Hepatic Topology of Glycosphingolipids in Schistosoma mansoni-Infected Hamsters.

Schistosomiasis is a neglected tropical disease caused by worm parasites of the genus Schistosoma. Upon infection, parasite eggs can lodge inside of host organs like the liver. This leads to granuloma formation, which is the main cause of the pathology of schistosomiasis. To better understand the different levels of host-pathogen interaction and pathology, our study focused on the characterization of glycosphingolipids (GSLs). For this purpose, GSLs in livers of infected and noninfected hamsters were studied by combining high-spatial-resolution atmospheric-pressure scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-SMALDI MSI) with nanoscale hydrophilic interaction liquid chromatography tandem mass spectrometry (nano-HILIC MS/MS). Nano-HILIC MS/MS revealed 60 GSL species with a distinct saccharide and ceramide composition. AP-SMALDI MSI measurements were conducted in positive- and negative-ion mode for the visualization of neutral and acidic GSLs. Based on nano-HILIC MS/MS results, we discovered no downregulated but 50 significantly upregulated GSLs in liver samples of infected hamsters. AP-SMALDI MSI showed that 44 of these GSL species were associated with the granulomas in the liver tissue. Our findings suggest an important role of GSLs during granuloma formation.

On the dipole moment distribution of rigid symmetric rotors.

The properties of dipolar molecules in electric fields have been studied for a long time. Their description often requires extensive numerical simulations, and only a limited number of analytically solvable models are known so far. Here, we derive an expression for the dipole probability density function of polar rigid symmetric rotors in the limit of high rotational temperatures and low electric field strengths, starting from the moment sequence of the probability distribution. Deviations of the distribution due to the non-spherical shape of the rotors are discussed in detail. In addition, it is demonstrated that there is a general connection between the average dipole moment of a thermal ensemble of rigid rotors in the high-temperature limit and the average dipole moment of an ensemble adiabatically entering the electric field.

Lipid Signatures and Inter-Cellular Heterogeneity of Naïve and Lipopolysaccharide-Stimulated Human Microglia-like Cells.

Microglia are non-neuronal cells, which reside in the central nervous system and are known to play an important role in health and disease. We investigated the lipidomic phenotypes of human naïve and stimulated microglia-like cells by atmospheric-pressure scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-SMALDI MSI). With lateral resolutions between 5 and 1.5 µm pixel size, we were able to chart lipid compositions of individual cells, enabling differentiation of cell lines and stimulation conditions. This allowed us to reveal local lipid heterogeneities in naïve and lipopolysaccharide (LPS)-stimulated cells. We were able to identify individual cells with elevated triglyceride (TG) levels and could show that the number of these TG-enriched cells increased with LPS stimulation as a hallmark for a proinflammatory phenotype. Additionally, the observed local abundance alterations of specific phosphatidylinositols (PIs) indicate a cell specific regulation of the PI metabolism.

IR-MALDI Mass Spectrometry Imaging with Plasma Post-Ionization of Nonpolar Metabolites.

Ambient mass spectrometry imaging (MSI) methods come with the advantage of visualizing biomolecules from tissues with no or minimal sample preparation and operation under atmospheric-pressure conditions. Similar to all other MSI methodologies, however, ambient MSI modalities suffer from a pronounced bias toward either polar or nonpolar analytes due to the underlying desorption and ionization mechanisms of the ion source. In this study, we present the design, construction, testing, and application of an in-capillary dielectric barrier discharge (DBD) module for post-ionization of neutrals desorbed by an ambient infrared matrix-assisted laser desorption/ionization (IR-MALDI) MSI source. We demonstrate that the DBD device enhances signal intensities of nonpolar compounds by up to 104 compared to IR-MALDI without affecting transmission of IR-MALDI ions. This allows performing MSI experiments of mouse tissue and Danaus plexippus caterpillar tissue sections, visualizing the distribution of sterols, fatty acids, monoglycerides, and diglycerides that are not detected in IR-MALDI MSI experiments. The pronounced signal enhancement due to IR-MALDI-DBD compared to IR-MALDI MSI enables mapping of nonpolar analytes with pixel resolutions down to 20 µm in mouse brain tissue and to discern the spatial distribution of sterol lipids characteristic for histological regions of D. plexippus.

Molecular Networking and On-Tissue Chemical Derivatization for Enhanced Identification and Visualization of Steroid Glycosides by MALDI Mass Spectrometry Imaging.

Spatial metabolomics describes the spatially resolved analysis of interconnected pathways, biochemical reactions, and transport processes of small molecules in the spatial context of tissues and cells. However, a broad range of metabolite classes (e.g., steroids) show low intrinsic ionization efficiencies in mass spectrometry imaging (MSI) experiments, thus restricting the spatial characterization of metabolic networks. Additionally, decomposing complex metabolite networks into chemical compound classes and molecular annotations remains a major bottleneck due to the absence of repository-scaled databases. Here, we describe a multimodal mass-spectrometry-based method combining computational metabolome mining tools and high-resolution on-tissue chemical derivatization (OTCD) MSI for the spatially resolved analysis of metabolic networks at the low micrometer scale. Applied to plant toxin sequestration in Danaus plexippus as a model system, we first utilized liquid chromatography (LC)-MS-based molecular networking in combination with artificial intelligence (AI)-driven chemical characterization to facilitate the structural elucidation and molecular identification of 32 different steroidal glycosides for the host-plant Asclepias curassavica. These comprehensive metabolite annotations guided the subsequent matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) analysis of cardiac-glycoside sequestration in D. plexippus. We developed a spatial-context-preserving OTCD protocol, which improved cardiac glycoside ion yields by at least 1 order of magnitude compared to results with untreated samples. To illustrate the potential of this method, we visualized previously inaccessible (sub)cellular distributions (2 and 5 µm pixel size) of steroidal glycosides in D. plexippus, thereby providing a novel insight into the sequestration of toxic metabolites and guiding future metabolomics research of other complex sample systems.

An enzymatic tandem reaction to produce odor-active fatty aldehydes.

Aldehydes represent a versatile and favored class of flavoring substances. A biocatalytic access to odor-active aldehydes was developed by conversion of fatty acids with two enzymes of the α-dioxygenase pathway. The recombinant enzymes α-dioxygenase (α-DOX) originating from Crocosphaera subtropica and fatty aldehyde dehydrogenase (FALDH) from Vibrio harveyi were heterologously expressed in E. coli, purified, and applied in a coupled (tandem) repetitive reaction. The concept was optimized in terms of number of reaction cycles and production yields. Up to five cycles and aldehyde yields of up to 26% were achieved. Afterward, the approach was applied to sea buckthorn pulp oil as raw material for the enzyme catalyzed production of flavoring/fragrance ingredients based on complex aldehyde mixtures. The most abundant fatty acids in sea buckthorn pulp oil, namely palmitic, palmitoleic, oleic, and linoleic acid, were used as substrates for further biotransformation experiments. Various aldehydes were identified, semi-quantified, and sensorially characterized by means of headspace-solid phase microextraction-gas chromatography-mass spectrometry-olfactometry (HS-SPME-GC-MS-O). Structural validation of unsaturated aldehydes in terms of double-bond positions was performed by multidimensional high-resolution mass spectrometry experiments of their Paternò-Büchi (PB) photoproducts. Retention indices and odor impressions of inter alia (Z,Z)-5,8-tetradecadienal (Z,Z)-6,9-pentadecadienal, (Z)-8-pentadecenal, (Z)-4-tridecenal, (Z)-6-pentadecenal, and (Z)-8-heptadecenal were determined for the first time. KEY POINTS: • Coupled reaction of Csα-DOX and VhFALDH yields chain-shortened fatty aldehydes. • Odors of several Z-unsaturated fatty aldehydes are described for the first time. • Potential for industrial production of aldehyde-based odorants from natural sources.

Influence of protein ion charge state on 213 nm top-down UVPD.

Ultraviolet photodissociation (UVPD) is a powerful and rapidly developing method in top-down proteomics. Sequence coverages can exceed those obtained with collision- and electron-induced fragmentation methods. Because of the recent interest in UVPD, factors that influence protein fragmentation and sequence coverage are actively debated in the literature. Here, we performed top-down 213 nm UVPD experiments on a 7 T Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR MS) for the model proteins ubiquitin, myoglobin and cytochrome c that were electrosprayed from native, denaturing and supercharging solutions in order to investigate the effect of protein charge states on UVPD fragments. By performing UVPD in ultrahigh vacuum, factors associated with collisional cooling and any ion activation during transfer between mass analyzers can be largely eliminated. Sequence coverage increased from <10% for low charge states to >60% for high charge states for all three proteins. This trend is influenced by the overall charge state, i.e., charges per number of amino acid residues, and to a lesser degree by associated structural changes of protein ions of different charge states based on comparisons to published collision-cross section measurements. To rationalize this finding, and correlate sequence ion formation and identity with the number and location of protons, UVPD results were compared to protonation sites predicted based on electrostatic modelling. Assuming confined protonation sites, these results indicate the presence of two general fragmentation types; i.e., charge remote and charge directed. For moderately high protein charge states, fragment ions mostly originate in regions between likely protonation sites (charge remote), whereas sequence ions of highly charge protein ions occur either near backbone amide protonation sites at low-basicity residues (charge directed) or at charge remote sites (i.e., high-basicity residues). Overall, our results suggest that top-down 213 UVPD performance in the zero-pressure limit depends strongly on protein charge states and protonation sites can influence the location of backbone cleavages.

Advanced tandem mass spectrometry in metabolomics and lipidomics-methods and applications.

Metabolomics and lipidomics are new drivers of the omics era as molecular signatures and selected analytes allow phenotypic characterization and serve as biomarkers, respectively. The growing capabilities of untargeted and targeted workflows, which primarily rely on mass spectrometric platforms, enable extensive charting or identification of bioactive metabolites and lipids. Structural annotation of these compounds is key in order to link specific molecular entities to defined biochemical functions or phenotypes. Tandem mass spectrometry (MS), first and foremost collision-induced dissociation (CID), is the method of choice to unveil structural details of metabolites and lipids. But CID fragment ions are often not sufficient to fully characterize analytes. Therefore, recent years have seen a surge in alternative tandem MS methodologies that aim to offer full structural characterization of metabolites and lipids. In this article, principles, capabilities, drawbacks, and first applications of these "advanced tandem mass spectrometry" strategies will be critically reviewed. This includes tandem MS methods that are based on electrons, photons, and ion/molecule, as well as ion/ion reactions, combining tandem MS with concepts from optical spectroscopy and making use of derivatization strategies. In the final sections of this review, the first applications of these methodologies in combination with liquid chromatography or mass spectrometry imaging are highlighted and future perspectives for research in metabolomics and lipidomics are discussed.

Multifunctional Reactive MALDI Matrix Enabling High-Lateral Resolution Dual Polarity MS Imaging and Lipid C═C Position-Resolved MS2 Imaging.

Local lipid variations in tissues are readily revealed with mass spectrometry imaging (MSI) methods, and the resulting lipid distributions serve as bioanalytical signatures to reveal cell- or tissue-specific lipids. Comprehensive MSI lipid mapping requires measurements in both ion polarities. Additionally, structural lipid characterization is necessary to link the lipid structure to lipid function. Whereas some structural elements of lipids are readily derived from high-resolution mass spectrometry (MS) and tandem-MS (MSn), the localization of C═C double bonds (DBs) requires specialized fragmentation and/or functionalization methods. In this work, we identify a multifunctional matrix-assisted laser desorption/ionization (MALDI) matrix for spatially resolved lipidomics investigations that reacts with lipids in Paternò-Büchi (PB) reactions during laser irradiation facilitating DB-position assignment and allows dual-polarity high-resolution MALDI-MSI and MALDI MS2I studies. By screening 12 compounds for improved ionization efficiency in positive-/negative-ion mode and the functionalization yield compared to the previously introduced reactive MALDI matrix benzophenone, 2-benzoylpyridine (BzPy) is identified as the best candidate. The new matrix enables DB localization of authentic standards belonging to 12 lipid classes and helps to assign 133/58 lipid features in positive-/negative-ion mode from mouse cerebellum tissue. The analytical capabilities of BzPy as a multifunctional MALDI-MSI matrix are demonstrated by imaging endogenous and PB-functionalized lipids in mouse kidney sections with 7 µm lateral resolution in both ion modes. Tracking diagnostic lipid DB-position fragment ions in mouse pancreatic tissue with down to 10 µm pixel size allows us to identify the islets of Langerhans associated with lipid isomer upregulation and depletion.

Atmospheric pressure MALDI mass spectrometry imaging of tissues and cells at 1.4-µm lateral resolution.

We report an atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) setup with a lateral resolution of 1.4 µm, a mass resolution greater than 100,000, and accuracy below ±2 p.p.m. We achieved this by coupling a focusing objective with a numerical aperture (NA) of 0.9 at 337 nm and a free working distance of 18 mm in coaxial geometry to an orbitrap mass spectrometer and optimizing the matrix application. We demonstrate improvement in image contrast, lateral resolution, and ion yield per unit area compared with a state-of-the-art commercial MSI source. We show that our setup can be used to detect metabolites, lipids, and small peptides, as well as to perform tandem MS experiments with 1.5-µm2 sampling areas. To showcase these capabilities, we identified subcellular lipid, metabolite, and peptide distributions that differentiate, for example, cilia and oral groove in Paramecium caudatum.

Autofocusing MALDI mass spectrometry imaging of tissue sections and 3D chemical topography of nonflat surfaces.

We describe an atmospheric pressure matrix-assisted laser desorption-ionization mass spectrometry imaging system that uses long-distance laser triangulation on a micrometer scale to simultaneously obtain topographic and molecular information from 3D surfaces. We studied the topographic distribution of compounds on irregular 3D surfaces of plants and parasites, and we imaged nonplanar tissue sections with high lateral resolution, thereby eliminating height-related signal artifacts.

Investigating C[double bond, length as m-dash]C positions and hydroxylation sites in lipids using Paternò-Büchi functionalization mass spectrometry.

Lipid oxidation plays a major role in biochemical processes and nutrition. Structural changes during oxidation can lead to alterations of lipid functions. Rancidification and production of secondary lipid messengers are well-known examples for the impact of oxidation on lipid function. Especially lipids with a high degree of unsaturation are prone to oxidize. In order to investigate structural changes of lipids upon oxidation, we here introduce a photochemical Paternò-Büchi functionalization workflow and subsequent mass spectrometric analysis for analysis of unsaturated, oxidized lipids. Results for hydroxylated fatty acids and triglycerides containing isolated and conjugated C[double bond, length as m-dash]C bonds will be presented making use of 3-acetylpyridine as a photochemically active compound. Photochemical derivatization is performed in nano-electrospray emitter tips in 30 s resulting in the formation of oxetanes without inducing light-triggered oxidation of analytes. Collisional-activation of photoproducts facilitates selective cleavage of oxetane moieties. Resulting fragment ions not only allow the determination of C[double bond, length as m-dash]C bond locations for isolated and conjugated C[double bond, length as m-dash]C bonds but also restrict the site of oxidation. By registering the mass shift in some fragment ions of +15.99 Da due to hydroxylation, the oxidized sections of lipids can be identified. In order to demonstrate its analytical robustness, the method is applied to determine the structural impact of non-selective ambient oxidation on fatty acids, triglycerides and complex triglyceride mixtures obtained from Sacha inchi oil.

Reactive Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging Using an Intrinsically Photoreactive Paternò-Büchi Matrix for Double-Bond Localization in Isomeric Phospholipids.

The location and identity of phospholipids (PLs) within tissues can serve as diagnostic markers for tissue types or diseases. Whereas mass spectrometry imaging (MSI) has emerged as a powerful bioanalytical tool to visualize PL distributions, inferring PL identities from MSI experiments is challenging. Especially, C═C double-bond (DB) positions are not identifiable in most MSI experiments. Herein, we introduce benzophenone (BPh) as a novel reactive matrix for matrix-assisted laser desorption/ionization (MALDI). BPh promotes desorption/ionization and simultaneously serves as derivatization reagent that allows functionalization of unsaturated PLs during the MALDI process via a laser-light driven Paternò-Büchi (PB) reaction without the need for additional equipment. Using BPh, PB product ions of numerous PL classes are readily generated to pinpoint the location of DBs. High lateral resolution MSI results of DB-position isomers are presented, highlighting the capabilities of BPh as a PB-reactive MALDI matrix to potentially unveil the impact of DB-position isomers in PL metabolism.

Relative Quantification of Phosphatidylcholine sn-Isomers Using Positive Doubly Charged Lipid-Metal Ion Complexes.

Phosphatidylcholines are the major phospholipid component of most eukaryotic cell membranes. Phosphatidylcholines have been shown to actively participate in regulatory and metabolic processes. Dysfunctional metabolic processes have been linked to human disease and can result in altered phosphatidylcholine structural features, such as permutation of fatty acid connectivity. Assignment and relative quantitation of structural isomers that arise from fatty acid permutation on the phosphatidylcholine backbone, so-called sn-isomers, is difficult with routine tandem mass spectrometry or with liquid chromatography without authentic standards. In this work, we report on the observation that phosphatidylcholines form abundant doubly charged metal ion complexes during electrospray ionization (ESI) and show that these complexes can be used to assign fatty acid moieties, relatively quantify sn-isomers in MS2 experiments, and mass spectrometrically separate phosphatidylcholines from other phospholipid classes in positive ion mode. Addition of Fe2+ salts (20 mol %) to ESI spray solutions affords highly abundant doubly charged metal ion phosphatidylcholine complexes (∼110% of protonated compounds) and allows sensitive fragment ion detection (limit of detection = 100 pM). Higher energy collisional dissociation, collision-induced dissociation, and ultraviolet photodissociation of doubly charged complexes yield two fragment ions for every fatty acid moiety. The latter two tandem MS methods preferentially yield sn-2 associated product ions enabling relative sn-isomer quantification. The analytical utility of doubly charged phosphatidylcholine-metal ion complexes is demonstrated for polar lipid extracts, including extracts from diabetes type 1 and type 2 mouse models, and sn-isomer abundances are derived.

Analysis of ketone-based neurosteroids by reactive low-temperature plasma mass spectrometry.

RATIONALE: Neurosteroids are important signalling molecules that modulate neuronal activity. Their low concentrations and low volatility make neurosteroid detection and quantification by ambient mass spectrometry challenging. Here we develop a reactive low-temperature plasma mass spectrometry (LTP-MS) method and demonstrate its potential for fast screening and quantification of neurosteroids in mouse brain. METHODS: Ketone-based neurosteroids were analysed with the LTP-MS method. The plasma of the LTP was heated in order to improve the desorption efficiency of low-volatility neurosteroids. Methylamine with a concentration of 500 ppbv was employed as the reactive reagent. Neurosteroids in mouse brain tissue extracts were detected in 70 s with mass errors less than ±3 ppm due to coupling of the ion source with a high-performance mass spectrometer. RESULTS: Reaction between neurosteroids and methylamine, seeded into the LTP gas stream, resulted in the formation of protonated methylamine-neurosteroid adducts with 5- to 100-fold abundances, compared to [M + H]+ ions detected in non-reactive LTP-MS. The lowest detectable concentrations of neurosteroid standards were in the range of ng/mL. Concentrations of neurosteroids in male and female mouse brain extracts as determined with reactive LTP-MS were on the level of ng/g, comparable to results obtained with high-performance liquid chromatography-tandem mass spectrometry. CONCLUSIONS: The developed reactive LTP-MS is capable of providing sensitive identification and quantification of ketone-based neurosteroids in mouse brain extracts with minimal sample treatment, and showcases the potential of reactive LTP-MS as a tool for fast screening of neurosteroid levels in brain.

Competition between salt bridge and non-zwitterionic structures in deprotonated amino acid dimers.

Structures of deprotonated Cys, Asp, Glu, Phe, Pro, His homo dimers as well as [2Cys - 3H]-, [Asp + Glu - H]- and [2Glu - 2H + Na]- are investigated with infrared multiple-photon dissociation (IRMPD) spectroscopy between 650 and 1850 cm-1 and theory. The IRMPD spectra of all investigated complexes but [2His - H]-, [2Phe - H]- and [2Pro - H]- indicate that the structures consist of a neutral non-zwitterionic (NZ) and a deprotonated form of the amino acids. In contrast, the spectrum of [2His - H]- is complex and indicates the presence of multiple isomers and/or interactions between His and [His - H]-, so that its structure differs from that of the other deprotonated amino acid dimers. For [2Phe - H]- and especially for [2Pro - H]-, some IRMPD bands can only be explained by the presence of salt bridge (SB) structures in the dimer in which a deprotonated amino acid interacts with a zwitterionic neutral amino acid. Computational results indicate that SB structures are lower in energy at 298 K than corresponding NZ structures for neutral-anion complexes in which SB formation is not disrupted by amino acid side chains or conformational constraints, such as in [2Glu - H]- and [2Cys - 3H]- for which NZ structures are most consistent with experimental results. For deprotonated amino acid dimers in which these interfering interactions are absent, such as in [2Phe - H]- and [2Pro - H]-, the higher number of hydrogen bonds in SB compared to NZ structures stabilize the formation of zwitterionic neutral amino acids and consequently SB structures in agreement with results from IRMPD spectroscopy. These results suggest that SB structures likely occur in deprotonated peptide or protein ions at hydrophobic sites, such as protein-protein interfaces or in the interior of proteins, where interfering functional groups will not disrupt SB formation.

Effects of wavelength, fluence, and dose on fragmentation pathways and photoproduct ion yield in 213 nm and 266 nm ultraviolet photodissociation experiments.

Ultraviolet photodissociation tandem mass spectrometry is a powerful tool to investigate the structure of biomolecules, due to its ability to generate rich fragmentation patterns or bond selective cleavage, as a function of used laser wavelength, laser fluence, dose (number of accumulated laser pulses), and available chromophores. Herein, we report first results obtained with a newly developed two-wavelength (266 nm and 213 nm) ultraviolet photodissociation setup coupled to a Fourier-transform ion cyclotron resonance mass spectrometer. Photoproduct yields for protonated 3-iodo-l-tyrosine were up to ∼75%. Dose and fluence dependent measurements for protonated 3-iodo-l-tyrosine, doubly charged protonated bradykinin and Fe(II) attached to 1,2-dioleoyl-sn-glycero-3-phosphocholine reveal that the ultraviolet photodissociation mechanism for photoproduct formation qualitatively differs between these model systems. Three derived photodissociation models were used to interpret the experimental results and show that while protonated 3-iodo-l-tyrosine and Fe(II) attached to 1,2-dioleoyl-sn-glycero-3-phosphocholine most likely dissociates via a single-photon process, fragmentation of doubly charged bradykinin ions was found to be most consistent with sequential two-photon dissociation (213 nm). The introduced dissociation models present an easy means to study the mechanism of ultraviolet photodissociation processes for a variety of analytes without prior knowledge of their photochemistry or to optimize experimental conditions by adjusting laser fluence or number of laser pulses.

Selective phosphatidylcholine double bond fragmentation and localisation using Paternò-Büchi reactions and ultraviolet photodissociation.

The effect of double bond functionalisation for selective double bond localisation by ultraviolet photodissociation of phosphatidylcholines is investigated. Paternò-Büchi reactions in nanoESI emitter tips enable attachment of acetophenone to double bonds of unsaturated phosphatidylcholines after 100 s of 254 nm light irradiation with about 50-80% reaction yield. Functionalized phosphatidylcholines dissociate upon 266 nm irradiation yielding double bond selective fragment ions in contrast to results for ultraviolet photodissociation of unmodified lipids. Ultraviolet photodissociation of Paternò-Büchi modified lipids results in a selectivity increase of up to 2.2 towards double bond localisation compared collision-induced dissociation experiments. Double bond localisation is also possible with ultraviolet photodissociation when alkali metal ion attachment to Paternò-Büchi modified phosphatidylcholines occurs in contrast to classic collision-induced dissociation experiments. The developed methodology is used to differentiate lipid double bond isomers and applied to phosphatidylcholines from egg yolk to identify 15 phosphatidylcholines. Results from this study demonstrate that locally depositing energy in close vicinity to cleavable bonds via ultraviolet photodissociation can result in increased dissociation selectivity. This method can help to disentangle contributions from different structural elements in complex tandem mass spectra of lipids and aid to the structural characterization of phospholipids in a "top-down" approach.

Role of water in stabilizing ferricyanide trianion and ion-induced effects to the hydrogen-bonding water network at long distance.

Structures and reactivities of gaseous Fe(CN)(6)(3-)(H(2)O)n were investigated using infrared photodissociation (IRPD) kinetics, spectroscopy, and computational chemistry in order to gain insights into how water stabilizes highly charged anions. Fe(CN)(6)(3-)(H(2)O)(8) is the smallest hydrated cluster produced by electrospray ionization, and blackbody infrared dissociation of this ion results in loss of an electron and formation of smaller dianion clusters. Fe(CN)(6)(3-)(H(2)O)(7) is produced by the higher activation conditions of IRPD, and this ion dissociates both by loss of an electron and by loss of a water molecule. Comparisons of IRPD spectra to those of computed low-energy structures for Fe(CN)(6)(3-)(H(2)O)(8) indicate that water molecules either form two hydrogen bonds to the trianion or form one hydrogen bond to the ion and one to another water molecule. Magic numbers are observed for Fe(CN)(6)(3-)(H(2)O)n for n between 58 and 60, and the IRPD spectrum of the n = 60 cluster shows stronger water molecule hydrogen-bonding than that of the n = 61 cluster, consistent with the significantly higher stability of the former. Remarkably, neither cluster has a band corresponding to a free O-H stretch, and this band is not observed for clusters until n ≥ 70, indicating that this trianion significantly affects the hydrogen-bonding network of water molecules well beyond the second and even third solvation shells. These results provide new insights into the role of water in stabilizing high-valency anions and how these ions can pattern the structure of water even at long distances.

Effects of electronic structure on the hydration of PbNO3(+) and SrNO3(+) ion pairs.

Hydration of PbNO3(+) and SrNO3(+) with up to 30 water molecules was investigated with infrared photodissociation (IRPD) spectroscopy and with theory. These ions are the same size, yet the IRPD spectra of these ion pairs for n = 2-8 are significantly different. Bands in the bonded O-H region (∼3000-3550 cm(-1)) indicate that the onset of a second hydration shell begins at n = 5 for PbNO3(+) and n = 6 for SrNO3(+). Spectra for [PbNO3](+)(H2O)2-5 and [SrNO3](+)(H2O)3-6 indicate that the structures of clusters with Pb(ii) are hemidirected with a void in the coordinate sphere. A natural bond orbital analysis of [PbNO3](+)(H2O)5 indicates that the anisotropic solvation of the ion is due to a region of asymmetric electron density on Pb(ii) that can be explained by charge transfer from the nitrate and water ligands into unoccupied p-orbitals on Pb(ii). There are differences in the IRPD spectra of PbNO3(+) and SrNO3(+) with up to 25 water molecules attached. IR intensity in the bonded O-H region is blue-shifted by ∼50 cm(-1) in nanodrops containing SrNO3(+) compared to those containing PbNO3(+), indicative of a greater perturbation of the water H-bond network by strontium. The free O-H stretches of surface water molecules in nanodrops containing 10, 15, 20, and 25 water molecules are red-shifted by ∼3-8 cm(-1) for PbNO3(+) compared to those for SrNO3(+), consistent with more charge transfer between water molecules and Pb(ii). These results demonstrate that the different electronic structure of these ions significantly influences how they are solvated.