Reaction In Aqueous Solution Research Articles

Ambient Aqueous Synthesis of Imine-Linked Covalent Organic Frameworks (COFs) and Fabrication of Freestanding Cellulose Nanofiber@COF Nanopapers.

Covalent organic frameworks (COFs) are usually synthesized under solvothermal conditions that require the use of toxic organic solvents, high reaction temperatures, and complicated procedures. Additionally, their insolubility and infusibility present substantial challenges in the processing of COFs. Herein, we report a facile, green approach for the synthesis of imine-linked COFs in an aqueous solution at room temperature. The key behind the synthesis is the regulation of the reaction rate. The preactivation of aldehyde monomers using acetic acid significantly enhances their reactivity in aqueous solutions. Meanwhile, the still somewhat lower imine formation rate and higher imine breaking rates in aqueous solution, in contrast to conventional solvothermal synthesis, allow for the modulation of the reaction equilibrium and the crystallization of the products. As a result, highly crystalline COFs with large surface areas can be formed in relatively high yields in a few minutes. In total, 16 COFs are successfully synthesized from monomers with different molecular sizes, geometries, pendant groups, and core structures, demonstrating the versatility of this approach. Notably, this method works well on the gram scale synthesis of COFs. Furthermore, the aqueous synthesis facilitates the interfacial growth of COF nanolayers on the surface of cellulose nanofibers (CNFs). The resulting CNF@COF hybrid nanofibers can be easily processed into freestanding nanopapers, demonstrating high efficiency in removing trace amounts of antibiotics from wastewater. This study provides a route to the green synthesis and processing of various COFs, paving the way for practical applications in diverse fields.

Anisotropic pressure effects on nanoconfined water within narrow graphene slit pores.

There is an increasing interest toward disclosing and explaining confinement effects on liquids, such as water or aqueous solutions, in slit pore setups. Particularly puzzling are the changes of physical and chemical properties in the nanoconfinement regime where no bulk-like water phase exists between the two interfacial water layers such that the density profile across the slit pore becomes highly stratified, ultimately leading to bilayer and monolayer water. These changes must be quantified with respect to some meaningful reference state of water, the most natural one being bulk water at the same pressure and temperature conditions. However, bulk water is a homogeneous liquid with isotropic properties, whereas water confined in slit pores is inhomogeneous, implying anisotropic properties as described by the perpendicular and parallel components of the respective tensors. In the case of pressure, the inhomogeneous nature of the setup results in a well-defined difference between the perpendicular and parallel pressure tensor components that is uniquely determined by the interfacial tension being a thermodynamic property. For bilayer water constrained in graphene slit pores that are only about 1 nm wide, we demonstrate that there exists a thermodynamic point where the pressure tensor of the inhomogeneous fluid, nanoconfined water, is effectively isotopic and the pressure is thus scalar as in the homogeneous fluid, bulk water. This specific point of vanishing effective interfacial tension is proposed to serve as a well-defined reference state to compare the properties of nanoconfined liquids to those of the corresponding bulk liquid at the same (isotropic) pressure and temperature conditions. In future work, this idea could be applied to assess confinement effects on chemical reactivity in aqueous solutions as well as to other nanoconfined liquids in other pores such as layered minerals.

Synthesis and Chromatographic Determination of S-Nitrosopantetheine: Exploring Reactivity and Stability in Different Aqueous Solutions

S-nitrosothiols (RSNOs) are a group of sulfur-containing compounds biologically involved in nitric oxide (NO˙) release and signalling pathway. NO˙ plays important physiological and pharmacological activities, particularly in vasodilation and in inducing muscle relaxation. Several RSNO compounds have been detected in biological systems, and many of them have been chemically synthesized in the laboratory. To date, no works describing the synthesis of the S-nitrosopantetheine (SNOPANT) are reported in the literature. Taking into account that pantetheine is a biological thiol with a crucial function in metabolism, its nitrosylation in vivo could play a role in various metabolic signalling pathways. In this paper, the synthesis and the chromatographic determination of SNOPANT is reported for the first time, as well as a brief investigation of its reactivity in aqueous solutions in the presence of factors known to affect its stability.

A Mechanistic Study on the Non‐enzymatic Hydrolysis of Kdn Glycosides

AbstractSialic acids are biologically important carbohydrates that are prevalent throughout nature. We are interested in their intrinsic reactivity in aqueous solution and how such reactivity affects the design of substrates for investigation of enzymes that process these sugars. To probe the reactivity differences between two sialic acid family members N‐acetylneuraminic acid and Kdn we measured the rate constants for hydrolysis of 4‐nitrophenyl 3‐deoxy‐d‐glycero‐α‐d‐galacto‐non‐2‐ulosonide in aqueous solution. The kinetic data is consistent with glycosidic C−O bond cleavage occurring via four mechanistic pathways, and these are: (i) hydronium ion‐catalyzed hydrolysis of the neutral sugar; (ii) hydronium ion‐catalyzed hydrolysis of the glycosidic carboxylate; (iii) water‐catalyzed hydrolysis of the anionic glycoside; and (iv) base‐promoted reaction of the anionic glycoside. To study the effects of C‐5 substitution on the Kdn glycoside we made 4‐nitrophenyl 5‐O‐methyl‐α‐Kdn glycoside and determined its rate constants for hydrolysis. All hydrolytic rate constants for both Kdn glycosides were larger than those reported for the parent N‐acetyl‐α‐neuraminide. The water‐catalyzed reaction (pathway iii) exhibited a βlg value of −1.3±0.1. We conclude that the larger rate constants associated with C5‐oxygen containing sialosides results from less steric congestion at the hydrolytic transition states than for the parent C‐5 acetamido glycoside.

Relative Contributions of Mg Hydration and Molecular Structural Restraints to the Barrier of Dolomite Crystallization: A Comparison of Aqueous and Non-Aqueous Crystallization in (BaMg)CO3 and (CaMg)CO3 Systems

Carbonate mineralization is reasonably well-understood in the Ca–CO2–H2O system but continuously poses difficulties to grasp when Mg is present. One of the outstanding questions is the lack of success in dolomite MgCa(CO3)2 crystallization at atmospheric conditions. The conventional view holds that hydration retards the reactivity of Mg2+ and is supported by solvation shell chemistry. This theory however is at odds with the easy formation of norsethite MgBa(CO3)2, a structural analogue of dolomite, leading to the premise that crystal or molecular structural constrains may also be at play. The present study represents our attempts to evaluate the separate contributions of the two barriers. Crystallization in the Mg–Ba–CO2 system was examined in a non-aqueous environment and in H2O to isolate the effect of hydration by determining the minimal relative abundance of Mg required for norsethite formation. The results, showing an increase from 1:5 to 6:4 in the solution Mg/Ba ratio, represented a ~88% reduction in Mg2+ reactivity, presumably due to the hydration effect. Further analyses in the context of transition state theory indicated that the decreased Mg2+ reactivity in aqueous solutions was equivalent to an approximately 5 kJ/mol energy penalty for the formation of the activated complex. Assuming the inability of dolomite to crystallizes in aqueous solutions originates from the ~40 kJ/mol higher (relative to norsethite) Gibbs energy of formation for the activated complex, a hydration effect was estimated to account for ~12% of the energy barrier. The analyses present here may be simplistic but nevertheless consistent with the available thermodynamic data that show the activated complex of dolomite crystallization reaction is entropically favored in comparison with that of norsethite formation but is significantly less stable due to the weak chemical bonding state.

Modeling the Hydrolysis of Iron-Sulfur Clusters.

Iron-sulfur (FeS) clusters are essential metal cofactors involved in a wide variety of biological functions. Their catalytic efficiency, biosynthesis, and regulation depend on FeS stability in aqueous solution. Here, molecular modeling is used to investigate the hydrolysis of an oxidized (ferric) mononuclear FeS cluster by bare dissociation and water substitution mechanisms in neutral and acidic solution. First, approximate electronic structure descriptions of FeS reactions by density functional theory are validated against high-level wave function CCSD(T) calculations. Solvation contributions are included by an all-atom model with hybrid quantum chemical/molecular mechanical (QM/MM) potentials and enhanced sampling molecular dynamics simulations. The free energy profile obtained for FeS cluster hydrolysis indicates that the hybrid functional M06 together with an implicit solvent correction capture the most important aspects of FeS cluster reactivity in aqueous solution. Then, 20 reaction channels leading to two consecutive Fe-S bond ruptures were explored with this calibrated model. For all protonation states, nucleophilic substitution with concerted bond breaking and forming to iron is the preferred mechanism, both kinetic and thermodynamically. In neutral solution, proton transfer from water to the sulfur leaving group is also concerted. Dissociative reactions show higher barriers and will not be relevant for FeS reactivity when exposed to solvent. These hydrolysis mechanisms may help to explain the stability and catalytic mechanisms of FeS clusters of multiple sizes and proteins.

Direct Observation of Nanoparticulate Goethite Recrystallization by Atom Probe Analysis of Isotopic Tracers

Goethite (α-FeOOH) is dispersed throughout the earth's surface, and its propensity to recrystallize in aqueous solutions determines whether this mineral is a source or sink for critical trace elements in the environment. Under reducing conditions, goethite commonly coexists with aqueous Fe(II) (Fe(II)aq), which accelerates recrystallization by coupled electron transfer and atom exchange. Quantifying the amount of the mineral phase that exchanges its structural Fe(III) atoms with Fe(II)aq is complicated by recrystallization models with untested assumptions of whether, and to what extent, the recrystallized portion of the mineral continues to interact with the solution. Here, we reacted nanoparticulate goethite with 57Fe-enriched Fe(II)aq and used atom probe tomography (APT) to resolve the three-dimensional distribution of Fe isotopes in goethite at the sub nm scale. We found that the 57Fe tracer isotope is enriched in the bulk structure (tens of nanometers deep), with some samples having 57Fe penetration throughout at a level that is similar to the isotopic composition of Fe(II)aq. This suggests that some particles undergo near-complete recrystallization. In other cases, however, the distribution of 57Fe is more heterogeneous and generally concentrates near the particle periphery. Nanoparticle encapsulation and subsequent APT can hence capture hidden recrystallization mechanisms which are critical to predicting mineral reactivity in aqueous solutions.

Stability and Catalase-Like Activity of a Mononuclear Non-Heme Oxoiron(IV) Complex in Aqueous Solution.

Heme-type catalase is a class of oxidoreductase enzymes responsible for the biological defense against oxidative damage of cellular components caused by hydrogen peroxide, where metal-oxo species are proposed as reactive intermediates. To get more insight into the mechanism of this curious reaction a non-heme structural and functional model was carried out by the use of a mononuclear complex [FeII(N4Py*)(CH3CN)](CF3SO3)2 (N4Py* = N,N-bis(2-pyridylmethyl)- 1,2-di(2-pyridyl)ethylamine) as a catalyst, where the possible reactive intermediates, high-valent FeIV=O and FeIII–OOH are known and spectroscopically well characterized. The kinetics of the dismutation of H2O2 into O2 and H2O was investigated in buffered water, where the reactivity of the catalyst was markedly influenced by the pH, and it revealed Michaelis–Menten behavior with KM = 1.39 M, kcat = 33 s−1 and k2(kcat/KM) = 23.9 M−1s−1 at pH 9.5. A mononuclear [(N4Py)FeIV=O]2+ as a possible intermediate was also prepared, and the pH dependence of its stability and reactivity in aqueous solution against H2O2 was also investigated. Based on detailed kinetic, and mechanistic studies (pH dependence, solvent isotope effect (SIE) of 6.2 and the saturation kinetics for the initial rates versus the H2O2 concentration with KM = 18 mM) lead to the conclusion that the rate-determining step in these reactions above involves hydrogen-atom transfer between the iron-bound substrate and the Fe(IV)-oxo species.

Detailed Investigation of the Outstanding Peroxyl Radical Scavenging Activity of Two Novel Amino-Pyridinol-Based Compounds.

The ability of two novel amino-pyridinol based compounds (NPyr6 and NPyr7) as peroxyl radical scavengers was investigated in silico. The gathered data indicate that they are exceptionally efficient in that role. However, solvent polarity influences their relative efficiency for that purpose. NPyr6 was identified as the best peroxyl radical scavenger in lipid solution, while NPyr7 takes that place in aqueous solution. Both compounds present two acid-base equilibria, which influence their reactivity in aqueous solution. The associated pKa values were estimated. Several reaction mechanisms were explored. Hydrogen transfer from the phenolic group was identified as the chemical route with the highest contribution to the antioxidant behavior of the investigated compounds in both, nonpolar medium and aqueous solution (at 2 ≤ pH ≤ 10). At higher pH other reaction pathways become the most relevant ones. In addition, their bioavailability, cell permeability, safety, and manufacturability were evaluated. According to these, particularly toxicity, NPyr7 seems to be a better candidate for use as an oral drug to fight oxidative stress than NPyr6.

Synthesis, reactivity and preliminary biological activity of iron(0) complexes with cyclopentadienone and amino‐appended N‐heterocyclic carbene ligands

Neutral and cationic cyclopentadienone (CpO) N‐heterocyclic carbene (NHC) bis‐carbonyl iron(0) complexes bearing, appended to the NHC ligand, either a terminal amino group on the lateral chain, [Fe(η4‐CpO)(CO)2(κC‐NHC(CH2)nNH2)] with n = 2 (2a) and 3 (2b), or a cationic NMe3+ fragment, [Fe(η4‐CpO)(CO)2(κC‐NHC(CH2)2NMe3)](I) (3), were prepared and characterized in terms of their structure, stability and reactivity. The photochemical properties of 2a and 2b were examined both in organic solvents and in water, revealing the photoactivated release of one CO ligand followed by the formation of the chelated complex [Fe(η4‐CpO)(CO)(κ2C,N‐NHC(CH2)2NH2)] (4), whose molecular structure was confirmed by single crystal X‐ray diffraction studies. This metallacyclization occurs only in the case of 2a, with the ethylene spacer between NHC ring and NH2 group in the lateral chain, allowing the formation of a stable 6‐membered ring. On the other hand, 2b undergoes decomposition upon irradiation. The reactivity in aqueous solutions revealed the chemical speciation of the complexes at different pH and especially under physiological conditions (phosphate buffer solution at pH 7.4 and 37 °C). The lack of data on the biological properties of iron(0) complexes prompted us to preliminarily investigate their cytotoxicity against model cancer cells (AsPC‐1 and HPAF‐II), along with a determination of their lipophilicity.

Pore scale modelling of calcite cement dissolution in a reservoir sandstone matrix

Long-term evolution of permeability and tortuosity due to porosity changes evoked by reactivity of aqueous solutions is of paramount importance for predicting water-rock interaction. This challenge is best tackled by introducing reactive transport modelling on the pore-scale, where the modeling domain is a high-resolution tomographic image of the porous media. We suggest to use a voxel based Navier-Stokes-Brinkman solver in a finite volume formulation coupled to the thermodynamic equilibrium code PhreeqC. High-performance parallelized computations using this coupled numerical reactive transport solver are performed directly on the voxel grid of the segmented micro-CT scans. Retreat of the calcite cement in a sandstone matrix due to dissolution reactions can be directly visualized by digital rock physics experiments.

Cyclodextrin-Driven Formation of Double Six-Ring (D6R) Silicate Cage: NMR Spectroscopic Characterization from Solution to Crystals

Identification and isolation of secondary building units (SBUs) from synthesis media of zeolites still represent a challenging task for chemists. The cage structure anion Si12O3012− known as the double six-ring (D6R) was synthesized from α-cyclodextrin (α-CD) mediated alkaline silicate solutions and conditions of its stability and reactivity in aqueous solution were studied by using nuclear magnetic resonance (NMR) spectroscopy. A single crystal X-ray diffraction (XRD) analysis revealed a novel polymorph of the hybrid complex K12Si12O30·2α-CD·nD2O (n ≈ 30–40), which crystallizes in the orthorhombic C2221 space group symmetry with a = 14.841(4) Å, b = 25.855(6) Å, and c = 41.91(1) Å. The supramolecular adduct of the silicate anion sandwiched by two α-CDs forms a perfect symmetry matching the H-bonding donor-acceptor system between the organic macrocycle and the D6R unit. The driving force of such a hybrid assembly has found to be strongly dependent on the nature of the cation present as large alkali counter ions (K+, Rb+ and Cs+), which stabilize the D6R structure acting as templates. Lastly, we provided the first 29Si MAS NMR measurement of 3Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and <SiOSi> bond angle for 3Q Si species in DnR cages (n = 3, 4, 6).

Electronic Tuning of Co, Ni‐Based Nanostructured (Hydr)oxides for Aqueous Electrocatalysis

AbstractCo, Ni‐based (hydr)oxides with stoichiometric or nonstoichiometric composites are playing significant roles in renewable energy technologies, such as electrocatalytic reactions for transforming earth abundant resources into value‐added chemicals. In the past several years, attributed to the development of novel synthesis strategies, characterization techniques, and theoretical calculations, the rational design and realization of a variety of Co, Ni‐based (hydr)oxides with exciting performances have been demonstrated. Particularly, the electrocatalytic reactivities of these Co, Ni‐based (hydr)oxides are highly dependent on their surface electronic structures, which can be well tuned by their nanostructures and compositions. This Review summarizes recent research advances in regulating electronic structures toward the optimization of electrochemical reactivity in aqueous solutions. These research works mainly focus on three important clean energy reactions: i) water oxidation, ii) oxygen reduction, and iii) carbon dioxide reduction, where the developments of advanced electrocatalysts can enable the production of value‐added chemicals or fuels. Furthermore, the electronic structure–property relationships behind the enhanced electrocatalytic properties of these Co, Ni‐based (hydr)oxides are also discussed.

Application of Channel Flow Double Electrode to pH Detection of Magnesium Dissolution during Anodic Polarization

An elucidation of magnesium dissolution mechanisms is one of the key issues for wider usage of magnesium and its alloys in industry because the magnesium has high reactivity in aqueous solutions. The hydrogen evolution rate is increased with positive polarization from the open circuit potential, which is the so called negative difference effect (NDE).1,2 The real-time hydrogen measurement methods have been developed to investigate the NDE.3-6 Authors7 developed a pH sensing channel flow double electrode (CFDE) in order to investigate the pH changes due to the magnesium dissolution. Our group8-10 have been applied the CFDE for analysis of dissolution mechanisms of copper and copper alloys because the oxidation state of the metal ions dissolved from the metal working electrode (WE) can be detected by arranging a detecting electrode (DE) downstream of the WE. In the pH sensing CFDE, the DE was used as the pH sensor, namely, the hydroxide ions generated during the anodic polarization of magnesium WE can be detected at the pH sensor. It indicates that this method allows for the estimation of the hydrogen evolution rate from pH values without the hydrogen gas collection. In addition, a video recording of the WE surface was performed during the measurement in this system. In the present study, the magnesium dissolution during anodic polarization was examined by the pH sensing CFDE. The relationship between the breakdown of the oxide film formed on the magnesium surface and increase of pH was discussed, and the hydrogen evolution rate was estimated from the pH values. References G. Song, A. Atrens, D. St John, X. Wu and J. Nairn, Corros. Sci., 39, 1981 (1997).G. S. Frankel, A. Samaniego and N. Birbilis, Corros. Sci., 70, 104 (2013).M. Curioni, Electrochim. Acta, 120, 284 (2014).S. Lebouil, A. Duboin, F. Monti, P. Tabeling, P. Volovitch and K. Ogle, Electrochim. Acta, 24, 176 (2014).S. Fajardo and G. S. Frankel, Electrochim. Acta, 165, 255 (2015).Y. Hoshi, R. Takemiya, I. Shitanda and M. Itagaki, J. Electrochem. Soc., 163, C303 (2016).Y. Hoshi, K. Miyazawa, I. Shitanda and M. Itagaki, J. Electrochem. Soc., 165, C243 (2018).M. Itagaki, M. Tagaki, T. Mori and K. Watanabe, Corros. Sci., 38, 601 (1996).Y. Hoshi, M. Itagaki, K. Tabei and I. Shitanda, J. Solid State Electrochem., 19, 3551 (2015).Y. Hoshi, T. Oda, I. Shitanda and M. Itagaki, J. Electrochem. Soc., 164, C450 (2017).

Catalytic Effect of Aqueous Solution in Water-Assisted Proton-Transfer Mechanism of 8-Hydroxy Guanine Radical.

Water-assisted proton-transfer process is a key step in guanine damage reaction by hydroxyl radical in aqueous solution. In this article, we quantitatively determine the solvent effect in water-assisted proton-transfer mechanism of 8-hydroxy guanine radical using combined quantum mechanics and molecular mechanism with an explicit solvation model. Atomic-level reaction pathway was mapped, which shows a synchronized two-proton-transfer mechanism between the assistant water molecule and 8-hydroxy guanine radical. The transition-state dipole moment is the largest along the reaction pathway, which electrostatically stabilizes the proton-transfer transition-state complex. The free-energy reaction barrier for this water-assisted proton-transfer reaction was calculated at 19.2 kcal/mol with the density functional theory/M08-SO/cc-pVTZ+/molecular mechanics level of theory. The solvent effect not only has a big impact on geometries, but also dramatically changes the energetics along the reaction pathway. Among the solvent effect contributions to the transition state, the solvent energy contribution is -28.5 kcal/mol and the polarization effect contribution is 19.9 kcal/mol. In total, the solvent effect contributes -8.6 kcal/mol to the free-energy barrier height, which means that the presence of aqueous solution has a catalytic effect on the reaction mechanism and enhances the proton-transfer reactivity in aqueous solution.

Nanosecond Dynamics at Protein Metal Sites: An Application of Perturbed Angular Correlation (PAC) of γ-Rays Spectroscopy.

Metalloproteins are essential to numerous reactions in nature, and constitute approximately one-third of all known proteins. Molecular dynamics of proteins has been elucidated with great success both by experimental and theoretical methods, revealing atomic level details of function involving the organic constituents on a broad spectrum of time scales. However, the characterization of dynamics at biomolecular metal sites on nanosecond time scales is scarce in the literature. The aqua ions of many biologically relevant metal ions exhibit exchange of water molecules on the nanosecond time scale or faster, often defining their reactivity in aqueous solution, and this is presumably also a relevant time scale for the making and breaking of coordination bonds between metal ions and ligands at protein metal sites. Ligand exchange dynamics is critical for a variety of elementary steps of reactions in metallobiochemistry, for example, association and dissociation of metal bound water, association of substrate and dissociation of product in the catalytic cycle of metalloenzymes, at regulatory metal sites which require binding and dissociation of metal ions, as well as in the transport of metal ions across cell membranes or between proteins involved in metal ion homeostasis. In Perturbed Angular Correlation of γ-rays (PAC) spectroscopy, the correlation in time and space of two γ-rays emitted successively in a nuclear decay is recorded, reflecting the hyperfine interactions of the PAC probe nucleus with the surroundings. This allows for characterization of molecular and electronic structure as well as nanosecond dynamics at the PAC probe binding site. Herein, selected examples describing the application of PAC spectroscopy in probing the dynamics at protein metal sites are presented, including (1) exchange of Cd2+ bound water in de novo designed synthetic proteins, and the effect of remote mutations on metal site dynamics; (2) dynamics at the β-lactamase active site, where the metal ion appears to jump between the two adjacent sites; (3) structural relaxation in small blue copper proteins upon 111Ag+ to 111Cd2+ transformation in radioactive nuclear decay; (4) metal ion transfer between two HAH1 proteins with change in coordination number; and (5) metal ion sensor proteins with two coexisting metal site structures. With this Account, we hope to make our modest contribution to the field and perhaps spur additional interest in dynamics at protein metal sites, which we consider to be severely underexplored. Relatively little is known about detailed atomic motions at metal sites, for example, how ligand exchange processes affect protein function, and how the amino acid composition of the protein may control this facet of metal site characteristics. We also aim to provide the reader with a qualitative impression of the possibilities offered by PAC spectroscopy in bioinorganic chemistry, especially when elucidating dynamics at protein metal sites, and finally present data that may serve as benchmarks on a relevant time scale for development and tests of theoretical molecular dynamics methods applied to biomolecular metal sites.

Synthesis of a Highly Azide-Reactive and Thermosensitive Biofunctional Reagent for Efficient Enrichment and Large-Scale Identification of O-GlcNAc Proteins by Mass Spectrometry.

O-linked β-N-acetylglucosamine (O-GlcNAc) is a ubiquitous post-translational modification of proteins in eukaryotic cells. Despite their low abundance, O-GlcNAc-modified proteins play many important roles in regulating gene expression, signal transduction, and cell cycle. Aberrant O-GlcNAc proteins are correlated with many major human diseases, such as Alzheimer's disease, diabetes, and cancer. Because of the extremely low stoichiometry of O-GlcNAc proteins, enrichment is required before mass spectrometry analysis for large-scale identification and in-depth understanding of their cellular function. In this work, we designed and synthesized a novel thermosensitive immobilized triarylphosphine reagent as a convenient tool for efficient enrichment of azide-labeled O-GlcNAc proteins from complex biological samples. Immobilization of triarylphosphine on highly water-soluble thermosensitive polymer largely increases its solubility and reactivity in aqueous solution. As a result, facilitated coupling is achieved between triarylphosphine and azide-labeled O-GlcNAc proteins via Staudinger ligation, due to the increased triarylphosphine concentration, reduced interfacial mass transfer resistance, and steric hindrance in homogeneous reaction. Furthermore, solubility of the polymer from complete dissolution to full precipitation can be easily controlled by simply adjusting the environmental temperature. Therefore, facile sample recovery can be achieved by increasing the temperature to precipitate the polymer-O-GlcNAc protein conjugates from solution. This novel immobilized triarylphosphine reagent enables efficient enrichment and sensitive detection of more than 1700 potential O-GlcNAc proteins from HeLa cell using mass spectrometry, demonstrating its potential as a general strategy for low-abundance target enrichment.

Force-induced chemical reactions on the metal centre in a single metalloprotein molecule.

Metalloproteins play indispensable roles in biology owing to the versatile chemical reactivity of metal centres. However, studying their reactivity in many metalloproteins is challenging, as protein three-dimensional structure encloses labile metal centres, thus limiting their access to reactants and impeding direct measurements. Here we demonstrate the use of single-molecule atomic force microscopy to induce partial unfolding to expose metal centres in metalloproteins to aqueous solution, thus allowing for studying their chemical reactivity in aqueous solution for the first time. As a proof-of-principle, we demonstrate two chemical reactions for the FeS4 centre in rubredoxin: electrophilic protonation and nucleophilic ligand substitution. Our results show that protonation and ligand substitution result in mechanical destabilization of the FeS4 centre. Quantum chemical calculations corroborated experimental results and revealed detailed reaction mechanisms. We anticipate that this novel approach will provide insights into chemical reactivity of metal centres in metalloproteins under biologically more relevant conditions.

Electronic Properties of III-V Semiconductor Photocathode-Water Interfaces: Predictions from First-Principles Calculations

Photoelectrochemical (PEC) cells based on III-V semiconductor photocathodes offer a promising avenue for hydrogen production from water and sunlight. The efficiency of these devices depends on the electronic structure of the interface between the photocathode and liquid water, including the alignment between the semiconductor band edges and the water redox potential. Accurate theoretical predictions of this quantity remain a challenging task, as conventional electronic structure methods such as density functional theory (DFT) often yield substantial errors [1]. In this talk, we present calculations of the electronic properties of two representative III-V semiconductor electrodes, i.e. GaP and InP, at the interface with water using a combination of first-principles molecular dynamics simulations and many-body perturbation theory (MBPT). We show that the use of MBPT is key to obtain band alignments in agreement with experimental measurements. In addition, we describe the relationship between interfacial structure, electronic properties of semiconductors and their reactivity in aqueous solutions. 1. T. A. Pham, D. Lee, E. Schwegler and G. Galli, J. Am. Chem. Soc. in press (2014). This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Hydrolytic chromium(III)–sulfur bond cleavage in chromium(III)–cysteine complexes

Two chromium(III)–cysteine complexes, [Cr(Cys)2]− and [Cr(Cys)(H2O)3]+, have been synthesized, and their reactivity in aqueous solution studied. Kinetics of the acid-catalyzed Cr–S bond fission was studied in strongly acidic medium, monitoring the absorbance decrease of the S → Cr CT band. The dependences of the pseudo-first-order rate constants on [H+] were determined. The spontaneous Cr–S bond cleavage in [Cr(Cys)2]− was studied in alkaline medium monitoring the spectral changes within the d–d transition bands. Based on the obtained kinetic and spectroscopic data, a reaction mechanism is proposed and factors controlling the rate of Cr–S cleavage are discussed.