Evolution of Hydrogen-Bond Networks in Protonated Water Clusters H(+)(H2O)n (n = 1 to 120) Studied by Cryogenic Ion Mobility-Mass Spectrometry

Hydrogen bond network structures of protonated dimethylamine clusters H+(DMA)n (n = 3-7)

Infrared spectroscopy of protonated dimethylamine clusters, H+(DMA)n, (n = 3-7), and their Ar-tagged clusters was performed in the NH and CH stretching vibrational region to explore their hydrogen bond network structures. A stable isomer search and vibrational spectral simulations of the clusters were also carried out to support the interpretations of the observed spectra. Weakly hydrogen-bonded NH stretching vibrational bands, which are characteristic of cyclic structures of small-sized protonated clusters, are observed in the spectra of the Ar-tagged clusters of n ≥ 5, while only linear chain type structures are suggested for the Ar-tagged clusters of n = 3-4 and the bare clusters of all the sizes. These results demonstrate that the size and temperature dependence of the hydrogen bond network structures of the protonated dimethylamine clusters is analogous to that of protonated monohydric alcohol clusters.

On the Arrival Time Distribution of Reacting Systems in Ion Mobility Spectrometry

Ion mobility spectrometry (IMS) is a widely used gas-phase separation technique, particularly when coupled with mass spectrometry (MS). Modern IMS instruments often apply elevated reduced field strengths for improved ion separation and ion focusing. These alter the collision dynamics and further drive ion reaction processes that can change the analyte's structure. As a result, the measured arrival time distribution (ATD) can change with the applied reduced field strengths. In this work, we systematically study how the ion collision dynamics and the ion reaction dynamics, as a function of the reduced field strength, can alter the ATD. To this end, we investigate 2,6-di-tert-butylpyridine, methanol, and ethyl acetate using a home-built drift tube IMS coupled to a home-built MS and extensive first-principles Monte Carlo modeling. We show how elevated reduced field strengths can actually lower resolving power through increased ion diffusion and how the field dependency of the ion mobility can introduce uncertainties to collision cross sections (CCS) calculated from the measured mobilities. On top of the collision dynamics, we show how chemical transformation processes that alter the analyte's CCS, e.g., dynamic clustering or fragmentation, can lead to broadened, shifted, or non-Gaussian ATDs and how sensitive these processes are to the applied field strengths. We highlight how first-principles ion dynamics simulations can help to understand and even harness the mentioned effects.

Exploration of the Existence Forms and Patterns of Dissolved Oxygen Molecules in Water

The structure of liquid water is primarily composed of three-dimensional networks of water clusters formed by hydrogen bonds, and dissolved oxygen is one of the most important indicators for assessing water quality. In this work, distilled water with different concentration of dissolved oxygen were prepared, and a clear negative correlation between the size of water clusters and dissolved oxygen concentration was observed. Besides, a phenomenon of rapid absorption and release of oxygen at the water interfaces was unveiled, suggesting that oxygen molecules predominantly exist at the interfaces of water clusters. Oxygen molecules can move rapidly through the interfaces among water clusters, allowing dissolved oxygen to quickly reach a saturation level at certain partial pressure of oxygen and temperature. Further exploration into the mechanism by molecular dynamics simulations of oxygen and water clusters found that oxygen molecules can only exist stably at the interfaces among water clusters. A semi-empirical formula relating the average number of water molecules in a cluster (n) to 17O NMR half-peak width (W) was summarized: n = 0.1 W + 0.85. These findings provide a foundation for exploring the structure and properties of water.

Threshold collision induced dissociation of protonated water clusters

We report threshold collision induced dissociation experiments on protonated water clusters thermalized at low temperature for sizes n = 19-23. Fragmentation cross sections are recorded as a function of the collision energy and analyzed with a statistical model. This model allows us to account for dissociation cascades and provides values for the dissociation energies of each cluster. These values, averaging around 0.47 eV, are in good agreement with theoretical predictions at various levels of theory. Furthermore, the dissociation energies show a trend for the n = 21 magic and n = 22 anti-magic numbers relative to their neighbours, which is also in agreement with theory. These results provide further evidence to resolve the disagreement between previously published experimental values. A careful quantitative treatment of cascade dissociation in this model introduces interdependence between the dissociation energies of neighboring sizes, which reduces the number of free fitting parameters and improves both reliability and uncertainties on absolute dissociation energies deduced from experiments.

Acidic Centers on the Surface of a Crystalline α-Sn(IV) Phosphate Characterized by the Solid-State 1H, 2H, 31P, and 119Sn MAS NMR Techniques

A layered crystalline phosphate α-Sn(HPO4)2·H2O (1), prepared and characterized in the present study by the multinuclear solid-state nuclear magnetic resonance (NMR), powder X-ray diffraction, and thermogravimetric analysis techniques, was treated with D2O and HOD imitating the reaction conditions in a water medium. The 2H solid-echo magic angle spinning NMR spectra of the products have revealed on their surface low mobile water molecules and hydronium ions, forming a structure close to the Zundel cation, [D2O···D-OD2]+. All the deuterons in the hydronium ions are tangled by hydrogen bonds with the water and the surface phosphate groups and stabilized by ionic interactions.

Photoelectron Circular Dichroism of Electrosprayed Gramicidin Anions

Many sophisticated approaches for analyzing properties of chiral matter have been developed in recent years. But in general, the available chiroptical methods are limited to either solvated or small gaseous molecules. Studying the chirality of large biopolymers in the gas phase, including aspects of the secondary structure, becomes accessible by combining the electrospray ionization technique with chiroptical detection protocols. Here, laser-induced photodetachment from gramicidin anions, a peptide consisting of 15 amino acids has been investigated. The angular distribution of photoelectrons is demonstrated to be sensitive to the substitution of protons by cesium ions, which is accompanied by a conformational change. The photoelectron circular dichroism (PECD) is -0.5% for bare gramicidin, whereas gramicidin with several Cs⁺ ions attached exhibits a PECD of +0.5%. The results are complemented and supported by ion mobility studies. The presented approach offers the prospect of studying chirality and the secondary structure of various biopolymers.

Large Conformational Change in the Isomerization of Flexible Crown Ether Observed at Low Temperature

The dynamic processes of conformational changes of supramolecules are important to understand the motion in synthetic supramolecules. Although a host-guest complex is the most basic supramolecule, a detailed mechanism of its conformational changes has rarely been studied. Here, we observed the large conformational change of a dibenzo-24-crown-8 complex with four guest ions (Ag⁺, Na⁺, K⁺, and NH₄⁺) at low temperature in the gas phase. The isomerization between the two types of conformers, which have different distances between the two benzene rings, proceeds even at 86 K. Using variable-temperature ion mobility-mass spectrometry (IM-MS) at 100-210 K, the activation energy for the isomerization is determined to be rather small (4.8-9.0 kJ mol^-1). Reaction pathway calculations revealed that the isomerization is caused by the sequential rotation of two single bonds in the crown ether ring. The present cryogenic IM-MS study of the host-guest complexes at the molecular level opens an approach to detailed understanding of the motion in supramolecules.

Conformer Separation of Dibenzo-Crown-Ether Complexes with Na+ and K+ Ions Studied by Cryogenic Ion Mobility-Mass Spectrometry

We performed cryogenic ion mobility-mass spectrometry (IM-MS) to study conformations of dibenzo-crown-ether complexes with Na⁺ and K⁺ ions at 86 K in the gas phase. Four dibenzo-crown-ethers (dibenzo-18-crown-6, dibenzo-21-crown-7, dibenzo-24-crown-8, and dibenzo-30-crown-10) with different cavity ring sizes were investigated. For dibenzo-18-crown-6 complexes with Na⁺ and K⁺, only one type of conformer was assigned by comparing the experimental collision cross sections with those predicted theoretically for candidate structures. In this conformer, the distance between two benzene rings in the complexes was long due to the open form of the dibenzo-18-crown-6. This open conformer was consistent with the previous laser spectroscopic studies of the cold complex ions in the gas phase. For dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with Na⁺ and K⁺, two types of conformers were clearly separated by IM-MS. These two conformer types were assigned to "open" and "closed" forms in which benzene-benzene distances were long and short, respectively. Observed relative abundances of the open and closed conformers qualitatively agreed with the Boltzmann distribution using Gibbs energies of the conformers calculated by quantum chemical calculations. For the Na⁺(dibenzo-30-crown-10) complex, open and closed conformers were also observed in IM-MS. On the other hand, only the closed conformer was observed for the K⁺(dibenzo-30-crown-10) complex. This closed conformer was similar to the "wraparound" structure, which was proposed in the previous studies in the solution. In conclusion, the closed conformers were formed by the deformation of flexible crown ethers with large cavity ring sizes. In addition, the diameter of the K⁺ ion was suitable to form the closed conformer by deformation of the molecular structure of dibenzo-30-crown-10.

Hydration of Guanidinium Ions: An Experimental Search for Like-Charged Ion Pairs

Guanidinium ions (GdmH⁺) are reported to form stable complexes (GdmH⁺/GdmH⁺) in aqueous solution despite strong repulsive interactions between the like-charged centers. These complexes are thought to play important roles in protein folding, membrane penetration, and formation of protein dimers. Although GdmH⁺ ions are weakly hydrated, semiempirical calculations provide evidence that these like-charged complexes are stabilized by water molecules, which serve important structural and energetic roles. Specifically, water molecules bridge between the GdmH⁺ ions of GdmH⁺/GdmH⁺ complexes as well as complexes involving the guanidinium side chains of arginine. Potential biological significances of like-charged complexes have been largely confirmed by ab initio molecular dynamics simulations and indirect experimental evidence. We report cryo-ion mobility-mass spectrometry results for the GdmH⁺/GdmH⁺ ion pair confined in a nanodroplet- the first direct experimental observation of this like-charged complex. A second like-charged complex, described as a water-mediated complex involving GdmH⁺ and H₃O⁺, was also observed.

Strategy for Modeling the Infrared Spectra of Ion-Containing Water Drops

Hydrated ions are ubiquitous in environmental and biological media. Understanding the perturbation exerted by an ion on the water hydrogen bond network is possible in the nanodrop regime by recording vibrational spectra in the O-H bond stretching region. This has been achieved experimentally in recent years by forming gaseous ions containing tens to hundreds of water molecules and recording their infrared photodissociation spectra. In this paper, we demonstrate the capabilities of a modeling strategy based on an extension of the AMOEBA polarizable force field to implement water atomic charge fluctuations along with those of intramolecular structure along the dynamics. This supplementary flexibility of nonbonded interactions improves the description of the hydrogen bond network and, therefore, the spectroscopic response. Finite temperature IR spectra are obtained from molecular dynamics simulations by computing the Fourier transform of the dipole moment autocorrelation function. Simulations of 1-2 ns are required for extensive sampling in order to reproduce the experimental spectra. Furthermore, bands are assigned with the driven molecular dynamics approach. This method package is shown to compare successfully with experimental spectra for 11 ions in water drops containing 36-100 water molecules. In particular, band frequency shifts of the free O-H stretching modes at the cluster surface are well reproduced as a function of both ion charge and drop size.

Design and Application of a High-Temperature Linear Ion Trap Reactor

A high-temperature linear ion trap reactor with hexapole design was homemade to study ion-molecule reactions at variable temperatures. The highest temperature for the trapped ions is up to 773 K, which is much higher than those in available reports. The reaction between V₂O₆^- cluster anions and CO at different temperatures was investigated to evaluate the performance of this reactor. The apparent activation energy was determined to be 0.10 ± 0.02 eV, which is consistent with the barrier of 0.12 eV calculated by density functional theory. This indicates that the current experimental apparatus is prospective to study ion-molecule reactions at variable temperatures, and more kinetic details can be obtained to have a better understanding of chemical reactions that have overall barriers. Graphical Abstract.

Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops

The effects of ion charge, polarity and size on the surface morphology of size-selected aqueous nanodrops containing a single ion and up to 550 water molecules are investigated with infrared photodissociation (IRPD) spectroscopy and theory.

The effects of ion charge, polarity and size on the surface morphology of size-selected aqueous nanodrops containing a single ion and up to 550 water molecules are investigated with infrared photodissociation (IRPD) spectroscopy and theory. IRPD spectra of M(H₂O)_n where M = La³⁺, Ca²⁺, Na⁺, Li⁺, I^–, SO₄^2– and supporting molecular dynamics simulations indicate that strong interactions between multiply charged ions and water molecules can disrupt optimal hydrogen bonding (H-bonding) at the nanodrop surface. The IRPD spectra also reveal that “free” OH stretching frequencies of surface-bound water molecules are highly sensitive to the ion's identity and the OH bond's local H-bond environment. The measured frequency shifts are qualitatively reproduced by a computationally inexpensive point-charge model that shows the frequency shifts are consistent with a Stark shift from the ion's electric field. For multiply charged cations, pronounced Stark shifting is observed for clusters containing ∼100 or fewer water molecules. This is attributed to ion-induced solvent patterning that extends to the nanodrop surface, and serves as a spectroscopic signature for a cation's ability to influence the H-bond network of water located remotely from the ion. The Stark shifts measured for the larger nanodrops are extrapolated to infinite dilution to obtain the free OH stretching frequency of a surface-bound water molecule at the bulk air–water interface (3696.5–3701.0 cm^–1), well within the relatively wide range of values obtained from SFG measurements. These cluster measurements also indicate that surface curvature effects can influence the free OH stretching frequency, and that even nanodrops without an ion have a surface potential that depends on cluster size.

Cryogenic Ion Mobility-Mass Spectrometry: Tracking Ion Structure from Solution to the Gas Phase

Electrospray ionization (ESI) combined with ion mobility-mass spectrometry (IM-MS) is adding new dimensions, that is, structure and dynamics, to the field of biological mass spectrometry. There is increasing evidence that gas-phase ions produced by ESI can closely resemble their solution-phase structures, but correlating these structures can be complicated owing to the number of competing effects contributing to structural preferences, including both inter- and intramolecular interactions. Ions encounter unique hydration environments during the transition from solution to the gas phase that will likely affect their structure(s), but many of these structural changes will go undetected because ESI-IM-MS analysis is typically performed on solvent-free ions. Cryogenic ion mobility-mass spectrometry (cryo-IM-MS) takes advantage of the freeze-drying capabilities of ESI and a cryogenically cooled IM drift cell (80 K) to preserve extensively solvated ions of the type [M + xH](x+)(H2O)n, where n can vary from zero to several hundred. This affords an experimental approach for tracking the structural evolution of hydrated biomolecules en route to forming solvent-free gas-phase ions. The studies highlighted in this Account illustrate the varying extent to which dehydration can alter ion structure and the overall impact of cryo-IM-MS on structural studies of hydrated biomolecules. Studies of small ions, including protonated water clusters and alkyl diammonium cations, reveal structural transitions associated with the development of the H-bond network of water molecules surrounding the charge carrier(s). For peptide ions, results show that water networks are highly dependent on the charge-carrying species within the cluster. Specifically, hydrated peptide ions containing lysine display specific hydration behavior around the ammonium ion, that is, magic number clusters with enhanced stability, whereas peptides containing arginine do not display specific hydration around the guanidinium ion. Studies on the neuropeptide substance P illustrate the ability of cryo-IM-MS to elucidate information about heterogeneous ion populations. Results show that a kinetically trapped conformer is stabilized by a combination of hydration and specific intramolecular interactions, but upon dehydration, this conformer rearranges to form a thermodynamically favored gas-phase ion conformation. Finally, recent studies on hydration of the protein ubiquitin reveal water-mediated dimerization, thereby illustrating the extension of this approach to studies of large biomolecules. Collectively, these studies illustrate a new dimension to studies of biomolecules, resulting from the ability to monitor snapshots of the structural evolution of ions during the transition from solution to gas phase and provide unparalleled insights into the intricate interplay between competing effects that dictate conformational preferences.

Stepwise formation of H3O(+)(H2O)n in an ion drift tube: Empirical effective temperature of association/dissociation reaction equilibrium in an electric field

We measured equilibrium constants for H3O(+)(H2O)n-1 + H2O↔H3O(+)(H2O)n (n = 4-9) reactions taking place in an ion drift tube with various applied electric fields at gas temperatures of 238-330 K. The zero-field reaction equilibrium constants were determined by extrapolation of those obtained at non-zero electric fields. From the zero-field reaction equilibrium constants, the standard enthalpy and entropy changes, ΔHn,n-1 (0) and ΔSn,n-1 (0), of stepwise association for n = 4-8 were derived and were in reasonable agreement with those measured in previous studies. We also examined the electric field dependence of the reaction equilibrium constants at non-zero electric fields for n = 4-8. An effective temperature for the reaction equilibrium constants at non-zero electric field was empirically obtained using a parameter describing the electric field dependence of the reaction equilibrium constants. Furthermore, the size dependence of the parameter was thought to reflect the evolution of the hydrogen-bond structure of H3O(+)(H2O)n with the cluster size. The reflection of structural information in the electric field dependence of the reaction equilibria is particularly noteworthy.

Fundamentals of Trapped Ion Mobility Spectrometry Part II: Fluid Dynamics

Trapped ion mobility spectrometry (TIMS) is a new high resolution (R up to ~300) separation technique that utilizes an electric field to hold ions stationary against a moving gas. Recently, an analytical model for TIMS was derived and, in part, experimentally verified. A central, but not yet fully explored, component of the model involves the fluid dynamics at work. The present study characterizes the fluid dynamics in TIMS using simulations and ion mobility experiments. Results indicate that subsonic laminar flow develops in the analyzer, with pressure-dependent gas velocities between ~120 and 170 m/s measured at the position of ion elution. One of the key philosophical questions addressed is: how can mobility be measured in a dynamic system wherein the gas is expanding and its velocity is changing? We noted previously that the analytically useful work is primarily done on ions as they traverse the electric field gradient plateau in the analyzer. In the present work, we show that the position-dependent change in gas velocity on the plateau is balanced by a change in pressure and temperature, ultimately resulting in near position-independent drag force. That the drag force, and related variables, are nearly constant allows for the use of relatively simple equations to describe TIMS behavior. Nonetheless, we derive a more comprehensive model, which accounts for the spatial dependence of the flow variables. Experimental resolving power trends were found to be in close agreement with the theoretical dependence of the drag force, thus validating another principal component of TIMS theory.

Gated Trapped Ion Mobility Spectrometry Coupled to Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Analysis of molecules by ion mobility spectrometry coupled with mass spectrometry (IMS-MS) provides chemical information on the three dimensional structure and mass of the molecules. The coupling of ion mobility to trapping mass spectrometers has historically been challenging due to the large differences in analysis time between the two devices. In this paper we present a modification of the trapped ion mobility (TIMS) analysis scheme termed "Gated TIMS" that allows efficient coupling to a Fourier Transform Ion Cyclotron Resonance (FT-ICR) analyzer. Analyses of standard compounds and the influence of source conditions on the TIMS distributions produced by ion mobility spectra of labile ubiquitin protein ions are presented. Ion mobility resolving powers up to 100 are observed. Measured collisional cross sections of ubiquitin ions are in excellent qualitative and quantitative agreement to previous measurements. Gated TIMS FT-ICR produces results comparable to those acquired using TIMS/time-of-flight MS instrument platforms as well as numerous drift tube IMS-MS studies published in the literature.

Water-Mediated Dimerization of Ubiquitin Ions Captured by Cryogenic Ion Mobility-Mass Spectrometry

The dynamics, structures, and functions of most biological molecules are strongly influenced by the nature of the peptide's or protein's interaction with water. Here, cryogenic ion mobility-mass spectrometry studies of ubiquitin have directly captured a water-mediated protein-protein binding event involving hydrated, noncovalently bound dimer ions in solution, and this interaction has potential relevance to one of the most important protein-protein interactions in nature. As solvent is removed, dimer ions, viz. [2 M + 14H](14+), can be stabilized by only a few attached water molecules prior to dissociation into individual monomeric ions. The hydrophobic patch of ubiquitin formed by the side chains of Leu-8, Ile-44, and Val-70 meet all the necessary conditions for a protein-protein binding "hot spot," including the requirement for occlusion of water to nearby hydrophilic sites, and it is suggested that this interaction is responsible for formation of the hydrated noncovalent ubiquitin dimer.

Going clean: structure and dynamics of peptides in the gas phase and paths to solvation

The gas phase is an artificial environment for biomolecules that has gained much attention both experimentally and theoretically due to its unique characteristic of providing a clean room environment for the comparison between theory and experiment. In this review we give an overview mainly on first-principles simulations of isolated peptides and the initial steps of their interactions with ions and solvent molecules: a bottom up approach to the complexity of biological environments. We focus on the accuracy of different methods to explore the conformational space, the connections between theory and experiment regarding collision cross section evaluations and (anharmonic) vibrational spectra, and the challenges faced in this field.

Diphenylalanine self assembly: novel ion mobility methods showing the essential role of water

The mechanism and driving forces behind the formation of diphenylalanine (FF) nanotubes have attracted much attention in the past decades. The hollow structure of the nanotubes suggests a role for water during the self-assembly process. Here, we use novel ion-mobility mass spectrometry methods to probe the early oligomers formed by diphenylalanine peptides. Interestingly, water-bound oligomers are observed in nano-electrospray ionization (ESI) mass spectra in the absence of bulk solvent. In addition, ligated water clusters transit the ion mobility cell but (often) dissociate before detection. These water molecules are shown to be essential for the formation of diphenylalanine oligomers larger than the dimer. The ligated water molecules exist in the solvent free environment either as neutral water or as protonated water clusters, depending on the composition of solvent from which they are sprayed. Water adduction helps stabilize conformers that are otherwise energetically unstable ultimately leading to the assembly of FF nanotubes.

Isolating the spectral signature of H3O+ in the smallest droplet of dissociated HCl acid

The smallest droplet of HCl acid, H₃O⁺(H₂O)₃Cl⁻, and its isolated H₃O⁺ infrared signature.

Fundamentals of trapped ion mobility spectrometry

Trapped ion mobility spectrometry (TIMS) is a relatively new gas-phase separation method that has been coupled to quadrupole orthogonal acceleration time-of-flight mass spectrometry. The TIMS analyzer is a segmented rf ion guide wherein ions are mobility-analyzed using an electric field that holds ions stationary against a moving gas, unlike conventional drift tube ion mobility spectrometry where the gas is stationary. Ions are initially trapped, and subsequently eluted from the TIMS analyzer over time according to their mobility (K). Though TIMS has achieved a high level of performance (R > 250) in a small device (<5 cm) using modest operating potentials (<300 V), a proper theory has yet to be produced. Here, we develop a quantitative theory for TIMS via mathematical derivation and simulations. A one-dimensional analytical model, used to predict the transit time and theoretical resolving power, is described. Theoretical trends are in agreement with experimental measurements performed as a function of K, pressure, and the axial electric field scan rate. The linear dependence of the transit time with 1/K provides a fundamental basis for determination of reduced mobility or collision cross section values by calibration. The quantitative description of TIMS provides an operational understanding of the analyzer, outlines the current performance capabilities, and provides insight into future avenues for improvement.