Exploring crown ethers as shift reagents for ion mobility spectrometry

Effect of the Temperature on the Process of Preferential Solvation of 1,4-Dioxane, 12-Crown-4, 15-Crown-5 and 18-Crown-6 Ethers in the Mixture of N-Methylformamide with Water: Composition of the Solvation Shell of the Cyclic Ethers

The aim of the work was to analyze the preferential solvation process, and determine the composition of the solvation shell of cyclic ethers using the calorimetric method. The heat of solution of 1,4-dioxane, 12-crown-4, 15-crown-5 and 18-crown-6 ethers in the mixture of N-methylformamide with water was measured at four temperatures, 293.15 K, 298.15 K, 303.15 K, and 308.15 K, and the standard partial molar heat capacity of cyclic ethers has been discussed. 18-crown-6 (18C6) molecules can form complexes with NMF molecules through the hydrogen bonds between -CH₃ group of NMF and the oxygen atoms of 18C6. Using the model of preferential solvation, the cyclic ethers were observed to be preferentially solvated by NMF molecules. It has been proved that the molar fraction of NMF in the solvation shell of cyclic ethers is higher than that in the mixed solvent. The exothermic, enthalpic effect of preferential solvation of cyclic ethers increases with increasing ring size and temperature. The increase in the negative effect of the structural properties of the mixed solvent with increase in the ring size in the process of preferential solvation of the cyclic ethers indicates an increasing disturbance of the mixed solvent structure, which is reflected in the influence of the energetic properties of the mixed solvent.

Charge Inversion Ion/Ion Reactions Coupled to Ion Mobility/Mass Spectrometry: Oligosaccharides

Various ion mobility-based separation techniques and instruments have been recently developed to increase the operational resolution of ion mobility separations, especially of isomers and isobars. In addition to developments in instrumentation, different covalent and noncovalent derivatization techniques have helped achieve effective separations by magnifying minor differences in collision cross section. Among these methodologies is host-guest complex formation and, a new development presented herein, charge inversion ion-ion reactions coupled to ion mobility separations. We used these methods to enable formation of complexes between isomeric deprotonated oligosaccharides and alkaline earth metals (in solution) and alkaline earth metal-trisphenanthroline complexes (in vacuo), observing minor shifts in ion mobility arrival times for the charge inversion reaction products as well as unique mobility fingerprints indicative of separations of α/β anomers of disaccharides. For example, we have demonstrated separations between reducing disaccharides such as lactose and lactulose and nonreducing disaccharides. We also observed separations based on the pyranose/furanose configurations of the isomers. These results suggest the potential for ion/ion reactions to enable isomer separation of biomolecules from various compound classes using ion mobility-mass spectrometry (IM-MS).

Composition of the Solvation Shell of the Selected Cyclic Ethers (1,4-Dioxane, 12-Crown-4, 15-Crown-5 and 18-Crown-6) in a Mixture of Formamide with Water at Four Temperatures

The solution enthalpy of 15-crown-5 and 18-crown-6 ethers in the mixture of formamide (F) and water (W) was measured at four temperatures: 293.15 K, 298.15 K, 303.15 K, 308.15 K. The standard molar enthalpy of solution, ΔsolHo, depends on the size of cyclic ethers molecules and the temperature. With increasing temperature, the values of ΔsolHo become less negative. The values of the standard partial molar heat capacity Cp,2o of cyclic ethers at 298.15 K have been calculated. The Cp,2o=f(xW) curve shape indicates the hydrophobic hydration process of cyclic ethers in the range of a high-water content in the mixture with formamide. The enthalpic effect of preferential solvation of cyclic ethers was calculated and the effect of temperature on the preferential solvation process was discussed. The process of complex formation between 18C6 molecules and formamide molecules is observed. The cyclic ethers molecules are preferentially solvated by formamide molecules. The mole fraction of formamide in the solvation sphere of cyclic ethers has been calculated.

Assessing the Use of Host-Guest Chemistry in Conjunction with Cyclic Ion Mobility Separations for the Linkage-Specific Characterization of Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) are a class of glycans that are highly abundant in human milk and contribute to the healthy growth of an infant's immune system. While new advancements in analytical methodologies have been made in glycomics, the high degree of isomeric heterogeneity and lack of authentic standards have made the high-resolution separation and accurate characterization of linkage positioning of all HMO species very challenging. Herein, we present an evaluation of the use of host-guest chemistry in conjunction with cyclic ion mobility spectrometry-mass spectrometry (cIMS-MS)-based separations for the identification of linkage positioning in three pairs of di-, tetra-, and hexasaccharide HMO isomers that only differ in the positioning of one glycosidic linkage (β1,3 versus β1,4). Suitable hosts, such as α/β cyclodextrins, cucurbit[n]urils (n = 5, 7), crown ethers, cyclic peptides, and an ionophore, were used to assess host-guest inclusion complex formation as well as linkage-specific cIMS-MS trends. Our results indicated a linkage-specific trend for the [M + 2α + 2H]2+ cyclodextrin-based host-guest inclusion complexes where the β1,3 linkage-containing isomers were always higher mobility than the β1,4 linkage-containing ones as well one for the [M + α + β + 2H]2+ complexes where the β1,4 linkage-containing isomers were always higher mobility than the β1,3 linkage-containing ones. We also observed diagnostic mobility fingerprints for the cucurbituril-based complexes. We anticipate that linkage-specific and mobility fingerprint trends can potentially aid in identifying linkage positioning for other HMO isomers as well as in complex human milk samples.

Ion Mobility Shift Reagents for Lipid Double Bonds Based on Paternò-Büchi Photoderivatization with Halogenated Acetophenones

The Paternò-Büchi (PB) reaction is a cycloaddition reaction between a carbon-carbon double bond (C═C) and a photochemically excited carbonyl-containing compound. The constrained ring formed between the C═C bond and the PB reagent is more susceptible to fragmentation by collision-induced dissociation, which facilitates identification of the C═C position within the fatty acyl tails of lipids. Although the original PB reaction using acetone had a low yield of derivatized lipids and therefore a low yield of diagnostic ions, a new generation of PB reagents based on halogenated acetophenones has improved the reaction yield substantially. In this study, we investigated the use of halogenated PB reagents and ion mobility to improve the identification of PB-derivatized lipids by shifting them out of the densely populated lipid region of ion mobility-mass spectrometry (IM-MS) space. Several halogenated PB reagents containing fluorine, chlorine and bromine were investigated for their ability to decrease the collision cross-section (CCS) values of derivatized lipids and yield sufficient intensity for both the derivatized lipid and its diagnostic ions. We found that 4'-chloro-2',6'-difluoroacetophenone (CDFAP) displayed the best performance, with an average decrease in CCS of 4.4% and yield of derivatized lipids and diagnostic ions comparable to the trifluorinated acetophenone reagent proposed by the Xia group. The unique isotope pattern resulting from the chlorine substituent aided in identification of the derivatized lipids and their diagnostic ions, as well. We further demonstrate that derivatization with CDFAP preserves the separation of lipids classes in IM-MS space.

Improved Ion Mobility Separation and Structural Characterization of Steroids using Derivatization Methods

Steroids are an important class of biomolecules studied for their role in metabolism, development, nutrition, and disease. Although highly sensitive GC- and LC-MS/MS-based methods have been developed for targeted quantitation of known steroid metabolites, emerging techniques including ion mobility (IM) have shown promise in improved analysis and capacity to better identify unknowns in complex biological samples. Herein, we couple LC-IM-MS/MS with structurally selective reactions targeting hydroxyl and carbonyl functional groups to improve IM resolution and structural elucidation. We demonstrate that 1,1-carbonyldiimidazole derivatization of hydroxyl stereoisomer pairs such as testosterone/epitestosterone and androsterone/epiandrosterone results in increased IM resolution with ΔCCS > 15%. Additionally, performing this in parallel with derivatization of the carbonyl group by Girard's Reagent P resulted in unique products based on relative differences in number of each functional group and C17 alkylation. These changes could be easily deciphered using the combination of retention time, collision cross section, accurate mass, and MS/MS fragmentation pattern. Derivatization by Girard's Reagent P, which contains a fixed charge quaternary amine, also increased the ionization efficiency and could be explored for its potential benefit to sensitivity. Overall, the combination of these simple and easy derivatization reactions with LC-IM-MS/MS analysis provides a method for improved analysis of known target analytes while also yielding critical structural information that can be used for identification of potential unknowns.

Simultaneous Differentiation of C═C Position Isomerism in Fatty Acids through Ion Mobility and Theoretical Calculations

Fatty acids play a pivotal role in biological processes and have many isomers, particularly at the C═C position, that influence their biological function. Distinguishing between isomers is crucial to investigating their role in health and disease. However, separating the isomers poses a significant analytical challenge. In this study, we developed a simple and rapid strategy combining ion mobility spectrometry and theoretical chemical calculations to differentiate and quantify the C═C positional isomers in 2-/3-butenoic acid (BA), 2-/3-/4-pentenoic acid (PA), and 2-/3-/5-hexenoic acid (HA). C═C positional isomerism was mobility-differentiated by simple complexation with crown ethers (12C4, 15C5, and 18C6) and divalent metal ions (Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Sr²⁺, and Ba²⁺), that is, converting C═C positional isomers with small structural differences into complexes with large structural differences through the interaction with metal ions and crown ethers. Metallized isomers were formed but could not be differentiated due to their complex and overlapping extracted ion mobiliograms (EIMs). Binary crown ether-isomer complexes were not observed, indicating that C═C positional isomers could not be separated by simple mixing with crown ethers. However, significant EIM differences were obtained for the formed ternary complexes, allowing baseline separation for the isomers. Notably, all crown ethers and metal ions have a separation effect with the isomers, with a calculated separation resolution (R_p-p) of 0.07-2.44. Theoretical chemical calculations were performed to provide in-depth structural information for the complexes and explain the separation principle. Theoretical conformational space showed that the divalent metal ions act as a bridge connecting the crown ether and the isomer. Additionally, the ternary complex becomes more compact as the distance between C═C and -COOH increases. Theoretical results can reflect the features of mobility experiments, with relative errors between the experiment collision cross-section (CCS) and theoretical CCS of no more than ±8.06%. This method was also evaluated in terms of quantification, accuracy, and precision repeatability. Overall, this study establishes that the crown ether-metal ion pair can function as a robust unit for differentiating C═C positional isomerism.

Host-Guest Chemistry for Simultaneous Imaging of Endogenous Alkali Metals and Metabolites with Mass Spectrometry

Sodium and potassium are biological alkali metal ions that are essential for the physiological processes of cells and organisms. In combination with small-molecule metabolite information, disturbances in sodium and potassium tissue distributions can provide a further understanding of the biological processes in diseases. However, methods using mass spectrometry are generally tailored toward either elemental or molecular detection, which limits simultaneous quantitative mass spectrometry imaging of alkali metal ions and molecular ions. Here, we provide a new method by including crown ether molecules in the solvent for nanospray desorption electrospray ionization mass spectrometry imaging (nano-DESI MSI) that combines host–guest chemistry targeting sodium and potassium ions and quantitative imaging of endogenous lipids and metabolites. After evaluation and optimization, the method was applied to an ischemic stroke model, which has highly dynamic tissue sodium and potassium concentrations, and we report 2 times relative increase in the detected sodium concentration in the ischemic region compared to healthy tissue. Further, in the same experiment, we showed the accumulation and depletion of lipids, neurotransmitters, and amino acids using relative quantitation with internal standards spiked in the nano-DESI solvent. Overall, we demonstrate a new method that with a simple modification in liquid extraction MSI techniques using host–guest chemistry provides the added dimension of alkali metal ion imaging to provide unique insights into biological processes.

Determination of drugs and drug metabolites by ion mobility-mass spectrometry: A review

Ion mobility-mass spectrometry (IM-MS) has gained increased applications in the characterization and identification of drugs and drug metabolites, largely owning to the complementary separation of analyte ions based on their gas-phase size and shape in the IM dimension in addition to their mass-to-charge ratios. In this review, we discuss recent advances in such applications. We first introduce various types of IM techniques, focusing on those that allow the measurement of collision cross section (CCS), the physical property of an ion that reflects its gas-phase size and shape. Next, we discuss the IM-MS landscape of the large chemical space of drugs and multimodal distributions of certain drugs in IM separation due to the presence of protomers. We then review drug metabolism reactions and discuss the application of IM-MS in separation and identification of isomeric drug metabolites. Subsequently, we discuss various approaches to generate theoretical and predicted CCS data, including theory-based calculation methods and data-driven prediction models, and currently available resources on these approaches. Finally, current limitations and future directions of application of IM-MS are discussed.

Crown ethers as shift reagents in peptide epimer differentiation –conclusions from examination of ac-(H)FRW-NH2 petide sequences

Crown ethers with different ring sizes and substituents (18-crown-6, dibenzo-18-crown-6, dicyclohexano-18-crown-6, a chiral tetracarboxylic acid-18-crown-6 ether, dibenzo-21-crown-7, and dibenzo-30-crown-10) were evaluated as shift reagents to differentiate epimeric model peptides (tri-and tetrapeptides) using ion mobility mass spectrometry (IM-MS). The stable associates of peptide epimers with crown ethers were detected and examined using traveling-wave ion mobility time-of-flight mass spectrometer (Synapt G2-S HDMS) equipped with an electrospray ion source. The overall decrease of the epimer separation upon crown ether complexation was observed. The increase of the effectiveness of the microsolvation of a basic moiety - guanidine or ammonium group in the peptide had no or little effect on the epimer discrimination. Any increase of the epimer separation, which referred to the specific association mode between crown substituents and a given peptide sequence, was drastically reduced for the longer peptide sequence (tetrapeptide). The obtained results suggest that the application of the crown ethers as shift reagents in ion mobility mass spectrometry is limited to the formation of complexes differing in stoichiometry rather than it refers to a specific coordination mode between a crown ether and a peptide molecule.

Distinguishing enantiomeric amino acids with chiral cyclodextrin adducts and structures for lossless ion manipulations

Enantiomeric molecular evaluations remain an enormous challenge for current analytical techniques. To date, derivatization strategies and long separation times are generally required in these studies, and the development and implementation of new approaches are needed to increase speed and distinguish currently unresolvable compounds. Herein, we describe a method using chiral cyclodextrin adducts and structures for lossless ion manipulations (SLIM) and serpentine ultralong path with extended routing (SUPER) ion mobility (IM) to achieve rapid, high resolution separations of d and l enantiomeric amino acids. In the analyses, a chiral cyclodextrin is added to each sample. Two cyclodextrins were found to complex each amino acid molecule (i.e. potentially sandwiching the amino acid in their cavities) and forming host-guest noncovalent complexes that were distinct for each d and l amino acid pair studied and thus separable with IM in SLIM devices. The SLIM was also used to accumulate much larger ion populations than previously feasible for evaluation and therefore allow enantiomeric measurements of higher sensitivity, with gains in resolution from our ultralong path separation capabilities, than previously reported by any other IM-based approach.

Rapid Ion Mobility Separations of Bile Acid Isomers Using Cyclodextrin Adducts and Structures for Lossless Ion Manipulations

Bile acids (BAs) constitute an important class of steroid metabolites often displaying changes associated with disease states and other health conditions. Current analyses for these structurally similar compounds are limited by a lack of sensitivity and long separation times with often poor isomeric resolution. To overcome these challenges and provide rapid analyses for the BA isomers, we utilized cyclodextrin adducts in conjunction with novel ion mobility (IM) separation capabilities provided by structures for lossless ion manipulations (SLIM). Cyclodextrin was found to interact with both the tauro- and glyco-conjugated BA isomers studied, forming rigid noncovalent host-guest inclusion complexes. Without the use of cyclodextrin adducts, the BA isomers were found to be nearly identical in their respective mobilities and thus unable to be baseline resolved. Each separation of the cyclodextrin-bile acid host-guest inclusion complex was performed in less than 1 s, providing a much more rapid alternative to current liquid chromatography-based separations. SLIM provided capabilities for the accumulation of larger ion populations and IM peak compression that resulted in much higher resolution separations and increased signal intensities for the BA isomers studied.

Buffer gas additives (modifiers/shift reagents) in ion mobility spectrometry: Applications, predictions of mobility shifts, and influence of interaction energy and structure

Ion mobility spectrometry (IMS) is an analytical technique used for fast and sensitive detection of illegal substances in customs and airports, diagnosis of diseases through detection of metabolites in breath, fundamental studies in physics and chemistry, space exploration, and many more applications. Ion mobility spectrometry separates ions in the gas-phase drifting under an electric field according to their size to charge ratio. Ion mobility spectrometry disadvantages are false positives that delay transportation, compromise patient's health and other negative issues when IMS is used for detection. To prevent false positives, IMS measures the ion mobilities in 2 different conditions, in pure buffer gas or when shift reagents (SRs) are introduced in this gas, providing 2 different characteristic properties of the ion and increasing the chances of right identification. Mobility shifts with the introduction of SRs in the buffer gas are due to clustering of analyte ions with SRs. Effective SRs are polar volatile compounds with free electron pairs with a tendency to form clusters with the analyte ion. Formation of clusters is favored by formation of stable analyte ion-SR hydrogen bonds, high analytes' proton affinity, and low steric hindrance in the ion charge while stabilization of ion charge by resonance may disfavor it. Inductive effects and the number of adduction sites also affect cluster formation. The prediction of IMS separations of overlapping peaks is important because it simplifies a trial and error procedure. Doping experiments to simplify IMS spectra by changing the ion-analyte reactions forming the so-called alternative reactant ions are not considered in this review and techniques other than drift tube IMS are marginally covered.

Ion mobility and gas phase H/D exchange: revealing the importance of a single hydrogen bond for the chiral recognition of crown ether ammonium complexes

Two new BINOL-based chiral crown ether/ammonium complexes are studied by travelling-wave ion-mobility spectrometry. Homo- and heterochiral crown ether/ammonium complexes differ in their collision cross sections, and these differences go along with changes in hydrogen bonding as revealed by gas phase H/D-exchange experiments. Applications for the determination of enantiomeric excess are discussed.

Quantitative Collision Cross-Sections from FTICR Linewidth Measurements: Improvements in Theory and Experiment

Two corrections to the equation used in the cross-sectional areas by Fourier transform ion cyclotron resonance ("CRAFTI") technique are identified. In CRAFTI, ion collision cross-sections are obtained from the pressure-dependent ion linewidths in Fourier transform mass spectra. The effects of these corrections on the accuracy of the cross-sections obtained using the CRAFTI technique are evaluated experimentally using the 20 biogenic amino acids and several crown ether complexes with protonated alkyl monoamines. Good absolute agreement is obtained between the CRAFTI cross-sections and the corresponding cross-sections obtained using both static drift ion mobility spectrometry and computational simulations. These results indicate that the CRAFTI cross-sections obtained using the updated equation presented here are quantitatively descriptive of the size and shape of the gas-phase ions. Cross-sections that differ by less than 3% are measured for the isobaric isomers n-butylamine and tert-butylamine complexed with the crown ethers. This level of precision is similar to what has been achieved previously using traveling wave ion mobility devices. These results indicate that CRAFTI can be used to probe subtle structural differences between ions with approximately the same precision as that achieved in traveling wave ion mobility devices. Graphical Abstract ᅟ.

Recent advances in ion mobility-mass spectrometry for improved structural characterization of glycans and glycoconjugates

Glycans and glycoconjugates are involved in regulating a vast array of cellular and molecular processes. Despite the importance of glycans in biology and disease, characterization of glycans remains difficult due to their structural complexity and diversity. Mass spectrometry (MS)-based techniques have emerged as the premier analytical tools for characterizing glycans. However, traditional MS-based strategies struggle to distinguish the large number of coexisting isomeric glycans that are indistinguishable by mass alone. Because of this, ion mobility spectrometry coupled to MS (IM-MS) has received considerable attention as an analytical tool for improving glycan characterization due to the capability of IM to resolve isomeric glycans before MS measurements. In this review, we present recent improvements in IM-MS instrumentation and methods for the structural characterization of isomeric glycans. In addition, we highlight recent applications of IM-MS that illustrate the enormous potential of this technology in a variety of research areas, including glycomics, glycoproteomics, and glycobiology.

Crown ethers attenuate aggregation of amyloid beta of Alzheimer's disease

In this report, we reasoned that non-covalent modification of amyloid beta (Aβ) by crown ethers could inhibit its aggregation. We demonstrated that PiB-C, a conjugate PiB and crown ether, could significantly reduce the aggregation in vitro. Additionally, two-photon imaging showed that PiB-C could efficiently label Aβ plaques and CAAs in AD mice.

Improved isobaric tandem mass tag quantification by ion mobility mass spectrometry

RATIONALE

Isobaric tandem mass tags are an attractive alternative to mass difference tags and label-free approaches for quantitative proteomics due to the high degree of multiplexing that can be performed with their implementation. A drawback of tandem mass tags are that the co-isolation and co-fragmentation of labeled peptide precursors can result in chimeric tandem mass (MS/MS) spectra that can underestimate the fold-change expression of each peptide. Ion mobility (IM) separations coupled to quadrupole time-of-flight (Q-TOF) instruments have the potential to mitigate MS/MS spectra chimeracy since IM-MS has the ability to separate ions based on charge, m/z, and collision cross section (CCS).

METHODS

Two complex protein mixtures, labeled with DiLeu isobaric tandem mass tags in opposite ratios, were mixed together and analyzed by data-dependent LC/IM-MS/MS. The accuracy of reporters from interfering pairs was compared with and without IM separation.

RESULTS

IM separation was able to mitigate isobaric interference from differentially charged interfering ion pairs, as well as pairs of the same charge. Of the eight example precursors shown, only one had reporters that remained compressed below the significance threshold after IM separation.

CONCLUSIONS

The results of this investigation demonstrate proof-of-principle that IM separation of tagged precursors prior to MS/MS fragmentation can help mitigate quantitative inaccuracies caused by isobaric interference. Future improvements of the method would include software for automated correction and use of higher resolution IM instrumentations.

Multiplexed Analysis of Peptide Functionality Using Lanthanide-based Structural Shift Reagents

Functionally selective lanthanide-based ion mobility shift reagents are presented as a method to elucidate protein or peptide structural information as well as relative quantitation of protein expression profiles. Sequence information and site localization of primary amines (n-terminus and lysine), phosphorylation sites, and cysteine residues can be obtained in a data dependent manner using ion mobility-mass spectrometry (IM-MS). The high mass of the incorporated lanthanide ensures a significant shift of where the signal occurs in IM-MS conformation space. Peptide sequence information provided by the use of IM-MS shift reagents allows for both a more confident identification of peptides from complex mixtures and site localization following tandem MS experiments. Stable isotopes of the lanthanide series may also be used as relative quantitation labels since several lanthanides can be utilized in differential sample analyses.

Biologically-inspired peptide reagents for enhancing IMS-MS analysis of carbohydrates

The binding properties of a peptidoglycan recognition protein are translated via combinatorial chemistry into short peptides. Non-adjacent histidine, tyrosine, and arginine residues in the protein's binding cleft that associate specifically with the glycan moiety of a peptidoglycan substrate are incorporated into linear sequences creating a library of 27 candidate tripeptide reagents (three possible residues permutated across three positions). Upon electrospraying the peptide library and carbohydrate mixtures, some noncovalent complexes are observed. The binding efficiencies of the peptides vary according to their amino acid composition as well as the disaccharide linkage and carbohydrate ring-type. In addition to providing a charge-carrier for the carbohydrate, peptide reagents can also be used to differentiate carbohydrate isomers by ion mobility spectrometry. The utility of these peptide reagents as a means of enhancing ion mobility analysis of carbohydrates is illustrated by examining four glucose-containing disaccharide isomers, including a pair that is not resolved by ion mobility alone. The specificity and stoichiometry of the peptide-carbohydrate complexes are also investigated. Trihistidine demonstrates both suitable binding efficiency and successful resolution of disaccharides isomers, suggesting it may be a useful reagent in IMS analyses of carbohydrates.

Shift reagents for multidimensional ion mobility spectrometry-mass spectrometry analysis of complex peptide mixtures: evaluation of 18-crown-6 ether complexes

18-Crown-6 ether (18C6) is evaluated as a shift reagent for multidimensional ion mobility spectrometry-mass spectrometry (IMS-IMS-MS) analyses of tryptic protein digests. In this approach, 18C6 is spiked into the solution-phase mixture and noncovalent peptide-crown ion complexes are formed by electrospraying the mixture into the gas phase. After an initial mobility separation in the first IMS drift region, complexes of similar mobility are selected and dissociated via collisional activation prior to entering the second drift region. These dissociation products (including smaller complexes, naked peptide ions, charge transfer products, and fragment ions) differ in mobility from their precursor ion complexes and (in favorable cases) from one another, allowing the mixture to resolve further in the second IMS region. We estimate an IMS-IMS peak capacity of ~2400 when shift reagents are employed. The approach is illustrated by examining a tryptic digest of cytochrome c and by identifying a peptide out of a complex mixture obtained by digestion of human plasma proteins. Disadvantages arising from increased complexity of data sets as well as other advantages of this approach are considered.

Complexation of amino compounds by 18C6 improves selectivity by IMS-IMS-MS: application to petroleum characterization

Complexation of a series of related amino compounds by 18-crown-6 ether (18C6) is studied as a means of improving the resolution of mixtures by combinations of ion mobility spectrometry (IMS) and mass spectrometry (MS) techniques. Mixtures of the isomeric amines n-octylamine (NOA), dibutylamine (DBA), and diisopropylethylamine (DIPEA) were electrosprayed to produce gaseous [M + H](+) ions. These species have overlapping mobilities and are not resolved by IMS. Addition of 18C6 yields [M + 18C6 + H](+) ion complexes that are resolved by IMS. In subsequent experiments, [M + 18C6 + H](+) ion complexes are separated according to their mobilities and specific species are selected and exposed to collisional activation. This analysis yields dissociation voltages that are inversely correlated with the number of separate substitutions on the nitrogen atom of the amino compounds; dissociation voltages of ~40, ~90, and ~150 V are obtained for the tri-, di-, and mono-substituted amino compounds DIPEA, DBA, and NOA, respectively. For these complexes, an inverse correlation is also observed with respect to the gas-phase basicities (GB) of the amino compounds (964, 935, and 895 kJ mol(-1), respectively). Studies of 18C6 complexes with a series of n-alkylamines (C( n )H(2n+3)N where n=3 to 18, respectively) show that dissociation voltages increase systematically (from ~140 to ~190 V) under the conditions employed. The sensitivity to collision energy provides an additional means of distinguishing between classes of compounds. The approach is extended as a means of separating nitrogen-containing compounds from petroleum.

Electrospray ionization tandem mass spectrometry of ammonium cationized polyethers

Quaternary ammonium salts (Quats) and amines are known to facilitate the MS analysis of high molar mass polyethers by forming low charge state adduct ions. The formation, stability, and behavior upon collision-induced dissociation (CID) of adduct ions of polyethers with a variety of Quats and amines were studied by electrospray ionization quadrupole time-of-flight, quadrupole ion trap, and linear ion trap tandem mass spectrometry (MS/MS). The linear ion trap instrument was part of an Orbitrap hybrid mass spectrometer that allowed accurate mass MS/MS measurements. The Quats and amines studied were of different degree of substitution, structure, and size. The stability of the adduct ions was related to the structure of the cation, especially the amine's degree of substitution. CID of singly/doubly charged primary and tertiary ammonium cationized polymers resulted in the neutral loss of the amine followed by fragmentation of the protonated product ions. The latter reveals information about the monomer unit, polymer sequence, and endgroup structure. In addition, the detection of product ions retaining the ammonium ion was observed. The predominant process in the CID of singly charged quaternary ammonium cationized polymers was cation detachment, whereas their doubly charged adduct ions provided the same information as the primary and tertiary ammonium cationized adduct ions. This study shows the potential of specific amines as tools for the structural elucidation of high molar mass polyethers.

A novel approach to collision-induced dissociation (CID) for ion mobility-mass spectrometry experiments

Collision induced dissociation (CID) combined with matrix assisted laser desorption ionization-ion mobility-mass spectrometry (MALDI-IM-MS) is described. In this approach, peptide ions are separated on the basis of mobility in a 15 cm drift cell. Following mobility separation, the ions exit the drift cell and enter a 5 cm vacuum interface with a high field region (up to 1000 V/cm) to undergo collisional activation. Ion transmission and ion kinetic energies in the interface are theoretically evaluated accounting for the pressure gradient, interface dimensions, and electric fields. Using this CID technique, we have successfully fragmented and sequenced a number of model peptide ions as well as peptide ions obtained by a tryptic digest. This instrument configuration allows for the simultaneous determination of peptide mass, peptide-ion sequence, and collision-cross section of MALDI-generated ions, providing information critical to the identification of unknown components in complex proteomic samples.

The use of shift reagents in ion mobility-mass spectrometry: studies on the complexation of an active pharmaceutical ingredient with polyethylene glycol excipients

Gas-phase ion mobility studies of mixtures containing polyethylene glycols (PEG) and an active pharmaceutical ingredient (API), lamivudine, have been carried out using electrospray ionization-ion mobility spectrometry-quadrupole-time-of-flight mass spectrometry (ESI-IMS-Q-TOF). In addition to protonated and cationized PEG oligomers, a series of high molecular weight ions were observed and identified as noncovalent complexes formed between lamivudine and PEG oligomers. The noncovalent complex ions were dissociated using collision induced dissociation (CID) after separation in the ion mobility drift tube to recover the protonated lamivudine free from interfering matrix ions and with a drift time associated with the precursor complex. The potential of PEG excipients to act as "shift reagents," which enhance selectivity by moving the mass/mobility locus to an area of the spectrum away from interferences, is demonstrated for the analysis of lamivudine in a Combivir formulation containing PEG and lamivudine.

Enhanced carbohydrate structural selectivity in ion mobility-mass spectrometry analyses by boronic acid derivatization

The boronic acid derivatization of carbohydrates is demonstrated as an ion mobility shift strategy to improve confidence in the identification and characterization of carbohydrate assignments using ion mobility-mass spectrometry.

Biomolecule analysis by ion mobility spectrometry

Although nonnative protein conformations, including intermediates along the folding pathway and kinetically trapped misfolded species that disfavor the native state, are rarely isolated in the solution phase, they are often stable in the gas phase, where macromolecular ions from electrospray ionization can exist in varying charge states. Differences in the structures of nonnative conformations in the gas phase are often large enough to allow different shapes and charge states to be separated because of differences in their mobilities through a gas. Moreover, gentle collisional activation can be used to induce structural transformations. These new structures often have different mobilities. Thus, there is the possibility of developing a multidimensional separation that takes advantage of structural differences of multiple stable states. This review discusses how nonnative states differ in the gas phase compared with solution and presents an overview of early attempts to utilize and manipulate structures in order to develop ion mobility spectrometry as a rapid and sensitive technique for separating complex mixtures of biomolecules prior to mass spectrometry.

Assessing the peak capacity of IMS-IMS separations of tryptic peptide ions in He at 300 K

Two-dimensional ion mobility spectrometry (IMS-IMS) coupled with mass spectrometry is examined as a means of separating mixtures of tryptic peptides (from myoglobin and hemoglobin). In this study, we utilize two distinct drift regions that are identical in that each contains He buffer gas at 300 K. The two-dimensional advantage is realized by changing the structures of the ions. As ions arrive at the end of the first drift region, those of a specified mobility are selected, exposed to energizing collisions, and then introduced into a second drift region. Upon collisional activation, some ions undergo structural transitions, leading to substantial changes in their mobilities; others undergo only slight (or no) mobility changes. Examination of peak positions and shapes for peptides that are separated in the first IMS dimension indicates experimental peak capacities ranging from approximately 60 to 80; the peak shapes and range of changes in mobility that are observed in the second drift region (after activation) indicate a capacity enhancement ranging from a factor of approximately 7 to 17. Thus, experimental (and theoretical) evaluation of the peak capacity of IMS-IMS operated in this fashion indicates that capacities of approximately 480 to 1360 are accessible for peptides. Molecular modeling techniques are used to simulate the range of structural changes that would be expected for tryptic peptide ions and are consistent with the experimental shifts that are observed.