Sep 16, 2014

Decoupled catalytic hydrogen evolution from a molecular metal oxide redox mediator in water splitting

  • Benjamin Rausch,
  • Mark D. Symes,
  • Greig Chisholm,
  • and Leroy Cronin
Science 12 September 2014: 1326-1330.
A silicotungstic acid cluster can store protons and electrons during water electrolysis and later rapidly release hydrogen.

 

Abstract

The electrolysis of water using renewable energy inputs is being actively pursued as a route to sustainable hydrogen production. Here we introduce a recyclable redox mediator (silicotungstic acid) that enables the coupling of low-pressure production of oxygen via water oxidation to a separate, catalytic hydrogen production step outside the electrolyzer that requires no post-electrolysis energy input. This approach sidesteps the production of high-pressure gases inside the electrolytic cell (a major cause of membrane degradation) and essentially eliminates the hazardous issue of product gas crossover at the low current densities that characterize renewables-driven water-splitting devices. We demonstrated that a platinum-catalyzed system can produce pure hydrogen over 30 times faster than state-of-the-art proton exchange membrane electrolyzers at equivalent platinum loading.

Isotope Effects, Dynamic Matching, and Solvent Dynamics in a Wittig Reaction. Betaines as Bypassed Intermediates

Publication Date (Web): September 11, 2014 (Communication)
DOI: 10.1021/ja506497b


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The mechanism of the Wittig reaction of anisaldehyde with a stabilized ylide was studied by a combination of 13C kinetic isotope effects, conventional calculations, and molecular dynamics calculations in a cluster of 53 THF molecules. The isotope effects support a cycloaddition mechanism involving two sequential transition states associated with separate C–C and P–O bond formations. However, the betaine structure in between the two transition states is bypassed as an equilibrated intermediate in most trajectories. The role of the dynamics of solvent equilibration in the nature of mechanistic intermediates is discussed.

Sep 10, 2014

Significant Expansion of Fluorescent Protein Sensing Ability through the Genetic Incorporation of Superior Photo-Induced Electron-Transfer Quenchers

Publication Date (Web): September 8, 2014 (Communication)
DOI: 10.1021/ja505219r
 
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Photo-induced electron transfer (PET) is ubiquitous for photosynthesis and fluorescent sensor design. However, genetically coded PET sensors are underdeveloped, due to the lack of methods to site-specifically install PET probes on proteins. Here we describe a family of acid and Mn(III) turn-on fluorescent protein (FP) sensors, named iLovU, based on PET and the genetic incorporation of superior PET quenchers in the fluorescent flavoprotein iLov. Using the iLovU PET sensors, we monitored the cytoplasmic acidification process, and achieved Mn(III) fluorescence sensing for the first time. The iLovU sensors should be applicable for studying pH changes in living cells, monitoring biogentic Mn(III) in the environment, and screening for efficient manganese peroxidase, which is highly desirable for lignin degradation and biomass conversion. Our work establishes a platform for many more protein PET sensors, facilitates the de novo design of metalloenzymes harboring redox active residues, and expands our ability to probe protein conformational dynamics.
 

Reversible H Atom Abstraction Catalyzed by the Radical S-Adenosylmethionine Enzyme HydG

Publication Date (Web): August 6, 2014 (Communication)
DOI: 10.1021/ja504618y
 
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The organometallic H-cluster at the active site of [FeFe]-hydrogenases is synthesized by three accessory proteins, two of which are radical S-adenosylmethionine enzymes (HydE, HydG) and one of which is a GTPase (HydF). In this work we probed the specific role of H atom abstraction in HydG-catalyzed carbon monoxide and cyanide production from tyrosine. The isotope distributions of 5′-deoxyadenosine and p-cresol were evaluated using deuterium-labeled tyrosine substrates in H2O and D2O. The observation of multiply deuterated 5′-deoxyadenosine and deuterated S-adenosylmethionine when the reaction is carried out in D2O provides evidence for a 5′-deoxyadenosyl radical-mediated abstraction of a hydrogen atom from a solvent-exchangeable position as a reversible event.
 

Sep 8, 2014

ATP Binding and Aspartate Protonation Enhance Photoinduced Electron Transfer in Plant Cryptochrome

Publication Date (Web): August 26, 2014 (Article)
DOI: 10.1021/ja506084f
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Abstract

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Cryptochromes are flavoproteins encountered in most vegetal and animal species. They play a role of blue-light receptors in plants and in invertebrates. The putative resting state of the FAD cofactor in these proteins is its fully oxidized form, FADox. Upon blue-light excitation, the isoalloxazine ring (ISO) may undergo an ultrafast reduction by a nearby tryptophan residue W400. This primary reduction triggers a cascade of electron and proton transfers, ultimately leading to the formation of the FADH° radical. A recent experimental study has shown that the yield of FADH° formation in Arabidopsis cryptochrome can be strongly modulated by ATP binding and by pH, affecting the protonation state of D396 (proton donor to FAD°). Here we provide a detailed molecular analysis of these effects by means of combined classical molecular dynamics simulations and time-dependent density functional theory calculations. When ATP is present and D396 protonated, FAD remains in close contact with W400, thereby enhancing electron transfer (ET) from W400 to ISO*. In contrast, deprotonation of D396 and absence of ATP introduce flexibility to the photoactive site prior to FAD excitation, with the consequence of increased ISO-W400 distance and diminished tunneling rate by almost two orders of magnitude. We show that under these conditions, ET from the adenine moiety of FAD becomes a competitive relaxation pathway. Overall, our data suggest that the observed effects of ATP and pH on the FAD photoreduction find their roots in the earliest stage of the photoreduction process; i.e., ATP binding and the protonation state of D396 determine the preferred pathway of ISO* relaxation.

Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D 29Si–29Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization

Publication Date (Web): August 25, 2014 (Article)
DOI: 10.1021/ja506688m


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Silica (SiO2) nanoparticles (NPs) were functionalized by silanization to produce a surface covered with organosiloxanes. Information about the surface coverage and the nature, if any, of organosiloxane polymerization, whether parallel or perpendicular to the surface, is highly desired. To this extent, two-dimensional homonuclear 29Si solid-state NMR could be employed. However, owing to the sensitivity limitations associated with the low natural abundance (4.7%) of 29Si and the difficulty and expense of isotopic labeling here, this technique would usually be deemed impracticable. Nevertheless, we show that recent developments in the field of dynamic nuclear polarization under magic angle spinning (MAS-DNP) could be used to dramatically increase the sensitivity of the NMR experiments, resulting in a timesaving factor of ∼625 compared to conventional solid-state NMR. This allowed the acquisition of previously infeasible data. Using both through-space and through-bond 2D 29Si–29Si correlation experiments, it is shown that the required reaction conditions favor lateral polymerization and domain growth. Moreover, the natural abundance correlation experiments permitted the estimation of 2JSi–O–Si-couplings (13.8 ± 1.4 Hz for surface silica) and interatomic distances (3.04 ± 0.08 Å for surface silica) since complications associated with many-spin systems and also sensitivity were avoided. The work detailed herein not only demonstrates the possibility of using MAS-DNP to greatly facilitate the acquisition of 2D 29Si–29Si correlation spectra but also shows that this technique can be used in a routine fashion to characterize surface grafting networks and gain structural constraints, which can be related to a system’s chemical and physical properties.

Sep 3, 2014

Electronic Control of the Protonation Rates of Fe–Fe Bonds

Publication Date (Web): August 12, 2014 (Article)
DOI: 10.1021/ja506693m


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Protonation at metal–metal bonds is of fundamental interest in the context of the function of the active sites of hydrogenases and nitrogenases. In diiron dithiolate complexes bearing carbonyl and electron-donating ligands, the metal–metal bond is the highest occupied molecular orbital (HOMO) with a “bent” geometry. Here we show that the experimentally measured rates of protonation (kH) of this bond and the energy of the HOMO as measured by the oxidation potential of the complexes (E1/2ox) correlate in a linear free energy relationship: ln kH = ((F(c – βE1/2ox))/(RT)), where c is a constant and β is the dimensionless Brønsted coefficient. The value of β of 0.68 is indicative of a strong dependence upon energy of the HOMO: measured rates of protonation vary over 6 orders of magnitude for a change in E1/2ox of ca. 0.55 V (ca. 11 orders of magnitude/V). This relationship allows prediction of protonation rates of systems that are either too fast to measure experimentally or that possess additional protonation sites. It is further suggested that the nature of the bridgehead in the dithiolate ligand can exert a stereoelectronic influence: bulky substituents destabilize the HOMO, thereby increasing the rate of protonation.
Publication Date (Web): August 15, 2014 (Perspective)
DOI: 10.1021/ja504441h

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In life science, water is the ubiquitous solvent, sometimes even called the “matrix of life”. There is increasing experimental and theoretical evidence that solvation water is not a passive spectator in biomolecular processes. New experimental techniques can quantify how water interacts with biomolecules and, in doing so, differs from “bulk” water. Terahertz (THz) absorption spectroscopy has turned out to be a powerful tool to study (bio)molecular hydration. The main concepts that have been developed in the recent years to describe the underlying solute-induced sub-picosecond dynamics of the hydration shell are discussed herein. Moreover, we highlight recent findings that show the significance of hydrogen bond dynamics for the function of antifreeze proteins and for molecular recognition. In all of these examples, a gradient of water motion toward functional sites of proteins is observed, the so-called “hydration funnel”. By means of molecular dynamics simulations, we provide new evidence for a specific water–protein coupling as the cause of the observed dynamical heterogeneity. The efficiency of the coupling at THz frequencies is explained in terms of a two-tier (short- and long-range) solute–solvent interaction.

Aug 28, 2014

Nitrite and Hydroxylamine as Nitrogenase Substrates: Mechanistic Implications for the Pathway of N2 Reduction

Publication Date (Web): August 19, 2014 (Article)
DOI: 10.1021/ja507123d


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Investigations of reduction of nitrite (NO2) to ammonia (NH3) by nitrogenase indicate a limiting stoichiometry, NO2 + 6e + 12ATP + 7H+ → NH3 + 2H2O + 12ADP + 12Pi. Two intermediates freeze-trapped during NO2 turnover by nitrogenase variants and investigated by Q-band ENDOR/ESEEM are identical to states, denoted H and I, formed on the pathway of N2 reduction. The proposed NO2 reduction intermediate hydroxylamine (NH2OH) is a nitrogenase substrate for which the H and I reduction intermediates also can be trapped. Viewing N2 and NO2 reductions in light of their common reduction intermediates and of NO2 reduction by multiheme cytochrome c nitrite reductase (ccNIR) leads us to propose that NO2 reduction by nitrogenase begins with the generation of NO2H bound to a state in which the active-site FeMo-co (M) has accumulated two [e/H+] (E2), stored as a (bridging) hydride and proton. Proton transfer to NO2H and H2O loss leaves M–[NO+]; transfer of the E2 hydride to the [NO+] directly to form HNO bound to FeMo-co is one of two alternative means for avoiding formation of a terminal M–[NO] thermodynamic “sink”. The N2 and NO2 reduction pathways converge upon reduction of NH2NH2 and NH2OH bound states to form state H with [−NH2] bound to M. Final reduction converts H to I, with NH3 bound to M. The results presented here, combined with the parallels with ccNIR, support a N2 fixation mechanism in which liberation of the first NH3 occurs upon delivery of five [e/H+] to N2, but a total of seven [e/H+] to FeMo-co when obligate H2 evolution is considered, and not earlier in the reduction process.

Facile Surface Functionalization of Hydrophobic Magnetic Nanoparticles

Publication Date (Web): August 20, 2014 (Communication)
DOI: 10.1021/ja5060324


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Nonpolar phase synthesized hydrophobic nanocrystals show attractive properties and have demonstrated prominent potential in biomedical applications. However, the preparation of biocompatible nanocrystals is made difficult by the presence of hydrophobic surfactant stabilizer on their surfaces. To address this limitation, we have developed a facile, high efficiency, single-phase and low-cost method to convert hydrophobic magnetic nanoparticles (MNPs) to an aqueous phase using tetrahydrofuran, NaOH and 3,4-dihydroxyhydrocinnamic acid without any complicated organic synthesis. The as-transferred hydrophilic MNPs are water-soluble over a wide pH range (pH = 3–12), and the solubility is pH-controllable. Furthermore, the as-transferred MNPs with carboxylate can be readily adapted with further surface functionalization, varying from small molecule dyes to oligonucleotides and enzymes. Finally, the strategy developed here can easily be extended to other types of hydrophobic nanoparticles to facilitate biomedical applications of nanomaterials.

Direct Comparison of Electrochemical and Spectrochemical Kinetics for Catalytic Oxygen Reduction

Publication Date (Web): August 19, 2014 (Communication)
DOI: 10.1021/ja505667t
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Aug 25, 2014

Design and Synthesis of New Stable Fluorenyl-Based Radicals

Publication Date (Web): August 18, 2014 (Article)
DOI: 10.1021/ja507005c

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Organic neutral radicals have long fascinated chemists with a fundamental understanding of structure–reactivity relationships in organic reactions and with applications as new functional materials. However, the elusive nature of these radicals makes the synthesis, isolation, and characterization very challenging. In this work, the synthesis of three long-lived, fluorenyl-based radicals are reported. The geometry and electronic structures of these radicals were systematically investigated with a combination of various experimental methods, besides density functional theory (DFT) calculations, which include X-ray crystallographic analysis, electron spin resonance (ESR), electron nuclear double resonance (ENDOR), cyclic voltammetry, and UV–vis–NIR measurements. Their half-life periods (τ1/2) in air-saturated solution under ambient conditions were also determined. Surprisingly, all three radicals showed remarkable stabilities: τ1/2 = 7, 3.5, and 43 days.

The Cyanide Ligands of [FeFe] Hydrogenase: Pulse EPR Studies of 13C and 15N-Labeled H-Cluster

Publication Date (Web): August 15, 2014 (Communication)
DOI: 10.1021/ja507046w


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The two cyanide ligands in the assembled cluster of [FeFe] hydrogenase originate from exogenous l-tyrosine. Using selectively labeled tyrosine substrates, the cyanides were isotopically labeled via a recently developed in vitro maturation procedure allowing advanced electron paramagnetic resonance techniques to probe the electronic structure of the catalytic core of the enzyme. The ratio of the isotropic 13C hyperfine interactions for the two CN ligands—a reporter of spin density on their respective coordinating iron ions—collapses from ≈5.8 for the Hox form of hydrogenase to <2 for the CO-inhibited form. Additionally, when the maturation was carried out using [15N]-tyrosine, no features previously ascribed to the nitrogen of the bridging dithiolate ligand were observed suggesting that this bridge is not sourced from tyrosine.

Homogeneous Photochemical Water Oxidation by Biuret-Modified Fe-TAML: Evidence of FeV(O) Intermediate

Publication Date (Web): August 13, 2014 (Article)
DOI: 10.1021/ja503753k
 
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Water splitting, leading to hydrogen and oxygen in a process that mimics natural photosynthesis, is extremely important for devising a sustainable solar energy conversion system. Development of earth-abundant, transition metal-based catalysts that mimic the oxygen-evolving complex of photosystem II, which is involved in oxidation of water to O2 during natural photosynthesis, represents a major challenge. Further, understanding the exact mechanism, including elucidation of the role of active metal-oxo intermediates during water oxidation (WO), is critical to the development of more efficient catalysts. Herein, we report FeIII complexes of biuret-modified tetra-amidomacrocyclic ligands (Fe-TAML; 1a and 1b) that catalyze fast, homogeneous, photochemical WO to give O2, with moderate efficiency (maximum TON = 220, TOF = 0.76 s–1). Previous studies on photochemical WO using iron complexes resulted in demetalation of the iron complexes with concomitant formation of iron oxide nanoparticles (NPs) that were responsible for WO. Herein, we show for the first time that a high valent FeV(O) intermediate species is photochemically generated as the active intermediate for the oxidation of water to O2. To the best of our knowledge, this represents the first example of a molecular iron complex catalyzing photochemical WO through a FeV(O) intermediate.
 

Aug 22, 2014

Direct Spectroscopic Observation of Closed-Shell Singlet, Open-Shell Singlet, and Triplet p-Biphenylyloxenium Ion

Publication Date (Web): August 14, 2014 (Article)
DOI: 10.1021/ja505447q


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The photophysics and photochemistry of p-biphenylyl hydroxylamine hydrochloride was studied using laser flash photolysis ranging from the femtosecond to the microsecond time scale. The singlet excited state of this photoprecursor is formed within 350 fs and partitions into three different transients that are assigned to the p-biphenyloxy radical, the open-shell singlet p-biphenylyloxenium ion, and the triplet p-biphenylyloxenium ion, having lifetimes of 40 μs, 45 ps, and 1.6 ns, respectively, in CH3CN. The open-shell singlet p-biphenylyloxenium ion predominantly undergoes internal conversion to produce the closed-shell singlet p-biphenylyloxenium ion, which has a lifetime of 5–20 ns. The longer-lived radical is unambiguously assigned by nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy, and the assignment of the short-lived singlet and triplet oxenium ion transient absorptions are supported by matching time-dependent density functional theory (TD-DFT) predictions of the absorptions of these species, as well as by product studies that implicate the intermediacy of charged electrophilic intermediates. Product studies from photolysis give p-biphenylol as the major product and a chloride adduct as the major product when NaCl is added as a trap. Thermolysis studies give p-biphenylol as a major product, as well as water, ammonium, and chloro adducts. These studies provide a rare direct look at a discrete oxenium ion intermediate and the first detection of open-shell singlet and triplet configurations of an oxenium ion, as well as providing an intriguing example of the importance of excited state dynamics in governing the electronic state population of reactive intermediates.