Protonation Of Species Research Articles

Mechanism of formate-nitrite transporters by dielectric shift of substrate acidity.

Bacterial formate-nitrite transporters (FNTs) regulate the metabolic flow of small, weak mono-acids. Recently, the eukaryotic PfFNT was identified as the malaria parasite's lactate transporter and novel drug target. Despite crystal data, central mechanisms of FNT gating and transport remained unclear. Here, we show elucidation of the FNT transport mechanism by single-step substrate protonation involving an invariant lysine in the periplasmic vestibule. Opposing earlier gating hypotheses and electrophysiology reports, quantification of total uptake by radiolabeled substrate indicates a permanently open conformation of the bacterial formate transporter, FocA, irrespective of the pH Site-directed mutagenesis, heavy water effects, mathematical modeling, and simulations of solvation imply a general, proton motive force-driven FNT transport mechanism: Electrostatic attraction of the acid anion into a hydrophobic vestibule decreases substrate acidity and facilitates protonation by the bulk solvent. We define substrate neutralization by proton transfer for transport via a hydrophobic transport path as a general theme of the Amt/Mep/Rh ammonium and formate-nitrite transporters.

Structural modifications of lanthanum silicate oxyapatite exposed to high water pressure

Dense lanthanum silicate oxyapatite samples La9.33+x(SiO4)6O2+3x/2 were exposed to high water pressure in autoclave, in order to study the effects of oxygen stoichiometry and treatment duration on the protonation of these materials. TG analyses showed that protonic species were successfully introduced into the bulk of the material, especially for La9.60(SiO4)6O2.4 sample after 84h of treatment at 550°C under 40bar water pressure. It was shown that the mass loss compares well with perovskite materials and increases when protonation time rises (0.66% above 600°C for 408h), with a time limit beyond which the microstructure is no more stable.Rietveld refinement and Raman spectroscopy studies confirmed some structural modifications which could be linked to the incorporation of protonic species. It was shown that autoclave treatment induced an increase of the cell volume related to an increase of the a and b lattice parameters (enlargement of the characteristic channels of oxyapatite).

Solid-state NMR study on changes of phosphate and proton species in metal pyrophosphate composite (MP2 O7 -MO2 ) ceramics.

In order to investigate the effects of composite production and the role of the counter cation for metal phosphate conductors, changes in the solid-state nuclear magnetic resonance (NMR) spectra and magnetic relaxation times caused by the removal of volatile products and water washing were examined for various metal pyrophosphate (MP2 O7 -MO2 ; M = Sn, Si, Ti, and Zr). Acidic species could be detected by 1 H MAS NMR spectra for all the composites except ZrP2 O7 -ZrO2 that had the lowest conductivity. The 31 P DD-MAS NMR spectra for MP2 O7 -MO2 composites showed different signal patterns depending on the counter cations participating in the ion exchange as a result of different microstructures. Combinational analysis of 31 P DD-MAS and 31 P CP-MAS NMR spectra of the composites indicated that protonic bulk phosphates were observed at slightly lower fields than non-protonic bulk phosphates in all of the MP2 O7 -MO2 composites. After water washing, the acidic species and the protonic bulk phosphates of MP2 O7 -MO2 composites disappeared or were reduced to trace amounts. The T1 H values of the water-washed composites lengthened because of removal of orthophosphoric acid, although the T1 P values remained almost unchanged. The results of the solid-state NMR studies suggest that the protonic bulk phosphates of MP2 O7 -MO2 composites do not generally distribute in the bulk but exist in the interface between excess H3 PO4 and the bulk. Copyright © 2016 John Wiley & Sons, Ltd.

Flow-Tube Investigations of Hypergolic Reactions of a Dicyanamide Ionic Liquid Via Tunable Vacuum Ultraviolet Aerosol Mass Spectrometry.

The unusually high heats of vaporization of room-temperature ionic liquids (RTILs) complicate the utilization of thermal evaporation to study ionic liquid reactivity. Although effusion of RTILs into a reaction flow-tube or mass spectrometer is possible, competition between vaporization and thermal decomposition of the RTIL can greatly increase the complexity of the observed reaction products. In order to investigate the reaction kinetics of a hypergolic RTIL, 1-butyl-3-methylimidazolium dicyanamide (BMIM+DCA-) was aerosolized and reacted with gaseous nitric acid, and the products were monitored via tunable vacuum ultraviolet photoionization time-of-flight mass spectrometry at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Reaction product formation at m/z 42, 43, 44, 67, 85, 126, and higher masses was observed as a function of HNO3 exposure. The identities of the product species were assigned to the masses on the basis of their ionization energies. The observed exposure profile of the m/z 67 signal suggests that the excess gaseous HNO3 initiates rapid reactions near the surface of the RTIL aerosol. Nonreactive molecular dynamics simulations support this observation, suggesting that diffusion within the particle may be a limiting step. The mechanism is consistent with previous reports that nitric acid forms protonated dicyanamide species in the first step of the reaction.

Investigation of Ion Conductivity and ORR on Metal/Water Interfaces in Proton Exchange Membrane Fuel Cells

The high cost of platinum (Pt) is a challenging barrier to broad commercialization of proton exchange membrane fuel cells (PEMFCs). Over recent decades efforts have been made to decrease Pt loading by increasing its mass and specific activities through nanostructuring and alloying. One such approach is the nanostructured thin film (NSTF) electrode1, which has Pt or Pt alloy catalyst deposited onto high surface area organic whiskers and lacks the addition of ionomer binder. NSTF cathodes typically have a lower Pt loading than conventional carbon supported Pt (Pt/C) catalyst cathodes, but tend to lose performance in low relative humidity and cold, flooded conditions. Despite the lack of ionomer, the ion conductivity for NSTF and ionomer-free Pt black electrodes has been shown to be good under high relative humidity conditions.2,3 However, the apparent proton transport mechanism is not well understood. The high mobility of adsorbed proton or hydroxyl species and the diffuse charge in the electric double layer could account for high ion conductivity on the metal surface away from the membrane.4 Such mechanisms are complicated by their sensitivities to water activity and interfacial electric potentials. The present work investigates the role of water on the ion conductivity and oxygen reduction reaction (ORR) activity of metal/water interfaces by measuring the dependence of accessible metal surface area on humidity and potential for two ionomer/binder-free metal surfaces: Pt and gold (Au). Pt is studied because of its relevance to PEMFCs and Au is selected as a model surface because of its close ideal behavior and oxide-free surface within the envelope of PEMFC cathode potentials. However, Au catalyst provides low ORR activity relative to Pt. A membrane electrode assembly (MEA) was fabricated by painting a layer of Pt black or Au micro powder (0.5 m2/g) ink on a gas diffusion layer (GDL). A commercial Pt gas diffusion electrode (GDE) was used as the anode. Cyclic voltammetry (CV) was conducted for different relative humidity conditions as shown for Au in Figure 1a. As the inlet gas relative humidity increases, the double layer and Au-OH redox currents increase, indicating increased electrochemical active surface area on the wetted Au. As Figure 1b shows, the increased active area also yields increased current density when operated the cell as an air/H2 fuel cell and polarization curves are measured. This indicates that water films on the Au surface play an essential role in ion conduction and that the active area made accessible by the water also facilitates the ORR. References (1) An, S. J., and Litster, S. (2013) In Situ, Ionic Conductivity Measurement of Ionomer/Binder-Free Pt Catalyst under Fuel Cell Operating Condition. ECS Trans. 58, 831–839. (2) McBreen, J. (1985) Voltammetric Studies of Electrodes in Contact with Ionomeric Membranes. J. Electrochem. Soc. 132, 1112. (3) Sinha, P. K., Gu, W., Kongkanand, A., and Thompson, E. (2011) Performance of Nano Structured Thin Film (NSTF) Electrodes under Partially-Humidified Conditions. J. Electrochem. Soc. 158, B831. (4) Zenyuk, I., and Litster, S. (2014) Modeling ion conduction and electrochemical reactions in water films on thin-film metal electrodes with application to low temperature fuel cells. Electrochim. Acta. Figure 1

EPR of Structural Phase Transition in Manganese- and Copper-Doped Formate Framework of [NH3(CH2)4NH3][Zn(HCOO)3]2

Electron paramagnetic resonance (EPR) and pulse electron–nuclear double resonance (ENDOR) spectroscopies are applied to investigate the structural phase transition and the low temperature phase in manganese- and copper-doped [NH3(CH2)4NH3][Zn(HCOO)3]2 dense metal–organic framework. Continuous-wave (CW) EPR measurements indicate successful incorporation of Mn2+ and Cu2+ ions at the Zn2+ lattice sites. Pulse ENDOR spectrum reveals at least four different proton species in the vicinity of the Mn2+ center showing excellent agreement with the X-ray diffraction experiments. Temperature-dependent CW EPR spectra demonstrate that Mn2+ and Cu2+ local paramagnetic probes are sensitive to the phase transition in the studied compounds. The analysis of the temperature dependence of the Cu2+ hyperfine coupling parameter reveals a first-order phase transition at Tc = 235 K into an antiferroelectric phase that is close to the tricritical point. The obtained logarithmic divergence of the line width of the Mn2+ EPR spectrum...

Anatase nanoparticles boundaries resulting from titanium tetrachloride hydrolysis

An important factor that governs solar energy transformation into electrical or chemical energy, when using nanoparticles-based devices, is the spatial location of traps limiting electron transport. Evidences have been presented indicating that the electron diffusion is strongly influenced by nanoparticle boundaries, whose characteristics depend on the particles preparation. In the present work we have studied the role of hydrated excess proton structures in the formation of anatase nanoparticles boundaries in samples prepared by TiCl4 hydrolysis at low temperature and strong acidic conditions. The samples, constituted by anatase nanocrystals but, mainly, by amorphous titania, are studied by 1H-MAS NMR, FT-IR and HRTEM. The results indicate that hydrated excess proton species, generated by solvation of bridging hydroxyls protons of highly defective anatase and/or amorphous titania chains, favor the chains bonding to anatase nanoparticles and/or their condensation at the nanoparticles surface. The chains’ bonding facilitates the anatase particles aggregation producing oriented self-assemblings. The preparation conditions determine a strong condensation of amorphous titania chains and it should favor anatase transformation into rutile.

Multistate empirical valence bond study of temperature and confinement effects on proton transfer in water inside hydrophobic nanochannels.

Microscopic characteristics of an aqueous excess proton in a wide range of thermodynamic states, from low density amorphous ices (down to 100 K) to high temperature liquids under the critical point (up to 600 K), placed inside hydrophobic graphene slabs at the nanometric scale (with interplate distances between 3.1 and 0.7 nm wide) have been analyzed by means of molecular dynamics simulations. Water-proton and carbon-proton forces were modeled with a multistate empirical valence bond method. Densities between 0.07 and 0.02 Å(-3) have been considered. As a general trend, we observed a competition between effects of confinement and temperature on structure and dynamical properties of the lone proton. Confinement has strong influence on the local structure of the proton, whereas the main effect of temperature on proton properties is observed on its dynamics, with significant variation of proton transfer rates, proton diffusion coefficients, and characteristic frequencies of vibrational motions. Proton transfer is an activated process with energy barriers between 1 and 10 kJ/mol for both proton transfer and diffusion, depending of the temperature range considered and also on the interplate distance. Arrhenius-like behavior of the transfer rates and of proton diffusion are clearly observed for states above 100 K. Spectral densities of proton species indicated that in all states Zundel-like and Eigen-like complexes survive at some extent. © 2016 Wiley Periodicals, Inc.

Insight into Hydrogen Bonding of Uranyl Hydroxide Layers and Capsules by Use of 1H Magic-Angle Spinning NMR Spectroscopy

Solid-state 1H magic-angle spinning (MAS) NMR was used to investigate local proton environments in anhydrous [UO2(OH)2] (α-UOH) and hydrated uranyl hydroxide [(UO2)4O(OH)6·5H2O (metaschoepite). For the metaschoepite material, proton resonances of the μ2-OH hydroxyl and interlayer waters were resolved, with two-dimensional (2D) double-quantum (DQ) 1H–1H NMR correlation experiments revealing strong dipolar interactions between these different proton species. The experimental NMR results were combined with first-principles CASTEP GIPAW (gauge including projector-augmented wave) chemical shift calculations to develop correlations between hydrogen-bond strength and observed 1H NMR chemical shifts. These NMR correlations allowed characterization of local hydrogen-bond environments in uranyl U24 capsules and of changes in hydrogen bonding that occurred during thermal dehydration of metaschoepite.

Deacidification of cranberry juice by electrodialysis: Impact of membrane types and configurations on acid migration and juice physicochemical characteristics

Abstract Cranberry is a typical fruit from North America having a very high acidity that makes raw juice hardly acceptable for consumers. In this study, the reduction of juice acidity was investigated using electrodialysis (ED) with different types of membranes and cell configurations. Electrodialysis is an ecofriendly membrane technology used in a large range of food applications. Three different ED configurations were tested at the laboratory scale: bipolar and anion-exchange membranes (ED2MB), bipolar and ultrafiltration membranes (ED2MBUF), and cation-exchange and ultrafiltration membranes (EDUF). Each configuration was evaluated in terms of juice physicochemical parameters (titratable acidity, conductivity, total soluble solids, color, anthocyanins, organic acids and mineral contents) and electrodialytic parameters (membrane conductivity and thicknesses, global system resistance). In ED2MB configuration, a 40% deacidification rate was reached after 3 h of treatment (80% after 6 h) whereas 0% and only 8% were obtained after 3 h with ED2MBUF and EDUF configurations, respectively. Furthermore, a selective migration of organic acids was observed with the ED2MB configuration: citric acid (22 ppm/min), malic acid (11 ppm/min) and quinic acid (6.5 ppm/min). Consequently, ED2MB configuration allows the deacidification of cranberry juice and the production of pure acids (no waste generated) without any chemical consumption due to the bipolar membrane in-situ generation of proton and hydroxyl species from water.

IRMPD signature of protonated pantothenic acid, an ubiquitous nutrient

Intrinsic properties of pantothenic acid, an essential nutraceutical, are examined. The effect of protonation on the energetic and geometric features of pantothenic acid, generated as gaseous protonated species, are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy over an extended frequency range (800–2000cm−1 and 2800–3700cm−1). DFT calculations are exploited to identify the possible structures and predict the absorption spectra at the B3LYP/6-311++G(d,p) level. Two amide-protonated structures, characterized by the most stable binding motifs, account well for the experimental spectrum, thus revealing structurally diagnostic features of potential benefit for the development of highly sensitive and selective nutrient screening.

Al-rich beta zeolites. Distribution of Al atoms in the framework and related protonic and metal-ion species

Al-rich BEA∗ zeolites with Si/Al 4.5 and 5.1, prepared by a template-free synthesis, were compared to a conventional (Si-rich) BEA∗ (Si/Al 11.3) regarding (i) the distribution of Al atoms in the framework, (ii) the behavior of the H-BEA∗ samples at high temperature, and (iii) the nature of charge balancing counter metal-ion species. 27Al and 29Si (CP) MAS NMR, and FTIR of adsorbed d3-acetonitrile and UV–Vis spectroscopy of Co(II) ions as probes of close Al atoms were used and the study was further supported by DFT molecular dynamic calculations of Co(II) sites in the BEA∗ zeolites. In contrast to the Si-rich BEA∗, the Al atoms in the Al-rich BEA∗ are mostly arranged in AlSiAl sequences with their Al atoms facing two different channels, which thus cannot bind bare divalent ions. Conversely, two Al atoms from two different AlSiAl sequences can face the same channel and they represent close unpaired Al atoms located in two different rings. They can be compensated by [Co(II)(H2O)6]2+, and in dehydrated zeolites by protons, monovalent metal-ions, and metal-oxo structures such as formally monovalent [Co(III)O]+. Only Al atoms in AlSiSiAl sequences in one ring and a minor fraction of AlSiAl sequences face the same channel and thus they can be balanced by bare divalent cations. Therefore, the Al-rich and conventional Si-rich BEA∗ differ substantially in the population of AlSiAl sequences, but not in the SiAl arrangement in the inner surface of zeolite channels which is relevant for binding counter-ion species. The concentration of acid and redox sites in Al-rich BEA∗, the potential catalytic active sites, is proportional to the increased Al content in the framework, but without marked change of their structure.

Arsenic Adsorption on Lanthanum-Impregnated Activated Alumina: Spectroscopic and DFT Study.

Rare earth-modified adsorbents (REMAs) have been widely used to remove oxyanion pollutants from water, including arsenic (As). However, the molecular-level structural information and reactions at the liquid/solid interface are still murky, which limits the design of applicable REMAs. Herein, a lanthanum-impregnated activated alumina (LAA) was synthesized as a representative REMA, and its As uptake mechanisms were explored using multiple complementary characterization techniques. Our adsorption experiments showed that LAA exhibited 2-3 times higher As adsorption capacity than AA. In contrast to the bidentate configuration formed on most metal oxide surfaces, our EXAFS and DFT results suggest that As(III) and As(V) form monodentate surface complexes on LAA through As-O-La coordinative bonding. In situ flow cell ATR-FTIR observed a strong dependence of As-O peak positions on pH, which could be interpreted as the change in the fractions of As(V) surface complexes with zero- to double-protonation on LAA, AA, and LaOOH. As(V) on LAA existed as singly and doubly protonated surface species, and the pKa of transition from double to single protonation (∼5.8) was lower than that for its soluble counterpart (6.97). The surface reaction and structural configuration were incorporated in a CD-MUSIC model to satisfactorily predict macroscopic As adsorption behaviors. The insights gained from the molecular-level reactions shed light on the design and application of REMAs in environmental remediation for As and its structural analogues.

Electrochemical dioxygen reduction catalyzed by a (nitro)cobalt(perfluorophthalocyanine) complex and the possibility of a peroxynitro complex intermediate

The (nitro)([Formula: see text],[Formula: see text]-dimethyl-4-aminopyridine) complex of perfluorinated cobalt(III) phthalocyanine Co(III)F16Pc(Me2Npy)(NO[Formula: see text] catalyzes the electrochemical oxygen reduction reaction (ORR) in pH 4.0, 7.0, and 10.0 buffer and 0.05 M sulfuric acid solution when deposited on a glassy carbon electrode. Cyclic voltammetry (CV), rotating disk electrode voltammetry (RDE), and rotating ring-disk electrode voltammetry (RRDE) have been used to determine the reduction product as hydrogen peroxide although in concentrations too small to observe by qualitative methods such as oxidation of NaI in solution. The dependence of the values of the peak potentials for the reduction on the pH of the solution and the -log[Me2Npy] are consistent with protonation up to pH 7.6 and pyridine ligand loss during the reduction. The addition of nitrite at 0.1 and 1 M to pH 7.0 solutions in contact with films of CoF16Pc on the glassy carbon electrode decreases the ORR current and shifts the peak potential of the ORR from -0.21 V vs. NHE to -0.19 V vs. NHE. The addition of nitrite at 0.1 and 1 M to films of Co(III)F16Pc(Me2Npy)(NO[Formula: see text] on glassy carbon, however, has no effect on either the current or the potential. While the electrochemical evidence for this proposal is not definitive, modeling has been used to examine the center of reduction in the alternative mechanisms by evaluation of the LUMOs of the hypothetical intermediates in both closed and open shell cases. The formation of five-coordinate Co(II)F16Pc(NO) is proposed to occur initially in the reduction mechanism. It is also possible that O2 reduction takes place at the NO ligand center by way of a nitrogen-bound peroxynitrite intermediate. The [Formula: see text] ligand appears to remain bound during the ORR. Direct coordination of O2 to the metal center requiring a six-coordinate species, Co(III)F16Pc(O[Formula: see text](NO[Formula: see text], Co(II)F16Pc(O[Formula: see text](NO) or [Co(II)F16Pc(O[Formula: see text](NO[Formula: see text]][Formula: see text] and has been considered in DFT modeling studies. The instability of the two-electron reduced, protonation species, [Co(I)F16Pc(NO2OH)][Formula: see text] in its loss of peroxynitrous acid suggests that the reduction of O2 may occur by two one-electron reduction steps rather than a two-electron step.

Proton transfer in liquid water confined inside graphene slabs.

The microscopic structure and dynamics of an excess proton in water constrained in narrow graphene slabs between 0.7 and 3.1 nm wide has been studied by means of a series of molecular dynamics simulations. Interaction of water and carbon with the proton species was modeled using a multistate empirical valence bond Hamiltonian model. The analysis of the effects of confinement on proton solvation structure and on its dynamical properties has been considered for varying densities. The system is organized in one interfacial and a bulk-like region, both of variable size. In the widest interplate separations, the lone proton shows a marked tendency to place itself in the bulk phase of the system, due to the repulsive interaction with the carbon atoms. However, as the system is compressed and the proton is forced to move to the vicinity of graphene walls it moves closer to the interface, producing a neat enhancement of the local structure. We found a marked slowdown of proton transfer when the separation of the two graphene plates is reduced. In the case of lowest distances between graphene plates (0.7 and 0.9 nm), only one or two water layers persist and the two-dimensional character of water structure becomes evident. By means of spectroscopical analysis, we observed the persistence of Zundel and Eigen structures in all cases, although at low interplate separations a signature frequency band around 2500 cm^{-1} suffers a blue shift and moves to characteristic values of asymmetric hydronium ion vibrations, indicating some unstability of the typical Zundel-Eigen moieties and their eventual conversion to a single hydronium species solvated by water.

Gd- and Pr-based double perovskite cobaltites as oxygen electrodes for proton ceramic fuel cells and electrolyser cells

Double perovskite oxides BaGd0.8La0.2Co2O6−δ (BGLC), BaGdCo1.8Fe0.2O6−δ (BGCF), BaPrCo2O6−δ (BPC) and BaPrCo1.4Fe0.6O6−δ (BPCF) were investigated as oxygen electrodes on mixed conducting BaZr0.7Ce0.2Y0.1O3 (BZCY72) electrolyte using impedance spectroscopy vs temperature, pO2, and pH2O. We propose and have applied a novel approach to extract and parameterise the charge transfer and diffusion impedances of the electrode reactions in a system comprising charge transport of protons, oxide ions, and electrons. Given by the properties of the BZCY72, transport of protons dominates at lower temperatures and high pH2O, oxide ions at higher temperatures, and electron holes increasingly at high temperatures and high pO2. The electrodes showed good performance, with the lowest total apparent polarisation resistance for BGLC/BZCY72 being 0.05 and 10Ωcm2 at 650 and 350°C, respectively. The low temperature rate limiting reaction step is a surface related process, involving protonic species, with an activation energy of approximately 50kJmol−1 for BGLC/BZCY72. The oxide ion transport taking over at higher temperatures exhibits a higher activation energy typical of SOFC cathodes. Thermogravimetric studies revealed that BGLC exhibits considerable protonation at 300–400°C, which may be interpreted as hydration with an enthalpy of approximately –50kJmol−1. The resulting mixed proton electron conduction may explain its good performance as electrode on BZCY72.

Water pressure enhanced sintering of alkaline-earth perovskite ceramics

The alkaline earth-based perovskite ceramics are potential electrolytes for components of electrolysers, fuel cells and CO2/syngas converters. Their high refractariness is an advantage from the chemical stability point of view but involves high sintering temperature (>1450°C), detrimental to the co-sintering procedure required to assemble device components. Since the presence of protonic species is not intrinsic to the perovskite structure, the Ln-modified perovskites have to be successfully protonated by exposition of dense (95–99% theoretical density) ceramic pieces to (pressurized) water. We demonstrate that densification of a AZr1−xLnxO3−δ ceramic can be enhanced by prior exposition of 1200 to 1300°C pre-fired (porous) ceramics to high water pressure using an autoclave. Autoclave temperature and treatment duration are considered. Thermal expansion measurements reveal that the protonation (200–300°C under 15–80bar of water vapour) of previously fired (70–80% of the theoretical density) porous perovskite body (SrZr1−xYbxO3, x=0.1) enhances the sintering and allows densification at lower temperature. The gain in temperature reaches 200–300°C. IR and Raman studies show that the film formed on surface consists of (hydrated) alkali-earth oxyhydroxides. Sintering enhancement is assigned to the modification of the material structure close to the grain surface/boundary and to the enhancement of ions mobility at the grain surface/interface.

Transfer Rate Edited experiment for the selective detection of Chemical Exchange via Saturation Transfer (TRE-CEST)

Chemical Exchange Saturation Transfer (CEST) magnetic resonance experiments have become valuable tools in magnetic resonance for the detection of low concentration solutes with far greater sensitivity than direct detection methods. Accurate measures of rates of chemical exchange provided by CEST are of particular interest to biomedical imaging communities where variations in chemical exchange can be related to subtle variations in biomarker concentration, temperature and pH within tissues using MRI. Despite their name, however, traditional CEST methods are not truly selective for chemical exchange and instead detect all forms of magnetization transfer including through-space NOE. This ambiguity crowds CEST spectra and greatly complicates subsequent data analysis. We have developed a Transfer Rate Edited CEST experiment (TRE-CEST) that uses two different types of solute labeling in order to selectively amplify signals of rapidly exchanging proton species while simultaneously suppressing ‘slower’ NOE-dominated magnetization transfer processes. This approach is demonstrated in the context of both NMR and MRI, where it is used to detect the labile amide protons of proteins undergoing chemical exchange (at rates⩾30s−1) while simultaneously eliminating signals originating from slower (∼5s−1) NOE-mediated magnetization transfer processes. TRE-CEST greatly expands the utility of CEST experiments in complex systems, and in-vivo, in particular, where it is expected to improve the quantification of chemical exchange and magnetization transfer rates while enabling new forms of imaging contrast.

Defect Chemistry of CeO2 Surfaces from First Principles and Space Charge Theory

CeO2 (ceria) is a diverse oxide with numerous technologically important functional properties. It is one of the most studied oxygen incorporation materials, and exhibits remarkable catalytic properties towards hydrocarbon and CO oxidation, and water splitting reactions.1-3 Further, acceptor doped ceria is of great interest as electrolyte for solid oxide fuel cells due to its considerable oxide ion conductivity at intermediate temperatures.4 Recently, nanocrystalline CeO2 ceramics were shown to display significant proton conduction even at room temperature,5 suggested to be due to surface, or sub-surface proton migration. The majority of these functional properties are affected by, or even originate from the rich surface, or interface defect chemistry of ceria, and corresponding space charge layers.3 , 6 In this contribution, we explore the defect chemistry of various surface terminations of CeO2 at finite temperatures, by combining first principles calculations and space charge theory. Plane-wave based electronic/atomic structure calculations at the GGA+U and hybrid HF-DFT levels are used to determine bulk defect formation energetics and the defects' surface segregation energies in CeO2. The first principles results are further combined with space charge theory through numerical evaluation of the space charge potential profiles and corresponding defect concentration profiles. The charged oxygen vacancies and protonic defects exhibit highly favorable surface segregation energies, and therefore tend to accumulate on CeO2 surfaces. Although such accumulation is partly charge compensated by oxide and hydroxide ad-ions and Ce3+ species at the surface, the surface turns out to be overall positively charged, and charge-compensated by a negative sub-surface space charge region, at least for low defect concentrations. Under humid conditions, especially at lower temperatures, the surfaces are saturated by protonic species, forming water layers. Finally, the effect of such surface saturation by protonic species, and also H2O monolayer and film formation on the sub-surface space charge potential and defect concentration profiles are discussed. 1. A. Trovarelli, Catalysis Reviews, 1996, 38, 439-520. 2. E. P. Murray, T. Tsai and S. A. Barnett, Nature, 1999, 400, 649-651. 3. Z. A. Feng, F. El Gabaly, X. Ye, Z.-X. Shen and W. C. Chueh, Nat Commun, 2014, 5. 4. H. L. Tuller and A. S. Nowick, Journal of The Electrochemical Society, 1975, 122, 255-259. 5. E. Ruiz-Trejo and J. Kilner, Journal of Applied Electrochemistry, 2009, 39, 523-528. 6. G. Gregori, M. Shirpour and J. Maier, Advanced Functional Materials, 2013, 23, 5861-5867.

The chemistry of sour taste and the strategy to reduce the sour taste of beer

The contributions of free hydrogen ions, undissociated hydrogen ions in protonated acid species, and anionic acid species to sour taste were studied through sensory experiments. According to tasting results, it can be inferred that the basic substance producing a sour taste is the hydrogen ion, including free hydrogen ions and undissociated hydrogen ions. The intensity of a sour taste is determined by the total concentration of free hydrogen ions and undissociated hydrogen ions. The anionic acid species (without hydrogen ions) does not produce a sour taste but can intensify or weaken the intensity of a sour taste. It seems that hydroxyl or conjugated groups in anionic acid species can intensify the sour taste produced by hydrogen ions. The following strategy to reduce the sensory sourness is advanced: not only reduce free hydrogen ions, namely elevate pH value, but also reduce the undissociated hydrogen ions contained in protonated acid species.