Hydration energies of divalent metal ions, Ca2+ (H2O)n (N = 5-7) and Ni2+ (H2O)m (N = 6-8), obtained by blackbody infrared radiative dissociation

A literature review of the increased intracellular free calcium concentration by biofield therapy or laser exposure. An explanation by using a theoretical study of hydrated calcium ions

INTRODUCTION

A revision of several experimental results on cells shows that electromagnetic radiation, either produced by biofield therapy (BFT) or laser, induced an increase in intracellular free calcium concentration. An explanation of this phenomenon is proposed.

METHODS

Quantum chemistry calculations were performed on Ca2+ with different degrees of hydration with the DFT/r2SCAN-3c method together with the implicit solvation model SMD.

RESULTS

Ca2+ dehydration energy by quantum calculations, in an aqueous medium, coincides with the experimental results of the energy of the photon emitted in biofield therapies and lasers. This strongly suggests that the increased intracellular free calcium concentration is because of calcium ion dehydration upon the application of radiation. The Ca2+ dehydration increases the membrane potential due to an augment of the net charge on Ca2+ and it moves near the membrane by the attraction of its negative ions. The voltage-dependent channels are also activated by this membrane potential.

CONCLUSION

The increased intracellular Ca2+ concentration occurs with biofield therapy (BFT) or laser. A novel explanation is given based on resonance-induced Ca2+ dehydration with applied radiation, supported by experimental data and theoretical calculations.

Periodic trends in the hydration energies and critical sizes of alkaline earth and transition metal dication water complexes

This review encompasses guided ion beam tandem mass spectrometry studies of hydrated metal dication complexes. Metals include the Group 2 alkaline earths (Mg, Ca, Sr, and Ba), late first-row transition metals (Mn, Fe, Co, Ni, Cu, and Zn), along with Cd. In all cases, threshold collision-induced dissociation experiments are used to quantitatively determine the sequential hydration energies for M2+ (H2 O)x complexes ranging in size from one to 11 water molecules. Periodic trends in these bond dissociation energies are examined and discussed. Values are compared to other experimental results when available. In addition to dissociation by simple water ligand loss, complexes at a select size (which differs from metal to metal) are also observed to undergo charge separation to yield a hydrated metal hydroxide cation and a hydrated proton. This leads to the concept of a critical size, xcrit , and the periodic trends in this value are also discussed.

Simplified Multiple-Well Approach for the Master Equation Modeling of Blackbody Infrared Radiative Dissociation of Hydrated Carbonate Radical Anions

Blackbody infrared radiative dissociation (BIRD) in a collision-free environment is a powerful method for the experimental determination of bond dissociation energies. In this work, we investigate temperature-dependent BIRD of CO₃^·-(H₂O)_1,2 at 250-330 K to determine water binding energies and assess the influence of multiple isomers on the dissociation kinetics. The ions are trapped in a Fourier-transform ion cyclotron resonance mass spectrometer, mass selected, and their BIRD kinetics are recorded at varying temperatures. Experimental BIRD rates as a function of temperature are fitted with rates obtained from master equation modeling (MEM), using the water binding energy as a fit parameter. MEM accounts for the absorption and emission of photons from black-body radiation, described with harmonic frequencies and infrared intensities from quantum chemical calculations. The dissociation rates as a function of internal energy are calculated by Rice-Ramsperger-Kassel-Marcus theory. Both single-well and multiple-well MEM approaches are used. Dissociation energies derived in this way from the experimental data are 56 ± 6 and 45 ± 3 kJ/mol for the first and second water molecules, respectively. They agree within error limits with the ones predicted by ab initio calculations done at the CCSD(T)/aug-cc-pVQZ//CCSD/aug-cc-pVDZ level of theory. We show that the multiple-well MEM approach described here yields superior results in systems with several low-lying minima, which is the typical situation for hydrated ions.

DEM-Based Approach for the Modeling of Gelation and Its Application to Alginate

The gelation of biopolymers is of great interest in the material science community and has gained increasing relevance in the past few decades, especially in the context of aerogels─lightweight open nanoporous materials. Understanding the underlying gel structure and influence of process parameters is of great importance to predict material properties such as mechanical strength. In order to improve understanding of the gelation mechanism in aqueous solution, this work presents a novel approach based on the discrete element method for the mesoscale for modeling gelation of hydrogels, similarly to an extremely coarse-grained molecular dynamics (MD) approach. For this, polymer chains are abstracted as dimer units connected by flexible bonds and interactions between units and with the environment, that is, diffusion in implicit water, are described. The model is based on Langevin dynamics and includes an implicit probabilistic ion model to capture the effects of ion availability during ion-mediated gelation. The model components are fully derived and parameterized using literature data and theoretical considerations based on a simplified representation of atomistic processes. The presented model enables investigations of the higher-scale network formation during gelation on the micrometer and millisecond scale, which are beyond classical modeling approaches such as MD. As a model system, calcium-mediated alginate gelation is investigated including the influence of ion concentration, polymer composition, polymer concentration, and molecular weight. The model is verified against numerous literature data as well as own experimental results for the corresponding Ca-alginate hydrogels using nitrogen porosimetry, NMR cryoporometry, and small-angle neutron scattering. The model reproduces both bundle size and pore size distribution in a reasonable agreement with the experiments. Overall, the modeling approach paves the way to physically motivated design of alginate gels.

Collision-induced dissociation of protonated uracil water clusters probed by molecular dynamics simulations

Collision-induced dissociation experiments of hydrated molecular species can provide a wealth of important information. However, they often need a theoretical support to extract chemical information. In the present article, in order to provide a detailed description of recent experimental measurements [Braud et al., J. Chem. Phys., 2019, 150, 014303], collision simulations between low-energy protonated uracil water clusters (H₂O)_1-7,11,12UH⁺ and an Ar atom were performed using a quantum mechanics/molecular mechanics formalism based on the self-consistent-charge density-functional based tight-binding method. The theoretical proportion of formed neutral vs. protonated uracil containing clusters, total fragmentation cross sections as well as the mass spectra of charged fragments are consistent with the experimental data which highlights the accuracy of the present simulations. They allow to probe which fragments are formed on the short time scale and rationalize the location of the excess proton on these fragments. We demonstrate that this latter property is highly influenced by the nature of the aggregate undergoing the collision. Analyses of the time evolution of the fragments populations and of their relative abundances demonstrate that, up to 7 water molecules, a direct dissociation mechanism occurs after collision whereas for 11 and 12 water molecules a statistical mechanism is more likely to participate. Although scarce in the literature, the present simulations appear as a useful tool to complement collision-induced dissociation experiments of hydrated molecular species.

Strontium increasing calcium accessibility from calcium citrate

Strontium chloride added to aqueous suspensions of metastable calcium citrate tetrahydrate increased calcium ion activity measured electrochemically without transition of metastable tetrahydrate to stable calcium citrate hexahydrate as shown by DSC. Calcium activity increase was explained by lower solubility of strontium citrate pentahydrate formed (8.9 × 10^-4 M at 25 °C) increasing with temperature compared to calcium citrate tetrahydrate (1.6 × 10^-3 M) decreasing with temperature. Strontium binding to citrate was found endothermic, ΔH⁰ = 45 kJ∙mol^-1 at 25 °C, while calcium binding shows variation from ΔH⁰ = 94 kJ∙mol^-1 at 10 °C becoming exothermic above physiological temperature with ΔH⁰ = -9 kJ∙mol^-1 at 45 °C as determined from temperature and concentration variation in electric conductivity. These differences in solution thermodynamics and pH effect on complex formation between calcium and strontium citrate are discussed in relation to biomineralization.

Nickel reduces calcium dependent dimerization in neural cadherin

Cadherins are the transmembrane component in adherens junctions, structures that link the actin cytoskeletons in adjacent cells within solid tissues including neurological synapses, epithelium and endothelium. Cell-cell adhesion by cadherins requires the binding of calcium ions to specific sites in the extracellular region. Given the complexity of the cell adhesion microenvironment, we are investigating whether other divalent cations might affect calcium-dependent dimerization of neural (N) cadherin. The studies reported herein characterize the impact of binding physiological magnesium(ii) or neurotoxic nickel(ii) on calcium-dependent N-cadherin function. Physiological levels of magnesium have only a small effect on the calcium-binding affinity and calcium-induced dimerization of N-cadherin. However, a tenfold lower concentration of nickel decreases the apparent calcium-binding affinity and calcium-induced dimerization of N-cadherin. Competitive binding studies indicate that the apparent dissociation constants for nickel and magnesium are 0.2 mM and 2.5 mM, respectively. These Kd values are consistent with concentrations observed for a range of divalent cations in the extracellular space. Results from these studies indicate that calcium-induced dimerization by N-cadherin is attenuated by natural and non-physiological divalent cations in the extracellular microenvironment.

Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2 (H2 O)n - , n=1-10, in the C-O Stretch Region

We investigate anionic [Co,CO2 ,nH2 O]- clusters as model systems for the electrochemical activation of CO2 by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250-2234 cm-1 using an FT-ICR mass spectrometer. We show that both CO2 and H2 O are activated in a significant fraction of the [Co,CO2 ,H2 O]- clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C-O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C-O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)2 - . However, calculations find Co(HCOO)(OH)- as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590-1730 cm-1 are most likely due to higher lying isomers of the composition Co(HOCO)(OH)- . Upon additional hydration, all species [Co,CO2 ,nH2 O]- , n≥2, undergo IRMPD through loss of H2 O molecules as a relatively weakly bound messenger. The main spectral features are the C-O stretching mode of the CO ligand around 1900 cm-1 , the water bending mode mixed with the antisymmetric C-O stretching mode of the HCOO- ligand around 1580-1730 cm-1 , and the symmetric C-O stretching mode of the HCOO- ligand around 1300 cm-1 . A weak feature above 2000 cm-1 is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.

The Solvation of Ca2+ with Gas Phase Clusters of Alcohol Molecules

A comprehensive examination of how the identity of an alcohol molecule can change the behavior of a solvated, alkaline earth dication has been undertaken. The metal dication of Ca²⁺ has been clustered with a range of different alcohols to form [Ca(ROH)_n]²⁺ complexes, where n lies in the range 2-20. Following collisional activation via electron capture from nitrogen gas, complexes for n in the range 2-6 exhibit a switch in reaction product as a function of n. For low values, solvated CaOH⁺ is the dominant fragment, but as n increases beyond 4, this is displaced by the appearance of solvated CaOR⁺. A separate study of unimolecular metastable decay by [Ca(ROH)_n]²⁺ complexes found evidence of charge separation to form CaOH⁺(ROH)_n-1 + R⁺. For two isomers of butanol, the n = 3 complexes were found to follow parallel, but different metastable pathways: one leading to the appearance of CaOH⁺ and another that resulted in proton abstraction to form ROH₂⁺. These differences have been attributed to the precursor complexes adopting geometries where one ROH molecule occupies a secondary solvation shell. Comparisons were made with a previous study of magnesium complexes; [Mg(ROH)_n]²⁺ show that the difference in second ionization energy Mg⁺ (15.09 eV) as opposed to Ca⁺ (11.88 eV) influences behavior. A complex between Ca²⁺ and 1-chloroethanol is shown to favor the formation of CaCl⁺ as opposed to CaOH⁺ as a unimolecular charge separation product, which is attributed to differences in bond energy in the precursor molecule.

Structure of amyloid β25-35 in lipid environment and cholesterol-dependent membrane pore formation

The amyloid β (Aβ) peptide and its shorter variants, including a highly cytotoxic Aβ_25–35 peptide, exert their neurotoxic effect during Alzheimer’s disease by various mechanisms, including cellular membrane permeabilization. The intrinsic polymorphism of Aβ has prevented the identification of the molecular basis of Aβ pore formation by direct structural methods, and computational studies have led to highly divergent pore models. Here, we have employed a set of biophysical techniques to directly monitor Ca²⁺-transporting Aβ_25–35 pores in lipid membranes, to quantitatively characterize pore formation, and to identify the key structural features of the pore. Moreover, the effect of membrane cholesterol on pore formation and the structure of Aβ_25–35 has been elucidated. The data suggest that the membrane-embedded peptide forms 6- or 8-stranded β-barrel like structures. The 8-stranded barrels may conduct Ca²⁺ ions through an inner cavity, whereas the tightly packed 6-stranded barrels need to assemble into supramolecular structures to form a central pore. Cholesterol affects Aβ_25–35 pore formation by a dual mechanism, i.e., by direct interaction with the peptide and by affecting membrane structure. Collectively, our data illuminate the molecular basis of Aβ membrane pore formation, which should advance both basic and clinical research on Alzheimer’s disease and membrane-associated pathologies in general.

Binding energies of hydrated cobalt(ii) by collision-induced dissociation and theoretical studies: evidence for a new critical size

The experimental sequential bond energies for loss of water from Co²⁺(H₂O)_x complexes, x = 5-11, are determined by threshold collision-induced dissociation (TCID) using a guided ion beam tandem mass spectrometer with a thermal electrospray ionization source. Kinetic energy dependent TCID cross sections are analyzed to yield 0 K thresholds for sequential loss of neutral water molecules. The thresholds are converted from 0 to 298 K values to give hydration enthalpies and free energies. Theoretical geometry optimizations and single point energy calculations at several levels of theory are performed for the reactant and product ion complexes. Theoretical bond energies for ground structures are used for direct comparison with experimental values to obtain structural information on these complexes. In addition, the dissociative charge separation process, Co²⁺(H₂O)_x → CoOH⁺(H₂O)_m + H⁺(H₂O)_x-m-1, is observed at x = 4, 6, and 7 in competition with primary water loss products. Energies for the charge separation rate-limiting transition states are calculated and compared to experimental threshold measurements. Results suggest that the critical size for which charge separation is energetically favored over water loss is x_crit = 6, in contrast to lower values in previous literature reports.

Optimization of electrospray ionization conditions to enhance formation of doubly protonated peptide ions with and without addition of chromium(III)

RATIONALE

Production of multiply protonated ions by electrospray ionization (ESI) is important to the analysis of peptides by mass spectrometry. For small neutral and acidic peptides, addition of chromium(III) greatly increases the intensity of doubly protonated ions. The current study examines instrumental and solution parameters that maximize peptide ion charge by ESI.

METHODS

The neutral and basic heptapeptides AAAAAAA (A7) and AAAKAAA (A3KA3) were used as test compounds and electrosprayed from a solution containing chromium(III) nitrate at a peptide to metal molar ratio of 1:10. Positive ion mode experiments were performed on a Bruker HCTultra PTM Discovery System quadrupole ion trap mass spectrometer. Source voltages and drying/nebulizer gases were systematically altered. The effects of rinsing, brand, and color of plastic microcentrifuge tubes (vials) employed were also investigated.

RESULTS

Nebulizer gas pressure and drying gas flow rate are crucial parameters for production of [M + 2H]²⁺ , while drying gas temperature alone has minimal effect. Optimization of the capillary exit and skimmer voltages are important both to maximize [M + 2H]²⁺ and reduce unwanted ion dissociation. Protonation is enhanced and fewer impurity peaks are observed when solutions are prepared in colorless plastic vials that have been rinsed briefly with propan-2-ol (isopropanol).

CONCLUSIONS

Optimization of instrument and sample preparation factors for enhanced protonation with and without Cr(III) is necessary to allow maximum formation of [M + 2H]²⁺ . Proteomics researchers should find these procedures to be of use for increasing multiply protonated signal intensity even in the absence of Cr(III). Copyright © 2017 John Wiley & Sons, Ltd.

Sequential water molecule binding enthalpies for aqueous nanodrops containing a mono-, di- or trivalent ion and between 20 and 500 water molecules

Sequential water molecule binding enthalpies, ΔH_n,n-1, are important for a detailed understanding of competitive interactions between ions, water and solute molecules, and how these interactions affect physical properties of ion-containing nanodrops that are important in aerosol chemistry. Water molecule binding enthalpies have been measured for small clusters of many different ions, but these values for ion-containing nanodrops containing more than 20 water molecules are scarce. Here, ΔH_n,n-1 values are deduced from high-precision ultraviolet photodissociation (UVPD) measurements as a function of ion identity, charge state and cluster size between 20-500 water molecules and for ions with +1, +2 and +3 charges. The ΔH_n,n-1 values are obtained from the number of water molecules lost upon photoexcitation at a known wavelength, and modeling of the release of energy into the translational, rotational and vibrational motions of the products. The ΔH_n,n-1 values range from 36.82 to 50.21 kJ mol^-1. For clusters containing more than ∼250 water molecules, the binding enthalpies are between the bulk heat of vaporization (44.8 kJ mol^-1) and the sublimation enthalpy of bulk ice (51.0 kJ mol^-1). These values depend on ion charge state for clusters with fewer than 150 water molecules, but there is a negligible dependence at larger size. There is a minimum in the ΔH_n,n-1 values that depends on the cluster size and ion charge state, which can be attributed to the competing effects of ion solvation and surface energy. The experimental ΔH_n,n-1 values can be fit to the Thomson liquid drop model (TLDM) using bulk ice parameters. By optimizing the surface tension and temperature change of the logarithmic partial pressure for the TLDM, the experimental sequential water molecule binding enthalpies can be fit with an accuracy of ±3.3 kJ mol^-1 over the entire range of cluster sizes.

Experimental measurements of water molecule binding energies for the second and third solvation shells of [Ca(H2O) n ]2+ complexes

Further understanding of the biological role of the Ca²⁺ ion in an aqueous environment requires quantitative measurements of both the short- and long-range interactions experienced by the ion in an aqueous medium. Here, we present experimental measurements of binding energies for water molecules occupying the second and, quite possibly, the third solvation shell surrounding a central Ca²⁺ ion in [Ca(H₂O) _n ]²⁺ complexes. Results for these large, previously inaccessible, complexes have come from the application of finite heat bath theory to kinetic energy measurements following unimolecular decay. Even at n = 20, the results show water molecules to be more strongly bound to Ca²⁺ than would be expected just from the presence of an extended network of hydrogen bonds. For n > 10, there is very good agreement between the experimental binding energies and recently published density functional theory calculations. Comparisons are made with similar data recorded for [Ca(NH₃) _n ]²⁺ and [Ca(CH₃OH) _n ]²⁺ complexes.

Experimental measurements of water molecule binding energies for the second and third solvation shells of [Ca(H2O) n ]2+ complexes

Further understanding of the biological role of the Ca²⁺ ion in an aqueous environment requires quantitative measurements of both the short- and long-range interactions experienced by the ion in an aqueous medium. Here, we present experimental measurements of binding energies for water molecules occupying the second and, quite possibly, the third solvation shell surrounding a central Ca²⁺ ion in [Ca(H₂O) _n ]²⁺ complexes. Results for these large, previously inaccessible, complexes have come from the application of finite heat bath theory to kinetic energy measurements following unimolecular decay. Even at n = 20, the results show water molecules to be more strongly bound to Ca²⁺ than would be expected just from the presence of an extended network of hydrogen bonds. For n > 10, there is very good agreement between the experimental binding energies and recently published density functional theory calculations. Comparisons are made with similar data recorded for [Ca(NH₃) _n ]²⁺ and [Ca(CH₃OH) _n ]²⁺ complexes.

Understanding ligand effects in gold clusters using mass spectrometry

This review summarizes recent research on the influence of phosphine ligands on the size, stability, and reactivity of gold clusters synthesized in solution. Sub-nanometer clusters exhibit size- and composition-dependent properties that are unique from those of larger nanoparticles. The highly tunable properties of clusters and their high surface-to-volume ratio make them promising candidates for a variety of technological applications. However, because "each-atom-counts" toward defining cluster properties it is critically important to develop robust synthesis methods to efficiently prepare clusters of predetermined size. For decades phosphines have been known to direct the size-selected synthesis of gold clusters. Despite the preparation of numerous species it is still not understood how different functional groups at phosphine centers affect the size and properties of gold clusters. Using electrospray ionization mass spectrometry (ESI-MS) it is possible to characterize the effect of ligand substitution on the distribution of clusters formed in solution at defined reaction conditions. In addition, ligand exchange reactions on preformed clusters may be monitored using ESI-MS. Collision induced dissociation (CID) may also be employed to obtain qualitative insight into the fragmentation of mixed ligand clusters and the relative binding energies of differently substituted phosphines. Quantitative ligand binding energies and cluster stability may be determined employing surface induced dissociation (SID) in a custom-built Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Rice-Ramsperger-Kassel-Marcus (RRKM) based modeling of the SID data allows dissociation energies and entropy values to be extracted. The charge reduction and reactivity of atomically precise gold clusters, including partially ligated species generated in the gas-phase by in source CID, on well-defined surfaces may be explored using ion soft landing (SL) in a custom-built instrument combined with in situ time of flight secondary ion mass spectrometry (TOF-SIMS). Jointly, this multipronged experimental approach allows characterization of the full spectrum of relevant phenomena including cluster synthesis, ligand exchange, thermochemistry, surface immobilization, and reactivity. The fundamental insights obtained from this work will facilitate the directed synthesis of gold clusters with predetermined size and properties for specific applications.

A density functional theory study of the hydration of calcium ions confined in the interlayer space of montmorillonites

The structures of Ca2+hydrates in the interlayer space of montmorillonites (MMT) were studied by periodic density functional theory (DFT) calculations under the GGA/PBE approximation. Affected by the internal surfaces, which are rich of negative charge, the Ca2+hydration exhibits different behaviors from that in gas phase. The Ca2+is located at the six-oxygen-ring (SOR) on the internal surface in dry MMT, while the incoming water molecules bind with the Ca2+, the O atoms on surface, and/or with each other. The water molecules have a tendency of forming a hydrogen bond (HB) network that connects the upper and lower surfaces. Attracted by surrounding water molecules, the Ca2+gradually moves outward with increasing number of water molecules. Moreover, the hydration energy (EH) of Ca2+is determined not only by the interaction between Ca2+and H2O , but also by that among Ca2+, H2O and the surfaces. As a result, the EHhas only small changes for additional incoming water molecules, in contrast to the great and monotonic decrease in gas phase.

Relative stability of peptide sequence ions generated by tandem mass spectrometry

We report the use of unimolecular dissociation by infrared radiation for gaseous multiphoton energy transfer to determine relative activation energy (E(a,laser)) for dissociation of peptide sequence ions. The sequence ions of interest are mass-isolated; the entire ion cloud is then irradiated with a continuous wave CO(2) laser, and the first order rate constant, k(d), is determined for each of a series of laser powers. Provided these conditions are met, a plot of the natural logarithm of k(d) versus the natural logarithm of laser power yields a straight line, whose slope provides a measure of E(a,laser). This method reproduces the E(a) values from blackbody radiative dissociation (BIRD) for the comparatively large, singly and doubly protonated bradykinin ions (nominally y ( 9 ) and y ( 9 ) ( 2+ )). The comparatively small sequence ion systems produce E(a,laser) values that are systematic underestimates of theoretical barriers calculated with density functional theory (DFT). However, the relative E(a,laser) values are in qualitative agreement with the mobile proton model and available theory. Additionally, novel protonated cyclic-dipeptide (diketopiperazine) fragmentation reactions are analyzed with DFT. FT-ICR MS provides access to sequence ions generated by electron capture dissociation, infrared multiphoton dissociation, and collisional activation methods (i.e., b ( n ), y ( m ) , c ( n ), z ( m ) ( • ) ions).

Coordination numbers of hydrated divalent transition metal ions investigated with IRPD spectroscopy

Hydration of the divalent transition metal ions, Mn, Fe, Co, Ni, Cu, and Zn, with 5-8 water molecules attached was investigated using infrared photodissociation spectroscopy and photodissociation kinetics. At 215 K, spectral intensities in both the bonded-OH and free-OH stretch regions indicate that the average coordination number (CN) of Mn(2+), Fe(2+), Co(2+), and Ni(2+) is ~6, and these CN values are greater than those of Cu(2+) and Zn(2+). Ni has the highest CN, with no evidence for any population of structures with a water molecule in a second solvation shell for the hexa-hydrate at temperatures up to 331 K. Mn(2+), Fe(2+), and Co(2+) have similar CN at low temperature, but spectra of Mn(2+)(H(2)O)(6) indicate a second population of structures with a water molecule in a second solvent shell, i.e., a CN < 6, that increases in abundance at higher temperature (305 K). The propensity for these ions to undergo charge separation reactions at small cluster size roughly correlates with the ordering of the hydrolysis constants of these ions in aqueous solution and is consistent with the ordering of average CN values established from the infrared spectra of these ions.

Ions in size-selected aqueous nanodrops: sequential water molecule binding energies and effects of water on ion fluorescence

The effects of water on ion fluorescence were investigated, and average sequential water molecule binding energies to hydrated ions, M(z)(H(2)O)(n), at large cluster size were measured using ion nanocalorimetry. Upon 248-nm excitation, nanodrops with ~25 or more water molecules that contain either rhodamine 590(+), rhodamine 640(+), or Ce(3+) emit a photon with average energies of approximately 548, 590, and 348 nm, respectively. These values are very close to the emission maxima of the corresponding ions in solution, indicating that the photophysical properties of these ions in the nanodrops approach those of the fully hydrated ions at relatively small cluster size. As occurs in solution, these ions in nanodrops with 8 or more water molecules fluoresce with a quantum yield of ~1. Ce(3+) containing nanodrops that also contain OH(-) fluoresce, whereas those with NO(3)(-) do not. This indirect fluorescence detection method has the advantages of high sensitivity, and both the size of the nanodrops as well as their constituents can be carefully controlled. For ions that do not fluoresce in solution, such as protonated tryptophan, full internal conversion of the absorbed 248-nm photon occurs, and the average sequential water molecule binding energies to the hydrated ions can be accurately obtained at large cluster sizes. The average sequential water molecule binding energies for TrpH(+)(H(2)O)(n) and a doubly protonated tripeptide, [KYK + 2H](2+)(H(2)O)(n), approach asymptotic values of ~9.3 (n ≥ 11) and ~10.0 kcal/mol (n ≥ 25), respectively, consistent with a liquidlike structure of water in these nanodrops.

Average sequential water molecule binding enthalpies of M(H2O)(19-124)2+ (M = Co, Fe, Mn, and Cu) measured with ultraviolet photodissociation at 193 and 248 nm

The average sequential water molecule binding enthalpies to large water clusters (between 19 and 124 water molecules) containing divalent ions were obtained by measuring the average number of water molecules lost upon absorption of an UV photon (193 or 248 nm) and using a statistical model to account for the energy released into translations, rotations, and vibrations of the products. These values agree well with the trend established by more conventional methods for obtaining sequential binding enthalpies to much smaller hydrated divalent ions. The average binding enthalpies decrease to a value of ~10.4 kcal/mol for n > ~40 and are insensitive to the ion identity at large cluster size. This value is close to that of the bulk heat of vaporization of water (10.6 kcal/mol) and indicates that the structure of water in these clusters may more closely resemble that of bulk liquid water than ice, owing either to a freezing point depression or rapid evaporative cooling and kinetic trapping of the initial liquid droplet. A discrete implementation of the Thomson equation using parameters for liquid water at 0 °C generally fits the trend in these data but provides values that are ~0.5 kcal/mol too low.

Structures, stability, vibration entropy and IR spectra of hydrated calcium ion clusters [Ca(H(2)O)(n)](2+) (n = 1-20, 27): a systematic investigation by density functional theory

The low-lying candidates of hydrated calcium ion clusters, [Ca(H(2)O)(n)](2+) with n = 1-20 and 27, have been extensively sought by using density functional theory (DFT) at BLYP/6-311+G(d,p) level. The results showed that the first hydration shell around the calcium ion was fully occupied by six water molecules, whereas the second hydration shell might be fully occupied with different numbers of water molecules. This just corresponds to different growth patterns of the hydrated calcium ion clusters. Furthermore, we revealed that the vibration entropy contributed to the free energy of an isomer significantly. As a result, the stability of some low-lying candidates at zero-temperature was not maintained at finite temperatures. Therefore, we suggested that, at finite temperatures, the realistic products of [Ca(H(2)O)(n)](2+) should be a mixture of the best candidate and some of metastable isomers for a given cluster size. For a cluster having second and/or third shell of water molecules, we found structural transitions between a low-lying structure and the lowest-energy structure undergoing much lower energy barriers. In addition, the IR spectra of the best candidates were predicted, in which the evolution of hydrogen-bond configurations with the cluster size was revealed.

Zn2+ has a primary hydration sphere of five: IR action spectroscopy and theoretical studies of hydrated Zn2+ complexes in the gas phase

Complexes of Zn(2+)(H(2)O)(n), where n = 6-12, are examined using infrared photodissociation (IRPD) spectroscopy, blackbody infrared radiative dissociation (BIRD), and theory. Geometry optimizations and frequency calculations are performed at the B3LYP/6-311+G(d,p) level along with single point energy calculations for relative energetics at the B3LYP, B3P86, and MP2(full) levels with a 6-311+G(2d,2p) basis set. The IRPD spectrum of Zn(2+)(H(2)O)(8) is most consistent with the calculated spectrum of the five-coordinate MP2(full) ground-state (GS) species. Results from larger complexes also point toward a coordination number of five, although contributions from six-coordinate species cannot be ruled out. For n = 6 and 7, comparisons of the individual IRPD spectra with calculated spectra are less conclusive. However, in combination with the BIRD and laser photodissociation kinetics as well as a comparison to hydrated Cu(2+) and Ca(2+), the presence of five-coordinate species with some contribution from six-coordinate species seems likely. Additionally, the BIRD rate constants show that Zn(2+)(H(2)O)(6) and Zn(2+)(H(2)O)(7) complexes are less stable than Zn(2+)(H(2)O)(8). This trend is consistent with previous work that demonstrates the enthalpic favorability of the charge separation process forming singly charged hydrated metal hydroxide and protonated water complexes versus loss of a water molecule for complexes of n ≤ 7. Overall, these results are most consistent with the lowest-energy structures calculated at the MP2(full) level of theory and disagree with those calculated at B3LYP and B3P86 levels.

Unimolecular reactions of dihydrated alkaline earth metal dications M2+(H2O)2, M = Be, Mg, Ca, Sr, and Ba: salt-bridge mechanism in the proton-transfer reaction M2+(H2O)2 --> MOH+ + H3O

The unimolecular reactivity of M(2+)(H(2)O)(2), M = Be, Mg, Ca, Sr, and Ba, is investigated by density functional theory. Dissociation of the complex occurs either by proton transfer to form singly charged metal hydroxide, MOH(+), and protonated water, H(3)O(+), or by loss of water to form M(2+)(H(2)O) and H(2)O. Charge transfer from water to the metal forming H(2)O(+) and M(+)(H(2)O) is not favorable for any of the metal complexes. The relative energetics of these processes are dominated by the metal dication size. Formation of MOH(+) proceeds first by one water ligand moving to the second solvation shell followed by proton transfer to this second-shell water molecule and subsequent Coulomb explosion. These hydroxide formation reactions are exothermic with activation energies that are comparable to the water binding energy for the larger metals. This results in a competition between proton transfer and loss of a water molecule. The arrangement with one water ligand in the second solvation shell is a local minimum on the potential energy surface for all metals except Be. The two transition states separating this intermediate from the reactant and the products are identified. The second transition state determines the height of the activation barrier and corresponds to a M(2+)-OH(-)-H(3)O(+) "salt-bridge" structure. The computed B3LYP energy of this structure can be quantitatively reproduced by a simple ionic model in which Lewis charges are localized on individual atoms. This salt-bridge arrangement lowers the activation energy of the proton-transfer reaction by providing a loophole on the potential energy surface for the escape of H(3)O(+). Similar salt-bridge mechanisms may be involved in a number of proton-transfer reactions in small solvated metal ion complexes, as well as in other ionic reactions.

Directly relating reduction energies of gaseous Eu(H2O)n(3+), n = 55-140, to aqueous solution: the absolute SHE potential and real proton solvation energy

In solution, half-cell potentials are measured relative to other half-cells resulting in a ladder of thermodynamic values that is anchored to the standard hydrogen electrode (SHE), which is assigned an arbitrary value of exactly 0 V. A new method for measuring the absolute SHE potential is introduced in which reduction energies of Eu(H(2)O)(n)(3+), from n = 55 to 140, are extrapolated as a function of the geometric dependence of the cluster reduction energy to infinite size. These measurements make it possible to directly relate absolute reduction energies of these gaseous nanodrops containing Eu(3+) to the absolute reduction enthalpy of this ion in bulk solution. From this value, an absolute SHE potential of +4.11 V and a real proton solvation energy of -269.0 kcal/mol are obtained. The infrared photodissociation spectrum of Eu(H(2)O)(119-124)(3+) indicates that the structure of the surface of the nanodrops is similar to that at the bulk air-water interface and that the hydrogen bonding of interior water molecules is similar to that in aqueous solution. These results suggest that the environment of Eu(3+) in these nanodrops and the surface potential of the nandrops are comparable to those of the condensed phase. This method for obtaining absolute potentials of redox couples has the advantage that no explicit solvation model is required, which eliminates uncertainties associated with these models, making this method potentially more accurate than previous methods.

Infrared consequence spectroscopy of gaseous protonated and metal ion cationized complexes

In this article, the new and exciting techniques of infrared consequence spectroscopy (sometimes called action spectroscopy) of gaseous ions are reviewed. These techniques include vibrational predissociation spectroscopy and infrared multiple photon dissociation spectroscopy and they typically complement one another in the systems studied and the information gained. In recent years infrared consequence spectroscopy has provided long-awaited direct evidence into the structures of gaseous ions from organometallic species to strong ionic hydrogen bonded structures to large biomolecules. Much is being learned with respect to the structures of ions without their stabilizing solvent which can be used to better understand the effect of solvent on their structures. This review mainly covers the topics with which the author has been directly involved in research: structures of proton-bound dimers, protonated amino acids and DNA bases, amino acid and DNA bases bound to metal ions and, more recently, solvated ionic complexes. It is hoped that this review reveals the impact that infrared consequence spectroscopy has had on the field of gaseous ion chemistry.

Nanocalorimetry in mass spectrometry: a route to understanding ion and electron solvation

A gaseous nanocalorimetry approach is used to investigate effects of hydration and ion identity on the energy resulting from ion-electron recombination. Capture of a thermally generated electron by a hydrated multivalent ion results in either loss of a H atom accompanied by water loss or exclusively loss of water. The energy resulting from electron capture by the precursor is obtained from the extent of water loss. Results for large-size-selected clusters of Co(NH(3))(6)(H(2)O)(n3)(+) and Cu(H(2)O)(n2)(+) indicate that the ion in the cluster is reduced on electron capture. The trend in the data for Co(NH(3))(6)(H(2)O)(n3)(+) over the largest sizes (n >/= 50) can be fit to that predicted by the Born solvation model. This agreement indicates that the decrease in water loss for these larger clusters is predominantly due to ion solvation that can be accounted for by using a model with bulk properties. In contrast, results for Ca(H(2)O)(n2)(+) indicate that an ion-electron pair is formed when clusters with more than approximately 20 water molecules are reduced. For clusters with n = approximately 20-47, these results suggest that the electron is located near the surface, but a structural transition to a more highly solvated electron is indicated for n = 47-62 by the constant recombination energy. These results suggest that an estimate of the adiabatic electron affinity of water could be obtained from measurements of even larger clusters in which an electron is fully solvated.