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1. Hydration State and Rheologic Stratification of the Lithospheric Mantle Beneath the North Anatolian Fault, Turkey

   https://doi.org/10.1029/2023GC011096  

   Lusk, A. D. ; Chatzaras, V. ; Aldanmaz, E. ; Tikoff, B. ( December 2023 , Geochemistry, Geophysics, Geosystems)

   We present constraints on the hydration state and rheology of the lithospheric mantle beneath the North Anatolian fault zone (NAFZ). Peridotite xenoliths from the Biyikali and Çorlu volcanic centers record deformational microstructures consistent with shearing in a lithosphere‐scale transcurrent fault system. Analysis by Fourier transform infrared spectroscopy indicates that nominally anhydrous phases retain some OH⁻, but bulk rock concentrations are generally restricted to <50 ppm H₂O by weight. From the rock microstructure, we determined differential stress magnitude and active deformation mechanism(s); combined with estimates of hydration state, we constrained the rheology. Recrystallized grain size piezometry shows that the mantle beneath the NAFZ sustained differential stresses of 10–20 MPa, largely independent of depth. The dominant deformation mechanism(s) change with depth; xenoliths extracted from shallower depths record evidence for grain size‐sensitive creep possibly in the presence of melt. At intermediate depths, both dislocation creep and grain size‐sensitive mechanisms were active, and we did not observe evidence for deformation in the presence of melt. The deepest samples were dominated by dislocation creep. The strong temperature sensitivity of creep mechanisms, combined with the low variability in differential stress, contributes to a stratified viscosity profile ranging from 10¹⁸ Pa s for the deepest samples to >10²² Pa s at shallower depths (assuming a melt‐free rheology). Although difficult to quantify from the rock record, melt likely reduced the viscosity of the shallow lithospheric mantle. Vertical stratification in viscosity beneath the NAFZ, the result of melt‐present deformation and/or transitions in the dominant deformation mechanism, has important consequences for the seismic cycle of strike‐slip fault systems.

    

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2. Highly efficient deoxydehydration and hydrodeoxygenation on MoS2-supported transition metal atoms through a C-H activation mechanism

   https://doi.org/10.1021/acscatal.0c02669  

   Xi, Yongjie ; Heyden, Andreas ( July 2020 , ACS Catalysis)

   Deoxydehydration (DODH) is an efficient process for the removal of vicinal OH groups of a diol or polyol. Conventional DODH reactions usually take place at a single-site MOx (M=Re, Mo, V etc.) active center, which proceed through a diol condensation step, an alkene extrusion step and a catalyst regeneration (or reduction) step. Here, we suggest that MoS2-supported transition metal atoms allow for the DODH reaction to occur through an alternative mechanism, whereby the C-H bond of a diol is activated first, which facilitates the C-OH bond cleavage on a neighboring carbon. The removal of the second OH group is also facile over the proposed catalysts. Our kinetic studies suggest that the DODH of ethylene glycol on Ru2/MoS2, Ir2/MoS2 and Ru3/MoS2 are highly active with predicted turnover frequencies of over 1/s. Thus, our study suggests a possible approach for the design of highly active DODH catalysts. Apart from being a DODH catalyst, the proposed MoS2-supported catalysts are also highly active as hydrodeoxygenation catalyst for the removal of alcohol OH groups. 

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3. Photoenzymatic Hydrogenation of Heteroaromatic Olefins Using ‘Ene’‐Reductases with Photoredox Catalysts

   https://doi.org/10.1002/anie.202003125  

   Nakano, Yuji ; Black, Michael J. ; Meichan, Andrew J. ; Sandoval, Braddock A. ; Chung, Megan M. ; Biegasiewicz, Kyle F. ; Zhu, Tianyu ; Hyster, Todd K. ( April 2020 , Angewandte Chemie International Edition)

   Flavin‐dependent ‘ene’‐reductases (EREDs) are highly selective catalysts for the asymmetric reduction of activated alkenes. This function is, however, limited to enones, enoates, and nitroalkenes using the native hydride transfer mechanism. Here we demonstrate that EREDs can reduce vinyl pyridines when irradiated with visible light in the presence of a photoredox catalyst. Experimental evidence suggests the reaction proceeds via a radical mechanism where the vinyl pyridine is reduced to the corresponding neutral benzylic radical in solution. DFT calculations reveal this radical to be “dynamically stable”, suggesting it is sufficiently long‐lived to diffuse into the enzyme active site for stereoselective hydrogen atom transfer. This reduction mechanism is distinct from the native one, highlighting the opportunity to expand the synthetic capabilities of existing enzyme platforms by exploiting new mechanistic models.

    

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4. Photoenzymatic Hydrogenation of Heteroaromatic Olefins Using ‘Ene’‐Reductases with Photoredox Catalysts

   https://doi.org/10.1002/ange.202003125  

   Nakano, Yuji ; Black, Michael J. ; Meichan, Andrew J. ; Sandoval, Braddock A. ; Chung, Megan M. ; Biegasiewicz, Kyle F. ; Zhu, Tianyu ; Hyster, Todd K. ( April 2020 , Angewandte Chemie)

   Flavin‐dependent ‘ene’‐reductases (EREDs) are highly selective catalysts for the asymmetric reduction of activated alkenes. This function is, however, limited to enones, enoates, and nitroalkenes using the native hydride transfer mechanism. Here we demonstrate that EREDs can reduce vinyl pyridines when irradiated with visible light in the presence of a photoredox catalyst. Experimental evidence suggests the reaction proceeds via a radical mechanism where the vinyl pyridine is reduced to the corresponding neutral benzylic radical in solution. DFT calculations reveal this radical to be “dynamically stable”, suggesting it is sufficiently long‐lived to diffuse into the enzyme active site for stereoselective hydrogen atom transfer. This reduction mechanism is distinct from the native one, highlighting the opportunity to expand the synthetic capabilities of existing enzyme platforms by exploiting new mechanistic models.

    

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5. The roles of chalcogenides in O 2 protection of H 2 ase active sites

   https://doi.org/10.1039/D0SC02584D  

   Yang, Xuemei ; Darensbourg, Marcetta Y. ( September 2020 , Chemical Science)

   null (Ed.)

   At some point, all HER (Hydrogen Evolution Reaction) catalysts, important in sustainable H 2 O splitting technology, will encounter O 2 and O 2 -damage. The [NiFeSe]-H 2 ases and some of the [NiFeS]–H 2 ases, biocatalysts for reversible H 2 production from protons and electrons, are exemplars of oxygen tolerant HER catalysts in nature. In the hydrogenase active sites oxygen damage may be extensive (irreversible) as it is for the [FeFe]–H 2 ase or moderate (reversible) for the [NiFe]–H 2 ases. The affinity of oxygen for sulfur, in [NiFeS]–H 2 ase, and selenium, in [NiFeSe]–H 2 ase, yielding oxygenated chalcogens results in maintenance of the core NiFe unit, and myriad observable but inactive states, which can be reductively repaired. In contrast, the [FeFe]–H 2 ase active site has less possibilities for chalcogen-oxygen uptake and a greater chance for O 2 -attack on iron. Exposure to O 2 typically leads to irreversible damage. Despite the evidence of S/Se-oxygenation in the active sites of hydrogenases, there are limited reported synthetic models. This perspective will give an overview of the studies of O 2 reactions with the hydrogenases and biomimetics with focus on our recent studies that compare sulfur and selenium containing synthetic analogues of the [NiFe]–H 2 ase active sites. 

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