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      Aerobic Damage to [FeFe]-Hydrogenases: Activation Barriers for the Chemical Attachment of O2**

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          Abstract

          [FeFe]-hydrogenases are the best natural hydrogen-producing enzymes but their biotechnological exploitation is hampered by their extreme oxygen sensitivity. The free energy profile for the chemical attachment of O 2 to the enzyme active site was investigated by using a range-separated density functional re-parametrized to reproduce high-level ab initio data. An activation free-energy barrier of 13 kcal mol −1 was obtained for chemical bond formation between the di-iron active site and O 2, a value in good agreement with experimental inactivation rates. The oxygen binding can be viewed as an inner-sphere electron-transfer process that is strongly influenced by Coulombic interactions with the proximal cubane cluster and the protein environment. The implications of these results for future mutation studies with the aim of increasing the oxygen tolerance of this enzyme are discussed.

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          Rapid planetesimal formation in turbulent circumstellar discs

          The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies (Safronov 1969). How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick together poorly (Benz 2000), and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas (Weidenschilling 1977). Gravitational collapse of the solid component has been suggested to overcome this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al. 2007), but turbulence must be present to explain observed gas accretion in protostellar discs (Hartmann 1998). Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are augmented a further order of magnitude by a streaming instability (Youdin & Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar discs.
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            The Dicke Quantum Phase Transition with a Superfluid Gas in an Optical Cavity

            A phase transition describes the sudden change of state in a physical system, such as the transition between a fluid and a solid. Quantum gases provide the opportunity to establish a direct link between experiment and generic models which capture the underlying physics. A fundamental concept to describe the collective matter-light interaction is the Dicke model which has been predicted to show an intriguing quantum phase transition. Here we realize the Dicke quantum phase transition in an open system formed by a Bose-Einstein condensate coupled to an optical cavity, and observe the emergence of a self-organized supersolid phase. The phase transition is driven by infinitely long-ranged interactions between the condensed atoms. These are induced by two-photon processes involving the cavity mode and a pump field. We show that the phase transition is described by the Dicke Hamiltonian, including counter-rotating coupling terms, and that the supersolid phase is associated with a spontaneously broken spatial symmetry. The boundary of the phase transition is mapped out in quantitative agreement with the Dicke model. The work opens the field of quantum gases with long-ranged interactions, and provides access to novel quantum phases.
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              Preparation of Artificial Plasma Membrane Mimicking Vesicles with Lipid Asymmetry

              Lipid asymmetry, the difference in lipid distribution across the lipid bilayer, is one of the most important features of eukaryotic cellular membranes. However, commonly used model membrane vesicles cannot provide control of lipid distribution between inner and outer leaflets. We recently developed methods to prepare asymmetric model membrane vesicles, but facile incorporation of a highly controlled level of cholesterol was not possible. In this study, using hydroxypropyl-α-cyclodextrin based lipid exchange, a simple method was devised to prepare large unilamellar model membrane vesicles that closely resemble mammalian plasma membranes in terms of their lipid composition and asymmetry (sphingomyelin (SM) and/or phosphatidylcholine (PC) outside/phosphatidylethanolamine (PE) and phosphatidylserine (PS) inside), and in which cholesterol content can be readily varied between 0 and 50 mol%. We call these model membranes “artificial plasma membrane mimicking” (“PMm”) vesicles. Asymmetry was confirmed by both chemical labeling and measurement of the amount of externally-exposed anionic lipid. These vesicles should be superior and more realistic model membranes for studies of lipid-lipid and lipid-protein interaction in a lipid environment that resembles that of mammalian plasma membranes.
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                Author and article information

                Affiliations
                Department of Physics and Astronomy, University College London Gower Street, London WC1E 6BT (UK)
                Department of Chemistry, Cambridge University Lensfield Road, Cambridge CB2 1EW (UK)
                Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health Bethesda, MD 20892-0520 (USA)
                Author notes
                [**]

                A.K. was supported by EPSRC grant EP/J015571/1 and D.D.S by EPSRC grant EP/J016764/1. R.B.B. was supported by the Intramural Research Program of the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health. J.B. thanks the Royal Society for a University Research Fellowship. This work was carried out on HECToR (Edinburgh), access to which was granted through the Materials Chemistry Consortium (EPSRC grants EP/F067496 and EP/L000202). We thank Christophe Léger for critical reading of the manuscript.

                Journal
                Angew Chem Int Ed Engl
                Angew. Chem. Int. Ed. Engl
                anie
                Angewandte Chemie (International Ed. in English)
                WILEY-VCH Verlag (Weinheim )
                1433-7851
                1521-3773
                14 April 2014
                11 March 2014
                : 53
                : 16
                : 4081-4084
                10.1002/anie.201400534
                4143129
                24615978
                © 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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                Communications

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