooling of the ions, while the other offers the ability to perform protomer-selective photodissociation spectroscopy. We show that both chromophores are generated as two protomers when using electrospray ionisation, and that the protomers have partially overlapping absorption profiles associated with the S₁ ← S₀ transition. The action spectra for both species span the 340-460 nm range, although the spectral onset for the pHBDI⁺ protomer with the proton residing on the carbonyl oxygen is red-shifted by ≈40 nm relative to the lower-energy imine protomer. Similarly, the imine and carbonyl protomers are the lowest energy forms of Cyan⁺, with the main band for the carbonyl protomer red-shifted by ≈60 nm relative to the lower-energy imine protomer. The present strategy for investigating protomers can be applied to a wide range of other biochromophore ions.

Bridging the Gap between Differential Mobility, Log S, and Log P Using Machine Learning and SHAP Analysis

Aqueous solubility, log S, and the water-octanol partition coefficient, log P, are physicochemical properties that are used to screen the viability of drug candidates and to estimate mass transport in the environment. In this work, differential mobility spectrometry (DMS) experiments performed in microsolvating environments are used to train machine learning (ML) frameworks that predict the log S and log P of various molecule classes. In lieu of a consistent source of experimentally measured log S and log P values, the OPERA package was used to evaluate the aqueous solubility and hydrophobicity of 333 analytes. With ion mobility/DMS data (e.g., CCS, dispersion curves) as input, we used ML regressors and ensemble stacking to derive relationships with a high degree of explainability, as assessed via SHapley Additive exPlanations (SHAP) analysis. The DMS-based regression models returned scores of R² = 0.67 and RMSE = 1.03 ± 0.10 for log S predictions and R² = 0.67 and RMSE = 1.20 ± 0.10 for log P after 5-fold random cross-validation. SHAP analysis reveals that the regressors strongly weighted gas-phase clustering in log P correlations. The addition of structural descriptors (e.g., # of aromatic carbons) improved log S predictions to yield RMSE = 0.84 ± 0.07 and R² = 0.78. Similarly, log P predictions using the same data resulted in an RMSE of 0.83 ± 0.04 and R² = 0.84. The SHAP analysis of log P models highlights the need for additional experimental parameters describing hydrophobic interactions. These results were achieved with a smaller dataset (333 instances) and minimal structural correlation compared to purely structure-based models, underscoring the value of employing DMS data in predictive models.

First-Principles Modeling of Preferential Solvation in Mixed-Modifier Differential Mobility Spectrometry

Differential mobility spectrometry (DMS) separates ions based on mobility differences between high and low electric field conditions. To enhance resolution, solvents such as water and acetonitrile are often used to modify the collision environment and take advantage of differing dynamic clustering behavior between analytes that coelute in hard-sphere environments (e.g., N₂). When binary solvent mixtures are used to modify the DMS environment, one solvent can have a dominant influence over the other with respect to ion trajectories. For example, for quinoline derivatives, a 9:1 water:acetonitrile solvent mixture exhibited identical behavior to an environment containing only acetonitrile as a modifier. It was hypothesized that this effect arises due to the significantly different binding strengths of the two solvents. Here, we utilize a first-principles model of DMS to study analytes in single and binary solvent mixtures and explore the effects governing the dominance of one solvent over the other. Computed DMS dispersion curves of quinoline derivatives are in excellent agreement with those measured experimentally. For mixed-modifier environments, the predicted cluster populations show a clear preferential solvation of ions with the stronger binding solvent. The influence of ion-solvent binding energies, solvent concentration, and solvent molecule size is discussed in the context of the observed DMS behavior. This work can guide the usage of binary solvent mixtures for improving ion separations in cases where compounds coelute in pure N₂ and in single-solvent modifier environments. Moreover, our results indicate that binary solvent mixtures can be used to create a relative scale for solvent binding energies.

Multianalytical Approach for Deciphering the Specific MS/MS Transition and Overcoming the Challenge of the Separation of a Transient Intermediate, Quinonoid Dihydrobiopterin

Despite recent technological developments in analytical chemistry, separation and direct characterization of transient intermediates remain an analytical challenge. Among these, separation and direct characterization of quinonoid dihydrobiopterin (qH2Bip), a transient intermediate of tetrahydrobiopterin (H4Bip)-dependent hydroxylation reactions, essential in living organisms, with important and varied human pathophysiological impacts, are a clear illustration. H4Bip regeneration may be impaired by competitive nonenzymatic autoxidation reactions, such as isomerization of qH2Bip into a more stable 7,8-H2Bip (H2Bip) isomer, and subsequent nonenzymatic oxidation reactions. The quinonoid qH2Bip intermediate thus plays a key role in H4Bip-dependent hydroxylation reactions. However, only a few experimental results have indirectly confirmed this finding while revealing the difficulty of isolating qH2Bip from H4Bip-containing solutions. As a result, no current H4Bip assay method allows this isomer to be quantified even by liquid chromatography-tandem mass spectrometry (MS/MS). Here, we report isolation, structural characterization, and abundance of qH2Bip formed upon H4Bip autoxidation using three methods integrated into MS/MS. First, we characterized the structure of the two observed H2B isomers using IR photodissociation spectroscopy in conjunction with quantum chemical calculations. Then, we used differential ion mobility spectrometry to fully separate all oxidized forms of H4Bip including qH2Bip. These data are consistent and show that qH2Bip can also be unambiguously identified thanks to its specific MS/MS transition. This finding paves the way for the quantification of qH2Bip with MS/MS methods. Most importantly, the half-life value of this intermediate is nearly equivalent to that of H4Bip (tens of minutes), suggesting that an accurate method of H4Bip analysis should include the quantification of qH2Bip.

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.

Conformational Changes Induced by Methyl Side-Chains in Protonated Tripeptides Containing Glycine and Alanine Residues

We present a systematic study of the conformational and isomeric populations in gas-phase protonated tripeptides containing glycine and alanine residues using infrared predissociation spectroscopy of cryogenically cooled ions. Specifically, the protonated forms of Gly-Gly-Gly, Ala-Gly-Gly, Gly-Ala-Gly, Gly-Gly-Ala, Ala-Ala-Gly, Ala-Gly-Ala, Gly-Ala-Ala, and Ala-Ala-Ala allow us to sample all permutations of the methyl side-chain position, providing a comprehensive view of the effects of this simple side-chain on the 3-D structure of the peptide. The individual structural populations for all but one of these peptide species are determined via conformer-specific IR-IR double-resonance spectroscopy and comparison with electronic structure predictions. The observed structures can be classified into three main families defined by the protonation site and the number of internal hydrogen bonds. The relative contribution of each structural family is highly dependent on the exact amino acid sequence of the tripeptide. These observed changes in structural population can be rationalized in terms of the electron-donating effect of the methyl side-chain modulating the local proton affinities of the amine and various carbonyl groups in the tripeptide.

Research Progress on Quantification Methods of Drug Concentration of Monoclonal Antibodies

Background:

With the development of monoclonal antibodies (mAbs) from the first generation of mice to the fourth generation of human origin, the efficacy and safety in the treatment of many diseases have been continuously improved. MAbs have been widely used in the treatment of cancer, chronic inflammatory diseases, and so on. However, the treatment response of mAbs varies greatly among individuals, and drug exposure may be affected by a variety of physiological and pathological factors, such as combined use of drugs and progression of disease. Therefore, studies tend to recommend therapeutic drug monitoring and individualized treatment strategies.

Objective:

In this paper, the commonly used methods of quantification of monoclonal antibodies were reviewed, especially liquid chromatography- mass spectrometry (LC-MS/MS) and enzyme-linked immunosorbent assay (ELISA), to provide technical support for therapeutic drug detection and individualize dosing for patients.

Conclusion:

For patients achieving mAbs treatment, it is necessary to carry out therapeutic drug monitoring and take it as a routine monitoring index. We recommend that for pharmaceutical laboratories in hospitals, establishing an appropriate assay formats, such as ELISA and LC-MS/MS is critical to determine drug concentration and antidrug antibody (ADA) for mAbs.

Improved First-Principles Model of Differential Mobility Using Higher Order Two-Temperature Theory

Differential mobility spectrometry is a separation technique that may be applied to a variety of analytes ranging from small molecule drugs to peptides and proteins. Although rudimentary theoretical models of differential mobility exist, these models are often only applied to small molecules and atomic ions without considering the effects of dynamic microsolvation. Here, we advance our theoretical description of differential ion mobility in pure N₂ and microsolvating environments by incorporating higher order corrections to two-temperature theory (2TT) and a pseudoequilibrium approach to describe ion-neutral interactions. When comparing theoretical predictions to experimentally measured dispersion plots of over 300 different compounds, we find that higher order corrections to 2TT reduce errors by roughly a factor of 2 when compared to first order. Model predictions are accurate especially for pure N₂ environments (mean absolute error of 4 V at SV = 4000 V). For strongly clustering environments, accurate thermochemical corrections for ion-solvent clustering are likely required to reliably predict differential ion mobility behavior. Within our model, general trends associated with clustering strength, solvent vapor concentration, and background gas temperature are well reproduced, and fine structure visible in some dispersion plots is captured. These results provide insight into the dynamic ion-solvent clustering process that underpins the phenomenon of differential ion mobility.

Rapid separation of cannabinoid isomer sets using differential mobility spectrometry and mass spectrometry

With legalization and decriminalization of cannabis in many parts of the world comes the need for rapid separation and quantitation of the psychoactive ingredients.

Action spectroscopy of the isolated red Kaede fluorescent protein chromophore

Incorporation of fluorescent proteins into biochemical systems has revolutionized the field of bioimaging. In a bottom-up approach, understanding the photophysics of fluorescent proteins requires detailed investigations of the light-absorbing chromophore, which can be achieved by studying the chromophore in isolation. This paper reports a photodissociation action spectroscopy study on the deprotonated anion of the red Kaede fluorescent protein chromophore, demonstrating that at least three isomers-assigned to deprotomers-are generated in the gas phase. Deprotomer-selected action spectra are recorded over the S1 ← S0 band using an instrument with differential mobility spectrometry coupled with photodissociation spectroscopy. The spectrum for the principal phenoxide deprotomer spans the 480-660 nm range with a maximum response at ≈610 nm. The imidazolate deprotomer has a blue-shifted action spectrum with a maximum response at ≈545 nm. The action spectra are consistent with excited state coupled-cluster calculations of excitation wavelengths for the deprotomers. A third gas-phase species with a distinct action spectrum is tentatively assigned to an imidazole tautomer of the principal phenoxide deprotomer. This study highlights the need for isomer-selective methods when studying the photophysics of biochromophores possessing several deprotonation sites.

UVPD Spectroscopy of Differential Mobility-Selected Prototropic Isomers of Rivaroxaban

Two ion populations of protonated Rivaroxaban, [C₁₉H₁₈ClN₃O₅S + H]⁺, are separated under pure N₂ conditions using differential mobility spectrometry prior to characterization in a hybrid triple quadrupole linear ion trap mass spectrometer. These populations are attributed to bare protonated Rivaroxaban and to a proton-bound Rivaroxaban-ammonia complex, which dissociates prior to mass-selecting the parent ion. Ultraviolet photodissociation (UVPD) and collision-induced dissociation (CID) studies indicate that both protonated Rivaroxaban ion populations are comprised of the computed global minimum prototropic isomer. Two ion populations are also observed when the collision environment is modified with 1.5% (v/v) acetonitrile. In this case, the protonated Rivaroxaban ion populations are produced by the dissociation of the ammonium complex and by the dissociation of a proton-bound Rivaroxaban-acetonitrile complex prior to mass selection. Again, both populations exhibit a similar CID behavior; however, UVPD spectra indicate that the two ion populations are associated with different prototropic isomers. The experimentally acquired spectra are compared with computed spectra and are assigned to two prototropic isomers that exhibit proton sharing between distal oxygen centers.

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.

Determining Collision Cross Sections from Differential Ion Mobility Spectrometry

The experimental determination of ion-neutral collision cross sections (CCSs) is generally confined to ion mobility spectrometry (IMS) technologies that operate under the so-called low-field limit or those that enable empirical calibration strategies (e.g., traveling wave IMS; TWIMS). Correlation of ion trajectories to CCS in other non-linear IMS techniques that employ dynamic electric fields, such as differential mobility spectrometry (DMS), has remained a challenge since its inception. Here, we describe how an ion's CCS can be measured from DMS experiments using a machine learning (ML)-based calibration. The differential mobility of 409 molecular cations (m/z: 86-683 Da and CCS 110-236 Å2) was measured in a N2 environment to train the ML framework. Several open-source ML routines were tested and trained using DMS-MS data in the form of the parent ion's m/z and the compensation voltage required for elution at specific separation voltages between 1500 and 4000 V. The best performing ML model, random forest regression, predicted CCSs with a mean absolute percent error of 2.6 ± 0.4% for analytes excluded from the training set (i.e., out-of-the-bag external validation). This accuracy approaches the inherent statistical error of ∼2.2% for the MobCal-MPI CCS calculations employed for training purposes and the <2% threshold for matching literature CCSs with those obtained on a TWIMS platform.

Clustering and Nonclustering Modifier Mixtures in Differential Mobility Spectrometry for Multidimensional Liquid Chromatography Ion Mobility-Mass Spectrometry Analysis

Modifiers provide fast and reliable tuning of separation in differential mobility spectrometry (DMS). DMS selectivity for separating isomeric molecules depends on the clustering modifier concentration, which is typically 1.5-3 mol % ratio of isopropanol or ethanol in nitrogen. Low concentrations (0.1%) of isopropanol were found to improve resolution and sensitivity but at the cost of practicality and robustness. Replacing the single-channel DMS pump with a binary high-performance liquid chromatography (HPLC) pump enabled the generation of modifier mixtures at a constant flow rate using an isocratic or gradient mode, and the analytical benefits of the system were investigated considering cyclohexane, n-hexane, or n-octane as nonclustering modifiers and isopropanol or ethanol as clustering modifiers. It was found that clustering and nonclustering modifier mixtures enable optimization of selectivity, resolution, and sensitivity for different positional isomers and diastereoisomers. Data further suggested different ion separation mechanisms depending on the modifier ratios. For 85 analytes, the absolute difference in compensation voltages (CoVs) between pure nitrogen and cyclohexane at 1.5 mol % ratio was below 4 V, demonstrating its potential as a nonclustering modifier. Cyclohexane's nonclustering behavior was further supported by molecular modeling using density functional theory (DFT) and calculated cluster binding energies, showing positive ΔG values. The ability to control analyte CoVs by adjusting modifier concentrations in isocratic and gradient modes is beneficial for optimizing multidimensional LCxDMS-MS. It is fast and effective for manipulating the DMS scanning window size to realize shorter mass spectrometry (MS) acquisition cycle times while maintaining a sufficient number of CoV steps and without compromising DMS separation performance.

The Charge-State and Structural Stability of Peptides Conferred by Microsolvating Environments in Differential Mobility Spectrometry

The presence of solvent vapor in a differential mobility spectrometry (DMS) cell creates a microsolvating environment that can mitigate complications associated with field-induced heating. In the case of peptides, the microsolvation of protonation sites results in a stabilization of charge density through localized solvent clustering, sheltering the ion from collisional activation. Seeding the DMS carrier gas (N₂) with a solvent vapor prevented nearly all field-induced fragmentation of the protonated peptides GGG, AAA, and the Lys-rich Polybia-MP1 (IDWKKLLDAAKQIL-NH₂). Modeling the microsolvation propensity of protonated n-propylamine [PrNH₃]⁺, a mimic of the Lys side chain and N-terminus, with common gas-phase modifiers (H₂O, MeOH, EtOH, iPrOH, acetone, and MeCN) confirms that all solvent molecules form stable clusters at the site of protonation. Moreover, modeling populations of microsolvated clusters indicates that species containing protonated amine moieties exist as microsolvated species with one to six solvent ligands at all effective ion temperatures (T_eff) accessible during a DMS experiment (ca. 375-600 K). Calculated T_eff of protonated GGG, AAA, and Polybia-MPI using a modified two-temperature theory approach were up to 86 K cooler in DMS environments seeded with solvent vapor compared to pure N₂ environments. Stabilizing effects were largely driven by an increase in the ion's apparent collision cross section and by evaporative cooling processes induced by the dynamic evaporation/condensation cycles incurred in the presence of an oscillating electric separation field. When the microsolvating partner was a protic solvent, abstraction of a proton from [MP1 + 3H]³⁺ to yield [MP1 + 2H]²⁺ was observed. This result was attributed to the proclivity of protic solvents to form hydrogen-bond networks with enhanced gas-phase basicity. Collectively, microsolvation provides analytes with a solvent "air bag," whereby charge reduction and microsolvation-induced stabilization were shown to shelter peptides from the fragmentation induced by field heating and may play a role in preserving native-like ion configurations.

A Data-Mining Approach for the Quantitative Assessment of Physicochemical Properties of Molecular Compounds in the Skin Flux

This paper aimed to provide an insight into the mechanism of transdermal penetration of drug molecules with respect to their physicochemical properties, such as solubility (S), the presence of enantiomer (ET) and logarithm of octanol-water partition coefficient (log P), molecular weight (MW), and melting point (MP). Propionic acid derivatives were evaluated for their flux through full-thickness skin excised from hairless mice upon being delivered from silicone-based pressure-sensitive adhesive (PSA) matrices in the presence or absence of various enhancers. The skin fluxes of model compounds were calculated based on the data obtained using the method engaged with the diffusion cell system. The statistical design of experiments (DoE) based on the factorial approach was used to find variables that have a significant impact on the outcomes. For the prediction of skin flux, a quantitative equation was derived using the data-mining approach on the relationship between skin permeation of model compounds (~125 mg/ml) and involved physicochemical variables. The most influential variables for the skin flux of propionic acid derivatives were the melting point (0.97) followed by the presence of enantiomer (0.95), molecular mass (0.93), log P values (0.86), and aqueous solubility (0.80). It was concluded that the skin flux of molecular compounds can be predicted based on the relationship between their physicochemical properties and the interaction with cofactors including additives and enhancers in the vehicles.

Computational analysis of the proton-bound acetonitrile dimer, (ACN)2 H. RAPID COMMUNICATIONS IN MASS SPECTROMETRY : RCM 2020;34:e8767. [PMID: 32115782 DOI: 10.1002/rcm.8767] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

RATIONALE

In atmospheric pressure ionization mass spectrometry the theoretical thermodynamic treatment of proton-bound cluster stabilities helps us to understand the prevailing chemical processes. However, such calculations are rather challenging because low-barrier internal rotations and strong anharmonicity of the hydrogen bonds cause the breakdown of the usually applied harmonic approximation. Even the implemented anharmonic treatment in standard ab initio software failed in the case of (ACN)₂ H⁺ .

METHODS

For a case study of the proton-bound acetonitrile dimer, (ACN)₂ H⁺ , we scan the potential energy surface (PES) for the internal rotation and the proton movement in all three spatial directions. We correct the partition functions by treating the internal rotation as a free rotor and by solving the nuclear Schrödinger equation explicitly for the proton movement. An additional PES scan for the dissociation surface further improves the understanding of the cluster behavior.

RESULTS

The internal rotation is essentially barrier free (V₀  = 2.6 × 10^-6 eV) and the proton's movement between the two nitrogen atoms follows a quartic rather than quadratic potential. As a figure of merit we calculate the free dissociation enthalpy of the dimer. Our description significantly improves the standard results from about 118.3 kJ/mol to 99.6 kJ/mol, compared with the experimentally determined value of 92.2 kJ/mol. The dissociation surface reveals strong crosstalk between modes and is essentially responsible for the observed errors.

CONCLUSIONS

The presented corrections to the partition functions significantly improve their accuracy and are rather easy to implement. In addition, this work stresses the importance of alternative theoretical methods for proton-bound cluster systems besides the standard harmonic approximations.

Discrimination between Protonation Isomers of Quinazoline by Ion Mobility and UV-Photodissociation Action Spectroscopy

The influence of oriented electric fields on chemical reactivity and photochemistry is an area of increasing interest. Within a molecule, different protonation sites offer the opportunity to control the location of charge and thus orientation of electric fields. New techniques are thus needed to discriminate between protonation isomers in order to understand this effect. This investigation reports the UV-photodissociation action spectroscopy of two protonation isomers (protomers) of 1,3-diazanaphthalene (quinazoline) arising from protonation of a nitrogen at either the 1- or 3-position. It is shown that these protomers are separable by field-asymmetric ion mobility spectrometry (FAIMS) with confirmation provided by UV-photodissociation (PD) action spectroscopy. Vibronic features in the UVPD action spectra and computational input allow assignment of the origin transitions to the S₁ and S₅ states of both protomers. These experiments also provide vital benchmarks for protomer-specific calculations and examination of isomer-resolved reaction kinetics and thermodynamics.

Understanding Nontraditional Differential Mobility Behavior: A Case Study of the Tricarbastannatrane Cation, N(CH2CH2CH2)3Sn. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2020;31:796-802. [PMID: 32129991 DOI: 10.1021/jasms.9b00042]

The effect of strong ion-solvent interactions on the differential mobility behavior of the tricarbastannatrane cation, N(CH₂CH₂CH₂)₃Sn⁺, has been investigated. Exotic "type D" dispersion behavior, which is intermediate to the more common types C and A behavior, is observed for gaseous N₂ environments that are seeded with acetone and acetonitrile vapor. Quantum chemical calculations and first-principles modeling show that under low-field conditions [N(CH₂CH₂CH₂)₃Sn + solvent]⁺ complexes containing a single solvent molecule survive the entire separation waveform duty cycle and interact weakly with the chemically modified environment. However, at high separation voltages, the ion-solvent bond dissociates and dynamic clustering ensues.

How Hot Are Your Ions in Differential Mobility Spectrometry?

Ions can experience significant field-induced heating in a differential mobility cell. To investigate this phenomenon, the fragmentation of several para-substituted benzylpyridinium "thermometer" ions (R = OMe, Me, F, Cl, H, CN) was monitored in a commercial differential mobility spectrometer (DMS). The internal energy of each benzylpyridinium derivative was characterized by monitoring the degree of fragmentation to obtain an effective temperature, T_eff, which corresponds to a temperature consistent with treating the observed fragmentation ratio using a unimolecular dissociation rate weighted by a Boltzmann distribution at a temperature T. It was found that ions are sufficiently thermalized after initial activation from the ESI process to the temperature of the bath gas, T_bath. Once a critical field strength was surpassed, significant fragmentation of the benzylpyridinium ions was detected. At the maximum bath gas temperature (450 K) and separation voltage (SV; 4400 V) for our instrument, T_eff for the benzylpyridinium derivatives ranged from 664 ± 9 K (p-OMe) to 759 ± 17 K (p-H). The extent of activation at a given SV depends on the ion's mass, degrees of freedom, (N_DoF), and collision frequency as represented by the ion's collision cross section. Plots of T_eff vs the product of ion mass and N_DoF and the inverse of collision cross section produce strong linear relationships. This provides an attractive avenue to estimate ion temperatures at a given SV using only intrinsic properties. Moreover, experimentally determined T_eff correlate with theoretically predicted T_eff using with a self-consistent method based on two-temperature theory. The various instrumental and external parameters that influence T_eff are additionally discussed.

Measuring Electronic Spectra of Differential Mobility-Selected Ions in the Gas Phase

We describe the modification of a commercially available tandem differential mobility mass spectrometer (DMS) that has been retrofitted to facilitate photodissociation (PD) of differential mobility-separated, mass-selected molecular ions. We first show that a mixture of protonated quinoline/isoquinoline (QH⁺/iQH⁺) can be separated using differential mobility spectrometry. Efficient separation is facilitated by addition of methanol to the DMS environment and increased residence time within the DMS. In action spectroscopy experiments, we gate each isomer using appropriate DMS settings, trap the ions in the third quadrupole of a triple quadrupole mass spectrometer, and irradiate them with tunable light from an optical parametric oscillator (OPO). The resulting mass spectra are recorded as the OPO wavelength is scanned, giving PD action spectra. We compare our PD spectra with previously recorded spectra for the same species and show that our instrument reproduces previous works faithfully.

Preferential Ion Microsolvation in Mixed-Modifier Environments Observed Using Differential Mobility Spectrometry

The preferential solvation behavior for eight different derivatives of protonated quinoline was measured in a tandem differential mobility spectrometer mass spectrometer (DMS-MS). Ion-solvent cluster formation was induced in the DMS by the addition of chemical modifiers (i.e., solvent vapors) to the N₂ buffer gas. To determine the effect of more than one modifier in the DMS environment, we performed DMS experiments with varying mixtures of water, acetonitrile, and isopropyl alcohol solvent vapors. The results show that doping the buffer gas with a binary mixture of modifiers leads to the ions binding preferentially to one modifier over another. We used density functional theory to calculate the ion-solvent binding energies, and in all cases, calculations show that the quinolinium ions bind most strongly with acetonitrile, then isopropyl alcohol, and most weakly with water. Computational results support the hypothesis that the quinolinium ions bind exclusively to whichever solvent they have the strongest interaction with, regardless of the presence of other modifier gases.

Separation of Sialylated Glycan Isomers by Differential Mobility Spectrometry

Mass spectrometry has proven itself to be an important technology for characterizing intact glycoproteins, glycopeptides, and released glycans. However, these molecules often present significant challenges during analysis. For example, glycans of identical molecular weights can be present in many isomeric forms, with one form having dramatically more biological activity than the others. Discriminating among these isomeric forms using mass spectrometry alone can be daunting, which is why orthogonal techniques, such as ion mobility spectrometry, have been explored. Here, we demonstrate the use of differential mobility spectrometry (DMS) to separate isomeric glycans differing only in the linkages of sialic acid groups (e.g., α 2,3 versus α 2,6). This ability extends from a small trisaccharide species to larger biantennary systems and is driven, in part, by the role of intramolecular solvation of the charge site(s) on these ions within the DMS environment.

Recommendations for reporting ion mobility Mass Spectrometry measurements

Here we present a guide to ion mobility mass spectrometry experiments, which covers both linear and nonlinear methods: what is measured, how the measurements are done, and how to report the results, including the uncertainties of mobility and collision cross section values. The guide aims to clarify some possibly confusing concepts, and the reporting recommendations should help researchers, authors and reviewers to contribute comprehensive reports, so that the ion mobility data can be reused more confidently. Starting from the concept of the definition of the measurand, we emphasize that (i) mobility values (K0 ) depend intrinsically on ion structure, the nature of the bath gas, temperature, and E/N; (ii) ion mobility does not measure molecular surfaces directly, but collision cross section (CCS) values are derived from mobility values using a physical model; (iii) methods relying on calibration are empirical (and thus may provide method-dependent results) only if the gas nature, temperature or E/N cannot match those of the primary method. Our analysis highlights the urgency of a community effort toward establishing primary standards and reference materials for ion mobility, and provides recommendations to do so. © 2019 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc.

A parallelized molecular collision cross section package with optimized accuracy and efficiency

A new parallelized calculation package predicts collision cross sections with high accuracy and efficiency.

Determining molecular properties with differential mobility spectrometry and machine learning

The fast and accurate determination of molecular properties is highly desirable for many facets of chemical research, particularly in drug discovery where pre-clinical assays play an important role in paring down large sets of drug candidates. Here, we present the use of supervised machine learning to treat differential mobility spectrometry - mass spectrometry data for ten topological classes of drug candidates. We demonstrate that the gas-phase clustering behavior probed in our experiments can be used to predict the candidates' condensed phase molecular properties, such as cell permeability, solubility, polar surface area, and water/octanol distribution coefficient. All of these measurements are performed in minutes and require mere nanograms of each drug examined. Moreover, by tuning gas temperature within the differential mobility spectrometer, one can fine tune the extent of ion-solvent clustering to separate subtly different molecular geometries and to discriminate molecules of very similar physicochemical properties.

Resolution and Assignment of Differential Ion Mobility Spectra of Sarcosine and Isomers

Due to their central role in biochemical processes, fast separation and identification of amino acids (AA) is of importance in many areas of the biomedical field including the diagnosis and monitoring of inborn errors of metabolism and biomarker discovery. Due to the large number of AA together with their isomers and isobars, common methods of AA analysis are tedious and time-consuming because they include a chromatographic separation step requiring pre- or post-column derivatization. Here, we propose a rapid method of separation and identification of sarcosine, a biomarker candidate of prostate cancer, from isomers using differential ion mobility spectrometry (DIMS) interfaced with a tandem mass spectrometer (MS/MS) instrument. Baseline separation of protonated sarcosine from α- and β-alanine isomers can be easily achieved. Identification of DIMS peak is performed using an isomer-specific activation mode where DIMS- and mass-selected ions are irradiated at selected wavenumbers allowing for the specific fragmentation via an infrared multiple photon dissociation (IRMPD) process. Two orthogonal methods to MS/MS are thus added, where the MS/MS(IRMPD) is nothing but an isomer-specific multiple reaction monitoring (MRM) method. The identification relies on the comparison of DIMS-MS/MS(IRMPD) chromatograms recorded at different wavenumbers. Based on the comparison of IR spectra of the three isomers, it is shown that specific depletion of the two protonated α- and β-alanine can be achieved, thus allowing for clear identification of the sarcosine peak. It is also demonstrated that DIMS-MS/MS(IRMPD) spectra in the carboxylic C=O stretching region allow for the resolution of overlapping DIMS peaks. Graphical Abstract ᅟ.

Probing the application range and selectivity of a differential mobility spectrometry-mass spectrometry platform for metabolomics

Metabolomics applications of differential mobility spectrometry (DMS)-mass spectrometry (MS) have largely concentrated on targeted assays and the removal of isobaric or chemical interferences from the signals of a small number of analytes. In the work reported here, we systematically investigated the application range of a DMS-MS method for metabolomics using more than 800 authentic metabolite standards as the test set. The coverage achieved with the DMS-MS platform was comparable to that achieved with chromatographic methods. High orthogonality was observed between hydrophilic interaction liquid chromatography and the 2-propanol-mediated DMS separation, and previously observed similarities were confirmed for the DMS platform and reversed-phase liquid chromatography. We describe the chemical selectivity observed for selected subsets of the metabolite test set, such as lipids, amino acids, nucleotides, and organic acids. Furthermore, we rationalize the behavior and separation of isomeric aromatic acids, bile acids, and other metabolites. Graphical abstract Differential mobility spectrometry-mass spectrometry (DMS-MS) facilitates rapid separation of metabolites of similar mass-to-charge ratio by distributing them across the compensation voltage range on the basis of their different molecular structures.

The structures and properties of anionic tryptophan complexes

IRMPD spectroscopy and electronic structure calculations are employed to identify π–π interactions in ionic tryptophan clusters.

Analyzing Glycopeptide Isomers by Combining Differential Mobility Spectrometry with Electron- and Collision-Based Tandem Mass Spectrometry

Differential mobility spectrometry (DMS) has been employed to separate isomeric species in several studies. Under the right conditions, factors such as separation voltage, temperature, the presence of chemical modifiers, and residence time can combine to provide unique signal channels for isomeric species. In this study, we examined a set of glycopeptide isomers, MUC5AC-3 and MUC5AC-13, which bear an N-acetyl-galactosamine (GalNAc) group on either threonine-3 or threonine-13. When analyzed as a mixture, the resulting MS and MS/MS spectra yield fragmentation patterns that cannot discern these convolved species. However, when DMS is implemented during the analysis of this mixture, two features emerge in the DMS ionogram representing the two glycopeptide isomers. In addition, by locking in DMS parameters at each feature, we could observe several low intensity CID fragments that contain the GalNAc functionality-specific amino acid residues - identifying the DMS separation of each isomer without standards. Besides conventional CID MS/MS, we also implemented electron-capture dissociation (ECD) after DMS separation, and clearly resolved both isomers with this fragmentation method, as well. The electron energy used in these ECD experiments could be tuned to obtain maximum sequence coverage for these glycopeptides; this was critical as these ions were present as doubly protonated species, which are much more difficult to fragment efficiently via electron-transfer dissociation (ETD). Overall, the combination of DMS with electron- or collision-based MS/MS methods provided enhanced separation and sequence coverage for these glycopeptide isomers. Graphical Abstract ᅟ.

Assessing Physicochemical Properties of Drug Molecules via Microsolvation Measurements with Differential Mobility Spectrometry

The microsolvated state of a molecule, represented by its interactions with only a small number of solvent molecules, can play a key role in determining the observable bulk properties of the molecule. This is especially true in cases where strong local hydrogen bonding exists between the molecule and the solvent. One method that can probe the microsolvated states of charged molecules is differential mobility spectrometry (DMS), which rapidly interrogates an ion's transitions between a solvated and desolvated state in the gas phase (i.e., few solvent molecules present). However, can the results of DMS analyses of a class of molecules reveal information about the bulk physicochemical properties of those species? Our findings presented here show that DMS behaviors correlate strongly with the measured solution phase pK_a and pK_b values, and cell permeabilities of a set of structurally related drug molecules, even yielding high-resolution discrimination between isomeric forms of these drugs. This is due to DMS's ability to separate species based upon only subtle (yet predictable) changes in structure: the same subtle changes that can influence isomers' different bulk properties. Using 2-methylquinolin-8-ol as the core structure, we demonstrate how DMS shows promise for rapidly and sensitively probing the physicochemical properties of molecules, with particular attention paid to drug candidates at the early stage of drug development. This study serves as a foundation upon which future drug molecules of different structural classes could be examined.

Determination of triacylglycerol regioisomers using differential mobility spectrometry

RATIONALE

Triacylglycerols (TG) contain three fatty acyls attached to the glycerol backbone in stereochemically numbered positions sn-1, 2 and 3. Isobaric TG with exchanged fatty acyl chains in positions sn-1/3 vs. sn-2 are referred to as regioisomers and the determination of their regioisomeric ratios is important for nutrition purposes.

METHODS

Differential mobility spectrometry (DMS) coupled to electrospray ionization mass spectrometry (ESI-MS) is applied for the separation of simple unsaturated TG regioisomers extracted from porcine adipose tissue using their silver-ion molecular adducts.

RESULTS

Four pairs of TG regioisomers containing combinations of unsaturated and saturated fatty acyl chains are successfully separated using DMS with 1-butanol or 1-propanol as the chemical modifier. Various experimental parameters are carefully optimized, such as the separation and compensation voltages applied to DMS electrodes, the type and flow rate of chemical modifier and the dwell time of analyte ions in the DMS cell. The optimized DMS approach is used for the characterization of TG regioisomers in less than one minute, compared to tens of minutes typical for silver-ion or reversed-phase high-performance liquid chromatography/mass spectrometry approaches.

CONCLUSIONS

The application of this method for the characterization of TG regioisomers in porcine adipose tissue shows the method suitability for analyses of other animal fats.