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Jeremy
Nugent
,
Carlos
Arroniz
,
Bethany R.
Shire
,
Alistair J.
Sterling
,
Helena D.
Pickford
,
Marie L. J.
Wong
,
Steven
Mansfield
,
Dimitri F. J.
Caputo
,
Benjamin
Owen
,
James J.
Mousseau
,
Fernanda
Duarte
,
Edward A.
Anderson
Abstract: Photoredox catalysis has transformed the landscape of radical-based synthetic chemistry. Additions of radicals generated through photoredox catalysis to carbon–carbon π-bonds are well-established; however, this approach has yet to be applied to the functionalization of carbon–carbon σ-bonds. Here, we report the first such use of photoredox catalysis to promote the addition of organic halides to the carbocycle [1.1.1]propellane; the product bicyclo[1.1.1]pentanes (BCPs) are motifs of high importance in the pharmaceutical industry and in materials chemistry. Showing broad substrate scope and functional group tolerance, this methodology results in the first examples of bicyclopentylation of sp2 carbon–halogen bonds to access (hetero)arylated BCPs, as well as the functionalization of nonstabilized sp3 radicals. Substrates containing alkene acceptors allow the single-step construction of polycyclic bicyclopentane products through unprecedented atom transfer radical cyclization cascades, while the potential to accelerate drug discovery is demonstrated through late-stage bicyclopentylations of natural productlike and druglike molecules. Mechanistic investigations demonstrate the importance of the photocatalyst in this chemistry and provide insight into the balance of radical stability and strain relief in the reaction cycle.
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Sep 2019
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I24-Microfocus Macromolecular Crystallography
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Diamond Proposal Number(s):
[17201]
Abstract: Norcoclaurine synthase (NCS) catalyzes a stereoselective
Pictet−Spengler reaction to give the key intermediate, (S)-norcoclaurine
in benzylisoquinoline alkaloid biosynthesis. This family of alkaloids
contains many bioactive molecules including morphine and berberine.
Recently, NCS has been demonstrated to accept a variety of aldehydes
and some ketones as substrates, leading to a range of chiral
tetrahydroisoquinoline (THIQ) products. Here, we report the unusual
acceptance of α-substituted aldehydes, in particular α-methyl-substituted
aldehydes, by wild-type Thalictrum flavum NCS (Δ33Tf NCS) to give
THIQ products. Moreover, the kinetic resolution of several α-
substituted aldehydes to give THIQs with two defined chiral centers
in a single step with high conversions was achieved. Several dopamine analogues were also accepted as substrates, and reactions
were amenable to scale up. Active site mutants of TfNCS were then used, which demonstrated the potential to enhance the
stereoselectivities in the reaction and improve yields. The rationale for the acceptance of these substrates and improved activity
with different mutants has been gained from a co-crystallized structure of Δ33TfNCS with a nonproductive mimic of a reaction
intermediate bound in the active site. Finally, molecular dynamics simulations were performed to study the binding of dopamine
and an α-substituted aldehyde and provided further insight into the reaction with these substrates.
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Sep 2019
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I03-Macromolecular Crystallography
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Diamond Proposal Number(s):
[10515]
Abstract: Hydrogenases are metalloenzymes that catalyze the redox conversion between H2 and protons. The so-called [NiFeSe] hydrogenases are highly active for both H2 production and oxidation, but like all hydrogenases, they are inhibited by O2. In the [NiFeSe] enzyme from Desulfovibrio vulgaris Hildenborough this results from the oxidation of an active site cysteine ligand. We designed mutations that constrict a hydrophilic channel that connects the protein surface to this active site cysteine. Two of the variants show markedly increased tolerance to O2 inactivation, while retaining high catalytic activities in both directions of the reaction, and structural studies confirm that these mutations prevent the oxidation of the cysteine. Our results indicate that the diffusion of O2 or ROS to the active site can occur through a hydrophilic water channel, in contrast to the widely held assumption that only hydrophobic channels are involved in active site inactivation. This provides an original strategy for optimizing the enzyme by protein engineering.
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Aug 2019
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I03-Macromolecular Crystallography
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Emma Zsófia Aletta
Nagy
,
Souad Diana
Tork
,
Pauline A.
Lang
,
Alina
Filip
,
Florin Dan
Irimie
,
László
Poppe
,
Monica Ioana
Toşa
,
Christopher J.
Schofield
,
Jurgen
Brem
,
Csaba
Paizs
,
Laszlo Csaba
Bencze
Diamond Proposal Number(s):
[19458]
Abstract: Modification of the hydrophobic binding pocket of phenylalanine ammonia-lyase from Petroselinum crispum (PcPAL) enables increased activity and selectivity towards phenylalanines and cinnamic acids mono-substituted with both electron donating (-CH3, -OCH3) and electron withdrawing (-CF3, -Br) groups at all positions (o-, m-, p-) of their aromatic ring. The results reveal specific residues involved in accommodating substituents at o-, m-, p-positions of the substrate’s phenyl ring. The predicted interactions were validated by crystallographic analysis of the binding mode of para-methoxy cinnamic acid complexed at the active site of PcPAL. The biocatalytic utility of the tailored PcPAL mutants was demonstrated by the efficient preparative scale synthesis of (S)-m-bromo-phenylalanine (ee: > 99%, yield: 60%) and (R)-p-methyl-phenylalanine (ee: 97%, yield: 49%), using the corresponding ammonia addition and ammonia elimination reactions catalyzed by the L134A and I460V PcPAL variants, respectively. Overall, the results reveal the potential for structure based protein engineering of PALs to provide enzymes with enhanced catalytic properties and which are specifically tailored for differently substituted phenylalanine analogues of high synthetic value.
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Aug 2019
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B18-Core EXAFS
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Diamond Proposal Number(s):
[16250]
Abstract: Energy storage solutions are a vital component of the global transition towards renewable energy sources. The Power-to-Gas (PtG) concept, which stores surplus renewable energy in the form of methane, has therefore become increasingly relevant in recent years. At present, supported Ni nanoparticles are preferred as industrial catalysts for CO2 methanation due to their low cost and high methane selectivity. However, commercial Ni catalysts are not active enough in CO2 methanation to reach the high CO2 conversion (>99 %) required by the specifications for injection in the natural gas grid. Herein we demonstrate the promise of promotion of Ni by Mn, another low-cost base metal, for obtaining very active CO2 methanation catalysts, with results comparable to more expensive precious metal-based catalysts. The origin of this improved performance is revealed by a combined approach of nanoscale characterization, mechanistic study, and density functional theory calculations. Nanoscale characterization with STEM-EDX and X-ray absorption spectroscopy shows that NiMn catalysts consist of metallic Ni particles decorated by oxidic Mn2+ species. A mechanistic study combining IR spectroscopy of surface adsorbates, transient kinetic analysis with isotopically labelled CO2, density functional theory calculations and microkinetics simulations ascertain that the MnO clusters enhance CO2 adsorption and facilitate CO2 activation. A macroscale perspective was achieved by simulating the Ni and NiMn catalytic activity in a Sabatier reactor, which revealed that NiMn catalysts have the potential to meet the demanding PtG catalyst performance requirements, and can largely replace the need for expensive and scarce noble metal catalysts.
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Jul 2019
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B18-Core EXAFS
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Diamond Proposal Number(s):
[16006]
Abstract: Different supporting procedures were followed to alter the nanoparticle-support interactions (NPSI) in two Co3O4/Al2O3 catalysts, prepared using the reverse micelle technique. The catalysts were tested in the dry preferential oxidation of carbon monoxide (CO-PrOx) while monitoring their phase stability using four complementary in situ techniques, viz. magnet-based characterisation, PXRD, combined XAS/DRIFTS as well as quasi in situ XPS, respectively. The catalyst with weak NPSI achieved higher CO2 yields and selectivities at temperatures below 225 °C compared to the sample with strong NPSI. However, relatively high degrees of reduction of Co3O4 to metallic Co were reached between 250 and 350 °C for the same catalyst. The presence of metallic Co led to the undesired formation of CH4, reaching a yield of over 90% above 300 °C. The catalyst with strong NPSI formed very low amounts of metallic Co (less than 1%) and low CH4 (yield of up to 20%) even at 350 °C. When the temperature was decreased from 350 to 50 °C under the reaction gas, both catalysts were slightly re-oxidised and gradually re-gained their CO oxidation activity while the formation of CH4 diminished. The present study, for the first time, shows a strong relationship between catalyst performance (i.e., activity and selectivity) and phase stability, both of which are affected by the strength of the NPSI. When using a metal oxide as the active CO-PrOx catalyst, it is important for it to have significant reduction resistance to avoid the formation of undesired products, e.g., CH4. However, the metal oxide should also be reducible (especially on the surface) to allow for a complete conversion of CO to CO2 via the Mars-van Krevelen mechanism.
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Jun 2019
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B22-Multimode InfraRed imaging And Microspectroscopy
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Diamond Proposal Number(s):
[13725, 16257, 20906]
Abstract: Synchrotron infrared microspectroscopy has identified with high temporal resolution (down to 0.25 s) the initial events occurring when methanol vapour is contacted with a crystal of zeolite HZSM-5. The first alkenes are generated directly from methoxy groups formed at the acid sites via their deprotonation. These alkenes can either desorb directly or oligomerise and cyclise to form dimethylcyclopentenyl cations. The oligomeric and dimethylcyclopentenyl cations are the first major components of the hydrocar-bon pool that precede aromatic hydrocarbons and lead to indirect alkene formation. This is the first direct observation of these events in real time.
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Jun 2019
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I04-Macromolecular Crystallography
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Diamond Proposal Number(s):
[9948]
Abstract: The recently identified pseudoephedrine and ephedrine dehydrogenases from Arthrobacter sp. TS-15, PseDH and EDH, are NADH-dependent members of the oxidoreductase superfamily of short-chain dehydrogenases/reductases (SDRs). They are specific for the enantioselective oxidation of (+)-(S) N-(pseudo)ephedrine and (-)-(R) N-(pseudo)ephedrine, respectively. Anti-Prelog stereospecific PseDH and Prelog-specific EDH catalyse the regio- and enantiospecific reduction of 1-phenyl-1,2-propanedione to (S)-phenylacetylcarbinol and (R)-phenylacetylcarbinol with full conversion and enantiomeric excess of >99%. Moreover, they perform the reduction of a wide range of aryl aliphatic carbonyl compounds, including keto amines, ketoesters and haloketones, to the corresponding enantiopure alcohols. The highest stability of PseDH and EDH was determined to be at a pH range of 6.0-8.0 and 7.5-8.5, respectively. PseDH was more stable than EDH at 25 °C, with half-lives of 279 h and 38 h respectively. However, EDH is more stable at 40 °C with a two-fold greater half-life than at 25 °C. The crystal structure of the PseDH-NAD+ complex, refined to a resolution of 1.83 Å, revealed a tetrameric structure, which was confirmed by solution studies. A model of the active site in complex with NAD+ and 1-phenyl-1,2-propanedione suggested key roles for S143 and W152 in recognition of the substrate and positioning for the reduction reaction. The wide substrate spectrum of these dehydrogenases, combined with their regio- and enantioselectivity, suggests excellent potential for the industrial production of valuable chiral compounds.
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May 2019
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I03-Macromolecular Crystallography
I04-1-Macromolecular Crystallography (fixed wavelength)
I24-Microfocus Macromolecular Crystallography
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Tobias
Hedison
,
Rajesh
Shenoy
,
Andreea Iulia
Iorgu
,
Derren
Heyes
,
Karl
Fisher
,
Gareth
Wright
,
Sam
Hay
,
Robert Roy
Eady
,
Svetlana
Antonyuk
,
S. Samar
Hasnain
,
Nigel S.
Scrutton
Diamond Proposal Number(s):
[11740]
Abstract: It is generally assumed that tethering enhances rates of electron harvesting and delivery to active sites in multi-domain enzymes by proximity and sampling mechanisms. Here, we explore this idea in a tethered 3-domain, trimeric copper-containing nitrite reductase. By reverse engineering, we find that tethering does not enhance the rate of electron delivery from its pendant cytochrome c to the catalytic copper-containing core. Using a linker that harbors a gatekeeper tyrosine in a nitrite access channel, the tethered haem domain enables catalysis by other mechanisms. Tethering communicates the redox state of the haem to the distant T2Cu center that helps initiate substrate binding for catalysis. It also tunes copper reduction potentials, suppresses reductive enzyme inactivation, enhances enzyme affinity for substrate and promotes inter-copper electron transfer. Tethering has multiple unanticipated beneficial roles, the combination of which fine-tunes function beyond simplistic mechanisms expected from proximity and restrictive sampling models.
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May 2019
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B18-Core EXAFS
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Diamond Proposal Number(s):
[15151]
Abstract: Herein we present a comparative study on the water-induced formation of metal-support compounds from metallic cobalt in a simulated high conversion Fischer-Tropsch environment. Literature on the deactivation of supported cobalt catalysts via oxidation to cobalt(II) oxide or cobalt-support compounds is contradictory due to a lack of use in suitable model catalysts and insufficient direct characterization of the metallic cobalt phase under reaction conditions. The particular carrier materials stabilize the active cobalt nanoparticles, but also dictate the likelihood of the formation of non-active cobalt-support compounds. In this study, well-defined cobalt nanoparticles of 5 nm were deposited on alumina, silica, and three titania carriers. The stability of the reduced nanoparticles against water-rich H2 atmospheres during exposure to simulated high Fischer-Tropsch conversion levels was monitored in an in situ magnetometer. Co/SiO2 was shown to be the most stable model catalyst, while various Co/TiO2 model systems readily formed large amounts of cobalt-support compounds at low ratios of the Fischer-Tropsch product H2O to reactant H2 or even during the preceding reduction of the oxidic precursor. Co/Al2O3 displayed a surprisingly high stability at industrially relevant conditions, in contradiction to thermodynamic predictions. However, cobalt aluminate forms at increased concentrations of water. The existence of hard-to-reduce metal-support compounds in the spent catalysts was confirmed and characterized by means of X-ray absorption near edge structure spectroscopy and high-resolution scanning transmission electron microscopy of the exposed and passivated model catalysts.
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Apr 2019
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