Microhydration of the Magnesium(II) Acetate Cation in the Gas Phase

Stepwise hydration of [CH3COOMg]+ studied by cold ion trap infrared spectroscopy: insights into interactions in the magnesium channel selection filters

The magnesium channel controls Mg2+ concentration in the cell and plays an indispensable role in biological functions. The crystal structure of the Magnesium Transport E channel suggested that Mg2+ hydrated by 6 water molecules is transported through a selection filter consisting of COO- groups on two Asp residues. This Mg2+ motion implies successive pairing with -OOC-R and dissociation mediated by water molecules. For another divalent ion, however, it is known that RCOO-⋯Ca2+ cannot be separated even with 12 water molecules. From this discrepancy, we probe the structure of Mg2+(CH3COO-)(H2O)4-17 clusters by measuring the infrared spectra and monitoring the vibrational frequencies of COO- with the help of quantum chemistry calculations. The hydration by (H2O)6 is not enough to induce ion separation, and partially-separated or separated pairs are formed from 10 water molecules at least. These results suggest that the ion separation between Mg2+ and carboxylate ions in the selection-filter of the MgtE channel not only results from water molecules in their first hydration shell, but also from additional factors including water molecules and protein groups in the second solvation shell of Mg2+.

Binding of divalent cations to acetate: molecular simulations guided by Raman spectroscopy

We combine Raman-MCR vibrational spectroscopy experiments with ab initio and classical MD simulations to gain molecular insights into carboxylate–cation binding.

Vacuum FTIR study on the hygroscopicity of magnesium acetate aerosols

Hygroscopicity and volatility of secondary organic aerosol (SOA) are two important properties, which determine the composition, concentration, size, phase state of SOA and thus chemical and optical properties for SOA. In this work, magnesium acetate (Mg(Ac)₂) aerosol was used as a simple SOA model in order to reveal relationship between hygroscopicity and volatility. A novel approach was set up based on a combination of a vacuum FTIR spectrometer and a home-made relative humidity (RH) controlling system. The striking advantage of this approach was that the RH and the compositions of aerosols could be obtained from a same IR spectrum, which guaranteed the synchronism between RH and spectral features on a sub-second scale. At the constant RH of 90% and 80% for 3000s, the water content within Mg(Ac)₂ aerosol particles decreased about 19.0% and 9.4% while there were 13.4% and 6.0% of acetate loss. This was attributed to a cooperation between volatile of acetic acid and Mg²⁺ hydrolysis in Mg(Ac)₂ aerosols, which greatly suppressed the hygroscopicity of Mg(Ac)₂ aerosols. When the RH changed with pulsed mode between ~70% and ~90%, hygroscopicity relaxation was observed for Mg(Ac)₂ aerosols. Diffuse coefficient of water in the relaxation process was estimated to be ~5×10^-12m²·s^-1 for the Mg(Ac)₂ aerosols. Combining the IR spectra analysis, the decrease in the diffuse coefficient of water was due to the formation of magnesium hydroxide accompanying acetic acid evaporation in the aerosols.

Infrared Multiphoton Dissociation Spectroscopy with Free-Electron Lasers: On the Road from Small Molecules to Biomolecules

Infrared multiphoton dissociation (IRMPD) spectroscopy is commonly used to determine the structure of isolated, mass-selected ions in the gas phase. This method has been widely used since it became available at free-electron laser (FEL) user facilities. Thus, in this Minireview, we examine the use of IRMPD/FEL spectroscopy for investigating ions derived from small molecules, metal complexes, organometallic compounds and biorelevant ions. Furthermore, we outline new applications of IRMPD spectroscopy to study biomolecules.

Coordination-Dependent Spectroscopic Signatures of Divalent Metal Ion Binding to Carboxylate Head Groups: H2- and He-Tagged Vibrational Spectra of M2+·RCO2¯ (M = Mg and Ca, R = -CD3, -CD2CD3) Complexes

We explore the intramolecular distortions present in divalent metal ion-carboxylate ion pairs using vibrational spectroscopy of the cryogenically cooled, mass-selected species isolated in the gas phase. The spectral signatures of the C-O stretching modes are identified using the perdeutero isotopologues of the acetate and propionate anions to avoid congestion arising from the CH₂ fundamentals. Both Ca²⁺ and Mg²⁺ are observed to bind in a symmetrical, so-called "bidentate" arrangement to the -CO₂¯ group. The very strong deformations of the head groups displayed by the binary complexes dramatically relax when either neutral water molecules or counterions are attached to the Mg²⁺RCO₂¯ cation. These results emphasize the critical role that local coordination plays when using the RCO₂¯ bands to deduce the metal ion complexation motif in condensed media.

Initial hydration processes of magnesium chloride: size-selected anion photoelectron spectroscopy and ab initio calculations

Separation of Cl⁻–Mg²⁺ ion pairs starts at n = 4 in MgCl₂(H₂O)_n⁻ anions and at n = 7 in neutral MgCl₂(H₂O)_n.

Reductions of oxygen, carbon dioxide, and acetonitrile by the magnesium(II)/magnesium(I) couple in aqueous media: theoretical insights from a nano-sized water droplet

Reductions of O2, CO2, and CH3CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg(2+) + e(-) → Mg(+•)) confined in a nanosized water droplet ([Mg(H2O)16](•+)) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in subpicoseconds. The reduction of O2 requires an overall doublet spin state of the system. The reductions of CO2 and CH3CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e., O2(•-), CO2(•-), and CH3CN(•-), are initially formed on the cluster surface. O2(•-) and CO2(•-) can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg(2+) as a solvent-separated ion pair (ssip) or directly coordinated to Mg(2+) as a contact-ion pair (cip) having the (1)η-[MgO2](•+) and (1)η-[MgOCO](•+) coordination modes. The (1)η-[MgO2](•+) core is more crowded than the (1)η-[MgOCO](•+) core. The reaction enthalpies of the formation of ssip and cip of [Mg(CO2)(H2O)16](•+) are -36 ± 4 kJ mol(-1) and -30 ± 9 kJ mol(-1), respectively, which were estimated based on the average temperature changes during the ion-molecule reaction between CO2 and [Mg(H2O)16](•+). The values for the formation of ssip and cip of [Mg(O2)(H2O)16](•+) are estimated to be -112 ± 18 kJ mol(-1) and -128 ± 28 kJ mol(-1), respectively. CH3CN(•-) undergoes protonation spontaneously to form the hydrophobic [CH3CN, H](•). Both CH3CN and [CH3CN, H](•) cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol(-1) and ∼40 kJ mol(-1), respectively. These results provide fundamental insights into the solvation dynamics of the Mg(2+)/Mg(•+) couple on the molecular level.

Copper mediated decyano decarboxylative coupling of cyanoacetate ligands: Pesci versus Lewis acid mechanism

A combination of gas-phase ion trap multistage mass spectrometry (MSⁿ) experiments and density functional theory (DFT) calculations have been used to examine the mechanisms of the sequential decomposition reactions of copper cyanoacetate anions, [(NCCH₂CO₂)₂Cu]⁻.

Gas phase structure and reactivity of doubly charged microhydrated calcium(II)-catechol complexes probed by infrared spectroscopy

Doubly charged microhydrated adducts formed from catechol and calcium(II) were produced in the gas phase using electrospray ionization (ESI) appearing as the most important ions in the mass spectra recorded. The gas phase structures of [Ca(catechol)2(H2O)](2+) and [Ca(catechol)2(H2O)2](2+) have been assayed by IR multiphoton dissociation (IRMPD) spectroscopy, recording their vibrational spectra in the 3450-3750 cm(-1) range (OH stretching region) and in the 900-1700 cm(-1) fingerprint spectral region. The agreement between experimental and calculated IR spectra of the selected cluster ions confirmed the suitability of the proposed geometries. In addition, quantum chemical calculations at the B3LYP/6-311+G(d,p) level of theory were performed for [Ca(catechol)2(H2O)](2+) to gain insight into the major routes of dissociation. The results suggest that loss of the water molecule is the lowest energy fragmentation channel followed by charge separation products and neutral loss of one catechol molecule, in agreement with the product ions observed upon collision-induced dissociation (CID).