Fraser A. Armstrong

University of Oxford, Oxford, England, United Kingdom

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Publications (213)1816.57 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Protein film electrochemistry has been used to investigate reactions of highly active nickel-containing carbon monoxide dehydrogenases (CODHs). When attached to a pyrolytic graphite electrode, these enzymes behave as reversible electrocatalysts, displaying CO2 reduction or CO oxidation at minimal overpotential. The O2 sensitivity of CODH is suppressed by adding cyanide, a reversible inhibitor of CO oxidation, or by raising the electrode potential. Reduction of N2O, isoelectronic with CO2, is catalyzed by CODH, but the reaction is sluggish, despite a large overpotential, and results in inactivation. Production of H2 and formate under highly reducing conditions is consistent with calculations predicting that a nickel-hydrido species might be formed, but the very low rates suggest that such a species is not on the main catalytic pathway.
    The Journal of Physical Chemistry B 07/2015; DOI:10.1021/acs.jpcb.5b03098 · 3.30 Impact Factor
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    ABSTRACT: Despite extensive studies on [NiFe]-hydrogenases, the mechanism by which these enzymes produce and activate H2 so efficiently remains unclear. A well-known EPR-active state produced under H2 and known as Ni-C is assigned as a Ni(III)-Fe(II) species with a hydrido ligand in the bridging position between the two metals. It has long been known that low-temperature photolysis of Ni-C yields distinctive EPR-active states, collectively termed Ni-L, that are attributed to migration of the bridging-H species as a proton; however, Ni-L has mainly been regarded as an artifact with no mechanistic relevance. It is now demonstrated, based on EPR and infrared spectroscopic studies, that the Ni-C to Ni-L interconversion in Hydrogenase-1 (Hyd-1) from Escherichia coli is a pH-dependent process that proceeds readily in the dark-proton migration from Ni-C being favored as the pH is increased. The persistence of Ni-L in Hyd-1 must relate to unassigned differences in proton affinities of metal and adjacent amino acid sites, although the unusually high reduction potentials of the adjacent Fe-S centers in this O2-tolerant hydrogenase might also be a contributory factor, impeding elementary electron transfer off the [NiFe] site after proton departure. The results provide compelling evidence that Ni-L is a true, albeit elusive, catalytic intermediate of [NiFe]-hydrogenases.
    Journal of the American Chemical Society 06/2015; 137(26). DOI:10.1021/jacs.5b03182 · 12.11 Impact Factor
  • Andreas Bachmeier · Fraser Armstrong
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    ABSTRACT: Metalloenzymes such as hydrogenases and carbon monoxide dehydrogenase can be attached to light-harvesting agents to produce informative photocatalytic systems of varying intricacy. Systematic studies yield important insight into mechanistic and design principles of artificial photosynthesis - one route to future renewable energy conversion, and the unconventional experiments reveal interesting new criteria for the catalytic performance of metals in biology. Recent advances are interpreted in terms of the importance of enzyme active centres that have evolved to perform fast and efficient catalysis using abundant elements, along with the ability of enzymes to trap photo-generated electrons by virtue of having receding, buried relay centres with low reorganisation energies. Copyright © 2015. Published by Elsevier Ltd.
    Current Opinion in Chemical Biology 04/2015; 25. DOI:10.1016/j.cbpa.2015.01.001 · 6.81 Impact Factor
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    ABSTRACT: Formaldehyde (HCHO), a strong electrophile and a rapid and reversible inhibitor of hydrogen production by [FeFe]-hydrogenases, is used to identify the point in the catalytic cycle at which a highly reactive metal-hydrido species is formed. Investigations of the reaction of Chlamydomonas reinhardtii [FeFe]-hydrogenase with formaldehyde using pulsed-EPR techniques including electron-nuclear double resonance spectroscopy establish that formaldehyde binds close to the active site. Density functional theory calculations support an inhibited super-reduced state having a short Fe-(13)C bond in the 2Fe subsite. The adduct forms when HCHO is available to compete with H(+) transfer to a vacant, nucleophilic Fe site: had H(+) transfer already occurred, the reaction of HCHO with the Fe-hydrido species would lead to methanol, release of which is not detected. Instead, Fe-bound formaldehyde is a metal-hydrido mimic, a locked, inhibited form analogous to that in which two electrons and only one proton have transferred to the H-cluster. The results provide strong support for a mechanism in which the fastest pathway for H2 evolution involves two consecutive proton transfer steps to the H-cluster following transfer of a second electron to the active site.
    Journal of the American Chemical Society 04/2015; 137(16). DOI:10.1021/ja513074m · 12.11 Impact Factor
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    ABSTRACT: Reduced forms of the C56S and C60S variants of the thioredoxin-like Clostridium pasteurianum [Fe2S2] ferredoxin (CpFd) provide the only known examples of valence-delocalized [Fe2S2]+ clusters, which constitute a fundamental building block of all higher nuclearity Fe-S clusters. In this work, we have revisited earlier work on the CpFd variants and carried out redox and spectroscopic studies on the [Fe2S2]2+,+ centers in wild-type and equivalent variants of the highly homologous and structurally characterized Aquifex aeolicus ferredoxin 4 (AaeFd4) using EPR, UV-visible-NIR absorption, CD and variable-temperature MCD, and protein-film electrochemistry. The results indicate that the [Fe2S2]+ centers in the equivalent AaeFd4 and CpFd variants reversibly interconvert between similar valence-localized S = 1/2 and valence-delocalized S = 9/2 forms as a function of pH, with pKa values in the range 8.3-9.0, due to protonation of the coordinated serinate residue. However, freezing high-pH samples results in partial or full conversion from valence-delocalized S = 9/2 to valence-localized S = 1/2 [Fe2S2]+ clusters. MCD saturation magnetization data for valence-delocalized S = 9/2 [Fe2S2]+ centers facilitated determination of transition polarizations and thereby assignments of low-energy MCD bands associated with the FeFe interaction. The assignments provide experimental assessment of the double exchange parameter, B, for valence-delocalized [Fe2S2]+ centers and demonstrate that variable-temperature MCD spectroscopy provides a means of detecting and investigating the properties of valence-delocalized S = 9/2 [Fe2S2]+ fragments in higher nuclearity Fe-S clusters. The origin of valence delocalization in thioredoxin-like ferredoxin Cys-to-Ser variants and Fe-S clusters in general is discussed in light of these results.
    Journal of the American Chemical Society 03/2015; 137(13). DOI:10.1021/jacs.5b01869 · 12.11 Impact Factor
  • Andreas Bachmeier · Bhavin Siritanaratkul · Fraser A. Armstrong
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    ABSTRACT: Water and sunlight are the most abundant resources on Earth: sunlight has more than enough energy to photolyse the oceans, producing H2 and O2, but this does not happen unassisted—light harvesters and catalysts are required. This chapter discusses how enzymes, which have evolved to be highly efficient catalysts for biological photosynthetic and energy conserving reactions, have an important role in designing and developing artificial photosynthesis systems at the model level. Enzymes facilitate the study of integrated model systems because they remove the burden of poor electrocatalytic rates and efficiencies that are characteristic of most systems (exception being platinum) and complicate the overall picture. In contrast to most artificial catalysts, enzymes operate efficiently at neutral pH and can hence be used to mimic conditions under which future technologies will have to operate.
    From Molecules to Materials - Pathways to Artificial Photosynthesis, Edited by Elena A. Rozhkova, Katsuhiko Ariga, 01/2015: chapter 4: pages 99-123; Springer International Publishing., ISBN: 978-3-319-13799-5
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    Lang Xu · Fraser A. Armstrong
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    ABSTRACT: The performance characteristics of simple enzyme-based membrane-less hydrogen fuel cells running on non-explosive H2-rich air mixtures have been established using an adjustable test bed that allows multiple unit cells to operate in series or parallel. Recent advances with ‘3D’ electrodes constructed from compacted porous carbon loaded with hydrogenase (anode) and bilirubin oxidase (cathode) have been extended in order to scale up fuel cell power to useful levels. One result is an appealing ‘classroom’ demonstration of a model house containing small electronic devices powered by H2 mixed with a small amount of air. The 3D electrodes work by greatly increasing catalyst loading (at both the anode and cathode) and selectively restricting the access of O2 (relative to H2) to enzymes embedded in pores at the anode. The latter property raises the possibility of using standard hydrogenases that are not O2-tolerant: however, experiments with such an enzyme reveal good short-term performance due to restricted O2 access, but low long-term stability because the root cause of O2 sensitivity has not been addressed. Hydrogenases that are truly O2 tolerant must therefore remain the major focus of any future enzyme-based hydrogen fuel cell technology.
    RSC Advances 12/2014; 5(5). DOI:10.1039/C4RA13565B · 3.84 Impact Factor
  • Vincent C-C Wang · Stephen W Ragsdale · Fraser A Armstrong
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    ABSTRACT: Carbon monoxide dehydrogenases (CODH) play an important role in utilizing carbon monoxide (CO) or carbon dioxide (CO2) in the metabolism of some microorganisms. Two distinctly different types of CODH are distinguished by the elements constituting the active site. A Mo-Cu containing CODH is found in some aerobic organisms, whereas a Ni-Fe containing CODH (henceforth simply Ni-CODH) is found in some anaerobes. Two members of the simplest class (IV) of Ni-CODH behave as efficient, reversible electrocatalysts of CO2/CO interconversion when adsorbed on a graphite electrode. Their intense electroactivity sets an important benchmark for the standard of performance at which synthetic molecular and material electrocatalysts comprised of suitably attired abundant first-row transition elements must be able to operate. Investigations of CODHs by protein film electrochemistry (PFE) reveal how the enzymes respond to the variable electrode potential that can drive CO2/CO interconversion in each direction, and identify the potential thresholds at which different small molecules, both substrates and inhibitors, enter or leave the catalytic cycle. Experiments carried out on a much larger (Class III) enzyme CODH/ACS, in which CODH is complexed tightly with acetyl-CoA synthase, show that some of these characteristics are retained, albeit with much slower rates of interfacial electron transfer, attributable to the difficulty in making good electronic contact at the electrode. The PFE results complement and clarify investigations made using spectroscopic investigations.
    Metal ions in life sciences 11/2014; 14:71-97. DOI:10.1007/978-94-017-9269-1_4
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    ABSTRACT: The class of [NiFe]-hydrogenases comprises oxygen-sensitive periplasmic (PH) and oxygen-tolerant membrane-bound (MBH) enzymes. For three PHs and four MBHs from six bacterial species, structural features of the nickel-iron active site of hydrogen turnover and of the iron-sulfur clusters functioning in electron transfer were determined using X-ray absorption spectroscopy (XAS). Fe-XAS indicated surplus oxidized iron and a lower number of ~2.7Å Fe-Fe distances plus additional shorter and longer distances in the oxidized MBHs compared to the oxidized PHs. This supported a double-oxidized and modified proximal FeS cluster in all MBHs with an apparent trimer-plus-monomer arrangement of its four iron atoms, in agreement with crystal data showing a [4Fe3S] cluster instead of a [4Fe4S] cubane as in the PHs. Ni-XAS indicated coordination of the nickel by the thiol group sulfurs of four conserved cysteines and at least one iron-oxygen bond in both MBH and PH proteins. Structural differences of the oxidized inactive [NiFe] cofactor of MBHs in the Ni-B state compared to PHs in the Ni-A state included a ~0.05Å longer Ni-O bond, a two times larger spread of the Ni-S bond lengths, and a ~0.1Å shorter Ni-Fe distance. The modified proximal [4Fe3S] cluster, weaker binding of the Ni-Fe bridging oxygen species, and an altered localization of reduced oxygen species at the active site may each contribute to O2 tolerance.
    Biochimica et Biophysica Acta (BBA) - Bioenergetics 10/2014; 1847(2). DOI:10.1016/j.bbabio.2014.06.011 · 5.35 Impact Factor
  • Andreas Bachmeier · Samuel Hall · Stephen W. Ragsdale · Fraser A. Armstrong
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    ABSTRACT: We present a photocathode assembly for the visible-light-driven selective reduction of CO2 to CO at potentials below the thermodynamic equilibrium in the dark. The photoelectrode comprises a porous p-type semiconducting NiO electrode modified with the visible-light-responsive organic dye P1 and the reversible CO2 cycling enzyme carbon monoxide dehydrogenase. The direct electrochemistry of the enzymatic electrocatalyst on NiO shows that in the dark the electrocatalytic behavior is rectified toward CO oxidation, with the reactivity being governed by the carrier availability at the semiconductor-catalyst interface.
    Journal of the American Chemical Society 09/2014; 136(39):13518. DOI:10.1021/ja506998b · 12.11 Impact Factor
  • Andreas Bachmeier · Bonnie J. Murphy · Fraser A. Armstrong
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    ABSTRACT: The enzyme flavocytochrome c3 (fcc3), which catalyzes hydrogenation across a C═C double bond (fumarate to succinate), is used to carry out the fuel-forming reaction in an artificial photosynthesis system. When immobilized on dye-sensitized TiO2 nanoparticles, fcc3 catalyzes visible-light-driven succinate production in aqueous suspension. Solar-to-chemical conversion using neutral water as the oxidant is achieved with a photoelectrochemical cell comprising an fcc3-modified indium tin oxide cathode linked to a cobalt phosphate-modified BiVO4 photoanode. The results reinforce new directions in the area of artificial photosynthesis, in particular for solar-energy-driven synthesis of organic chemicals and commodities, moving away from simple fuels as target molecules.
    Journal of the American Chemical Society 09/2014; 136(37):12876. DOI:10.1021/ja507733j · 12.11 Impact Factor
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    ABSTRACT: Under anaerobic conditions, Escherichia coli can carry out a mixed-acid fermentation that ultimately produces molecular hydrogen. The enzyme directly responsible for hydrogen production is the membrane-bound formate hydrogenlyase (FHL) complex, which links formate oxidation to proton reduction and has evolutionary links to Complex I, the NADH:quinone oxidoreductase. Although the genetics, maturation, and some biochemistry of FHL are understood, the protein complex has never been isolated in an intact form to allow biochemical analysis. In this work, genetic tools are reported that allow the facile isolation of FHL in a single chromatographic step. The core complex is shown to comprise HycE (a [NiFe] hydrogenase component termed Hyd-3), FdhF (the molybdenum-dependent formate dehydrogenase-H), and three iron-sulfur proteins: HycB, HycF, and HycG. A proportion of this core complex remains associated with HycC and HycD, which are polytopic integral membrane proteins believed to anchor the core complex to the cytoplasmic side of the membrane. As isolated, the FHL complex retains formate hydrogenlyase activity in vitro. Protein film electrochemistry experiments on Hyd-3 demonstrate that it has a unique ability among [NiFe] hydrogenases to catalyze production of H2 even at high partial pressures of H2. Understanding and harnessing the activity of the FHL complex is critical to advancing future biohydrogen research efforts.
    Proceedings of the National Academy of Sciences 08/2014; 111(38). DOI:10.1073/pnas.1407927111 · 9.67 Impact Factor
  • Suzannah V Hexter · Min-Wen Chung · Kylie A Vincent · Fraser A Armstrong
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    ABSTRACT: Cyanide reacts rapidly with [NiFe]-hydrogenases (hydrogenase-1 and hydrogenase-2 from Escherichia coli) under mild oxidizing conditions, inhibiting the electrocatalytic oxidation of hydrogen as recorded by protein film electrochemistry. Electrochemical, EPR, and FTIR measurements show that the final enzyme product, formed within a second (even under 100% H2), is the resting state known as Ni-B, which contains a hydroxido-bridged species, Ni(III)-μ(OH)-Fe(II), at the active site. "Cyanide inhibition" is easily reversed because it is simply the reductive activation of Ni-B. This paper brings back into focus an observation originally made in the 1940s that cyanide inhibits microbial H2 oxidation and addresses the interesting mechanism by which cyanide promotes the formation of Ni-B. As a much stronger nucleophile than hydroxide, cyanide binds more rapidly and promotes oxidation of Ni(II) to Ni(III); however, it is quickly replaced by hydroxide which is a far superior bridging ligand.
    Journal of the American Chemical Society 07/2014; 136(29). DOI:10.1021/ja504942h · 12.11 Impact Factor
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    Philip Wulff · Christopher C Day · Frank Sargent · Fraser A Armstrong
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    ABSTRACT: An oxygen-tolerant respiratory [NiFe]-hydrogenase is proven to be a four-electron hydrogen/oxygen oxidoreductase, catalyzing the reaction 2 H2 + O2 = 2 H2O, equivalent to hydrogen combustion, over a sustained period without inactivating. At least 86% of the H2O produced by Escherichia coli hydrogenase-1 exposed to a mixture of 90% H2 and 10% O2 is accounted for by a direct four-electron pathway, whereas up to 14% arises from slower side reactions proceeding via superoxide and hydrogen peroxide. The direct pathway is assigned to O2 reduction at the [NiFe] active site, whereas the side reactions are an unavoidable consequence of the presence of low-potential relay centers that release electrons derived from H2 oxidation. The oxidase activity is too slow to be useful in removing O2 from the bacterial periplasm; instead, the four-electron reduction of molecular oxygen to harmless water ensures that the active site survives to catalyze sustained hydrogen oxidation.
    Proceedings of the National Academy of Sciences 04/2014; 111(18). DOI:10.1073/pnas.1322393111 · 9.67 Impact Factor
  • Bonnie J. Murphy · Frank Sargent · Fraser A. Armstrong
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    ABSTRACT: Many hydrogenases are highly electroactive when attached to an electrode, and most exhibit reversible 2H(+)/H-2 electrocatalysis, i.e. only a minuscule overpotential is required to drive the reaction in either direction. A notable exception is an important class of membrane-bound O-2-tolerant [NiFe] hydrogenases that appear only to catalyse H-2 oxidation (the uptake reaction), at a substantial overpotential and with little activity for H-2 production, yet possess an active site that is structurally very similar to that of standard, reversible [NiFe] hydrogenases (Volbeda et al., Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 5305-5310). In a discovery providing important insight into this puzzle, we show that the O-2-tolerant [NiFe] hydrogenase (Hyd-1) from E. coli converts into a reversible electrocatalyst as the pH is lowered from 8 to 3 and becomes an efficient H-2 producer below pH 4. The transformation to a reversible electrocatalyst is not due, trivially, to the higher substrate (H+ aq) availability at low pH but to a large shift in the enzyme's catalytic bias. Systematic investigations provide compelling evidence that the factor controlling this behaviour is the distal [4Fe-4S] cluster, a spectroscopically elusive site that provides the natural entry point for electrons into the enzyme. In E. coli cells, Hyd-1 is located in the periplasmic (extracytoplasmic) compartment and thus, being exposed to the pH extremes of the gastrointestinal tract or the external environment, is a potential catalyst for H-2 production by these bacteria. In a wider context, the observation and proposal are highly relevant for biohydrogen production and catalysis.
    Energy & Environmental Science 04/2014; 7(4):1426. DOI:10.1039/c3ee43652g · 20.52 Impact Factor
  • Rhiannon M Evans · Fraser A Armstrong
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    ABSTRACT: Protein film electrochemistry is a technique which allows the direct control of redox-active enzymes, providing particularly detailed information on their catalytic properties. The enzyme is deposited onto a working electrode tip, and through control of the applied potential the enzyme activity is monitored as electrical current, allowing for direct study of inherent activity as electrons are transferred to and from the enzyme redox center(s). No mediators are used. Because the only enzyme present in the experiment is bound at the electrode surface, gaseous and liquid phase inhibitors can be introduced and removed whilst the enzyme remains in situ. Potential control means that kinetics and thermodynamics are explored simultaneously; the kinetics of a reaction can be studied as a function of potential. Steady-state catalytic rates are observed directly as current (for a given potential) and non-steady-state rates (such as interconversions between different forms of the enzyme) are observed from the change in current with time. The more active the enzyme, the higher the current and the better the signal-to-noise. In this chapter we outline the practical aspects of PFE for studying electroactive enzymes, using the Escherichia coli [NiFe]-hydrogenase 1 (Hyd-1) as an example.
    Methods in molecular biology (Clifton, N.J.) 03/2014; 1122:73-94. DOI:10.1007/978-1-62703-794-5_6 · 1.29 Impact Factor
  • Suzannah V Hexter · Thomas F Esterle · Fraser A Armstrong
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    ABSTRACT: Despite being so large, many enzymes are not only excellent electrocatalysts - making possible chemical transformations under almost reversible conditions - but they also facilitate our understanding of electrocatalysis by allowing complex processes to be dissected systematically. The electrocatalytic voltammograms obtained for enzymes attached to an electrode expose fundamental aspects of electrocatalysis that can be addressed in ways that are not available to conventional molecular or surface electrocatalysts. The roles of individual components, each characterisable by diffraction or spectroscopy, can be tested and optimised by genetic engineering. Importantly, unlike small-molecule electrocatalysts (RMM < 1000) that are structurally well-defined but invariably altered by being attached to a surface, the enzyme is a giant, multi-component assembly in which the active site is buried and relatively insensitive to the presence of the electrode and solvent interface. A central assertion is that for a given driving force (electrode potential) a true catalyst has no influence on the direction of the reaction; consequently, 'catalytic bias', i.e. the common observation that an enzyme or indeed any electrocatalyst operates preferentially in one direction, must arise from secondary effects beyond the elementary catalytic cycle. This Perspective highlights and extends a general model for electrocatalysis by surface-confined enzymes, and explains how two secondary effects control the bias: (i) the electrode potential at which electrons enter or leave the catalytic cycle; (ii) potential-dependent interconversions between states of the catalyst differing in catalytic activity due to changes in the composition and arrangements of atoms. The model, which is easily applied to enzymes that have been studied recently, highlights important considerations for understanding and developing surface-confined electrocatalysts.
    Physical Chemistry Chemical Physics 02/2014; 16(24). DOI:10.1039/c3cp55230f · 4.49 Impact Factor
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    Mehmet Can · Fraser A Armstrong · Stephen W Ragsdale
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    ABSTRACT: The structure, function, and mechanism of the nickel metalloenzymes, CO dehydrogenase (CODH), and acetyl-CoA synthase was reviewed. CODH also is a wonderful system to explore how chemical bond forming and breaking interfaces with redox chemistry. This enzyme, especially coupled to ACS and other enzymes of the Wood-Ljungdahl pathway, offers great potential for biotechnology through the conversion of simple abundant compounds into needed chemicals and fuels. It will be extremely important to understand how the activities of CODH and ACS are coordinated in the complex and to increase our understanding of the dynamics and mechanics of the tunnel that carries CO from the C-cluster to the A-cluster. With both CODH and ACS, it is important to understand the movement of domains and how these proteins interact with other components of the Wood-Ljungdahl pathway, especially the CFeSP. Future high-impact papers will emerge that provide an understanding of the structures of complexes between CODH/ACS and the CFeSP.
    Chemical Reviews 02/2014; 114(8). DOI:10.1021/cr400461p · 46.57 Impact Factor
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  • Fraser A. Armstrong
    Interface focus: a theme supplement of Journal of the Royal Society interface 10/2013; 3(5). DOI:10.1098/rsfs.2013.0039 · 2.63 Impact Factor

Publication Stats

9k Citations
1,816.57 Total Impact Points


  • 1982–2015
    • University of Oxford
      • • Inorganic Chemistry Laboratory
      • • Department of Chemistry
      Oxford, England, United Kingdom
  • 2011
    • University of Cambridge
      • Department of Chemistry
      Cambridge, ENG, United Kingdom
  • 2006–2011
    • Humboldt-Universität zu Berlin
      • Department of Biology
      Berlin, Land Berlin, Germany
    • Oxford College
      Oxford, Ohio, United States
  • 2007
    • Leiden University
      • Leiden Institute of Chemistry
      Leyden, South Holland, Netherlands
  • 2004
    • University of Amsterdam
      Amsterdamo, North Holland, Netherlands
  • 2003
    • The Scripps Research Institute
      • Department of Cell and Molecular Biology
      لا هویا, California, United States
    • Boston University
      Boston, Massachusetts, United States
  • 2002
    • The University of Arizona
      • Department of Chemistry and Biochemistry (College of Science)
      Tucson, Arizona, United States
  • 2000
    • The University of Edinburgh
      • Institute of Cell Biology
      Edinburgh, Scotland, United Kingdom
  • 1996–1998
    • University of East Anglia
      • School of Biological Sciences
      Norwich, England, United Kingdom
  • 1992–1994
    • University of California, Irvine
      • Department of Chemistry
      Irvine, CA, United States
  • 1991–1993
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1990
    • University of Liverpool
      • Department of Chemistry
      Liverpool, England, United Kingdom