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Publication highlights

Picks of our best science in 2019!

October 18 2019: Nicolas Queyriaux, Wesley B. Swords, Hemlata Agarwala,Ben A. Johnson, Sascha Ott and Leif Hammarström published an article in Dalton Transactions:

Mechanistic insights on the non-innocent role of electron donors: reversible photocapture of CO2 by RuII-polypyridyl complexes.

Abstract: The ability of [RuII(tButpy)(dmbpy)(MeCN)]2+ (1-MeCN) to capture CO2, with the assistance of triethanolamine (TEOA), has been assessed under photocatalytically-relevant conditions. The photolability of 1-MeCN has proven essential to generate a series of intermediates which only differ by the nature of their monodentate ligand. In DMF, ligand photoexchange of 1-MeCN to give [RuII(tButpy)(dmbpy)(DMF)]2+ (1-DMF) proceeds smoothly with a quantum yield of 0.011. However, in the presence of TEOA, this process was disrupted, leading to the formation of a mixture of 1-DMF and [RuII(tButpy)(dmbpy)(TEOA)]+ (1-TEOA). An equilibrium constant of 3 was determined. Interestingly, 1-TEOA demonstrated an ability to reversibly catch and release CO2 making it a potentially crucial intermediate towards CO2 reduction.

September 11 2019: Souvik Roy, Zhehao Huang, Asamanjoy Bhunia, Ashleigh Castner, Arvind K. Gupta, Xiaodong Zou, and Sascha Ott published an article in Journal of the American Chemical Society:

Electrocatalytic Hydrogen Evolution from a Cobaloxime-Based Metal–Organic Framework Thin Film.

Abstract: Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as integral linker units in metal–organic frameworks (MOFs) can remedy these shortcomings. Moreover, the extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime linkers promote electron transport through the film as well as function as molecular HECs. The obtained turnover numbers are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the cobaloxime catalysts is maintained for at least 18 h of electrocatalysis. Being one of the very few hydrogen evolving electrocatalytic MOFs based on a redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity and charge transport pathways.

July 31 2019:  Pilla Sankara Krishna, Stenbjörn Styring and Fikret Mamedov published an article in Green chemistry: 

Photosystem ratio imbalance promotes direct sustainable H2 production in Chlamydomonas reinhardtii.

Abstract: The green alga Chlamydomonas reinhardtii can photoproduce H2 gas for only a few minutes under anaerobic conditions due to the inhibition of hydrogenase by O2 produced by Photosystem II (PSII). A few days of sustained H2 production can only be achieved when O2 and H2 production are temporally separated under two-stage processes such as sulfur deprivation. Under sulfur deprivation, H2 production is initiated after the over-reduction of the plastoquinone pool and decreased PSII activity in the thylakoid membrane. As a result, activated hydrogenase consumes the excess of electrons produced by PSII [Volgusheva et al., Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 7223]. Here, we report that similar conditions can be achieved by simply altering the ratio between photosystem I (PSI) and PSII. In the C3 mutant of C. reinhardtii, we found a lower PSI/PSII ratio than in the wild type, 0.33 vs. 0.85, respectively. This imbalance of photosystems resulted in the over-reduced state of the plastoquinone pool and activation of hydrogenase in the C3 mutant that allowed the photoproduction of H2 continuously for 42 days. This is an unprecedented duration of H2 production in green algae under standard growth conditions without any nutrient limitation. Photosynthetic electron flow from PSII to hydrogenase was closely regulated during this long-term H2 production. The amount of PSII was decreased and the amount of PSI was increased reaching a PSI/PSII ratio of more than 5 as shown by EPR and fluorescence spectroscopy. This fine-tuning of photosystems allows to sustain the long-term production of H2 in C. reinhardtii by a direct photosynthetic pathway.

July 16 2019:  Xufeng Liu,  Rui Miao,  Pia Lindberg  and  Peter Lindblad published an article in Energy & Environmental Science:

Modular Engineering for Efficient Photosynthetic Biosynthesis of 1-Butanol from CO2 in Cyanobacteria

Abstract: Cyanobacteria are photoautotrophic microorganisms which can be engineered to directly convert CO2 and water into biofuels and chemicals via photosynthesis using sunlight as energy. However, product titers and rates are main challenges that need to be overcome for industrial applications. Here we present a systematic modular engineering of the cyanobacterium Synechocystis PCC 6803, enabling efficient biosynthesis of 1-butanol, an attractive commodity chemical and gasoline substitute. Through introducing and re-casting the 1-butanol biosynthetic pathway at gene and enzyme levels, optimizing the 5’-regions of expression units for tuning transcription and translation, rewiring carbon flux and rewriting the photosynthetic central carbon metabolism to enhance the precursor supply, and performing process development, we were able to reach a cumulative 1-butanol titer of 4.8 g∙L-1 with a maximal rate of 302 mg∙L-1∙day-1 from the engineered Synechocystis. This represents the highest 1-butanol production from CO2 reported so far. Our multi-level modular strategy for high-level production of chemicals and advanced biofuels represents a blue-print for future systematic engineering in photosynthetic microorganisms.

June 11 2019:  Jing Huang, Bo Xu, Lei Tian, Palas Baran Pati, Ahmed S. Etman, Junliang Sun, Leif Hammarström and Haining Tian published an article in Chemical Communications: 

A heavy metal-free CuInS2 quantum dot sensitized NiO photocathode with a Re molecular catalyst for photoelectrochemical CO2 reduction.

Abstract: Heavy metal-free CuInS2 quantum dots (QDs) were employed as a photosensitizer on a NiO photocathode to drive an immobilized molecular Re catalyst for photoelectrochemical CO2 reduction for the first time. A photocurrent of 25 μA cm−2 at −0.87 V vs. NHE was obtained, providing a faradaic efficiency of 32% for CO production.

May 17 2019: Yaxiao Guo, Zhaoyang Yao, Brian J. Timmer, Xia Sheng, Lizhou Fan, YuanYuan Li, Fuguo Zhang, and Licheng Sun, published an article in Nano Energy:

Boosting nitrogen reduction reaction by bio-inspired FeMoS containing hybrid electrocatalyst over a wide pH range.

Abstract: A facile preparation of bio-inspired and morphology controllable catalytic electrode FeS@MoS2/CFC, featuring a carbon fiber cloth (CFC) covered with FeS dotted MoS2 nanosheets, has been established. Synergy between the CFC as a self-standing conductive substrate and the FeS nanoparticle dotted MoS2 nanosheets with abundant active sites makes the noble-metal-free catalytic electrode FeS@MoS2/CFC highly efficient in nitrogen reduction reaction (NRR), with an ammonia production rate of 8.45 μg h−1 cm−2 and excellent long-term stability at −0.5 V in pH neutral electrolyte. Further electrolysis in acidic and alkaline electrolytes revealed the overall NRR catalytic activity of this electrode over a wide pH range.

May 2 2019:  Alexander Aster, Shihuai Wang, Mohammad Mirmohades, Charlène Esmieu, Gustav BerggrenLeif Hammarström and Reiner Lomoth published an article in Chemical Science: 

Metal vs. ligand protonation and the alleged proton-shuttling role of the azadithiolate ligand in catalytic H2 formation with FeFe hydrogenase model complexes.

Abstract: Electron and proton transfer reactions of diiron complexes [Fe2adt(CO)6] (1) and [Fe2adt(CO)4(PMe3)2] (4), with the biomimetic azadithiolate (adt) bridging ligand, have been investigated by real-time IR- and UV-vis-spectroscopic observation to elucidate the role of the adt-N as a potential proton shuttle in catalytic H2 formation. Protonation of the one-electron reduced complex, 1−, occurs on the adt-N yielding 1H and the same species is obtained by one-electron reduction of 1H+. The preference for ligand vs. metal protonation in the Fe2(I,0) state is presumably kinetic but no evidence for tautomerization of 1H to the hydride 1Hy was observed. This shows that the adt ligand does not work as a proton relay in the formation of hydride intermediates in the reduced catalyst. A hydride intermediate 1HHy+ is formed only by protonation of 1H with stronger acid. Adt protonation results in reduction of the catalyst at much less negative potential, but subsequent protonation of the metal centers is not slowed down, as would be expected according to the decrease in basicity. Thus, the adtH+ complex retains a high turnover frequency at the lowered overpotential. Instead of proton shuttling, we propose that this gain in catalytic performance compared to the propyldithiolate analogue might be rationalized in terms of lower reorganization energy for hydride formation with bulk acid upon adt protonation.

April 11 2019:  Giovanny A. Parada, Zachary K. Goldsmith, Scott Kolmar, Belinda Pettersson Rimgard, Brandon Q. Mercado, Leif Hammarström, Sharon Hammes-Schiffer, and James M. Mayer published an article in Science: 

Concerted proton-electron transfer reactions in the Marcus inverted region.

Abstract: Electron transfer (ET) reactions slow down when they become thermodynamically very favorable, a counterintuitive interplay of kinetics and thermodynamics termed the inverted region in Marcus theory. Here we report inverted region behavior for proton-coupled electron transfer (PCET). Specifically, photochemical studies of anthracene-phenol-pyridine triads give rate constants for PCET charge recombination that are slower for the more thermodynamically favorable reactions. Photoexcitation forms an anthracene excited state that undergoes PCET to create a charge separated state. The rate constants for return charge recombination show an inverted dependence on the driving force upon changing pyridine substituents and the solvent. Calculations using vibronically nonadiabatic PCET theory yield rate constants for simultaneous tunneling of the electron and proton that account for the results.

February 21 2019:  Sonja Pullen, Somnath Maji, Matthias Stein and Sascha Ott published an article in Dalton transactions: 

Restricted rotation of an Fe(CO)2(PL3)-subunit in [FeFe]-hydrogenase active site mimics by intramolecular ligation.

Abstract: A new series of homodinuclear iron complexes as models of the [FeFe]-hydrogenase active site was prepared and characterized. The complexes of the general formula [Fe2(mcbdt)(CO)5PPh2R] (mcbdt = benzene-1,2-dithiol-3-carboxylic acid) feature covalent tethers that link the mcbdt ligand with the phosphine ligands which are terminally coordinated to one of the Fe centres. The synthetic feasability of the concept is demonstrated with the preparation of three novel complexes. A detailed theoretical investigation showes that by introducing a rigid covalent link between the phosphine and the bridging dithiolate ligands, the rotation of the Fe(CO)2P unit is hindered and higher rotation barriers were calculated compared to nonlinked reference complexes. The concept of restricting Fe(L)3 rotation is an approach to kinetically stabilize terminal hydrides which are reactive intermediates in catalytic proton reduction cycles of the enzymes.

February 18 2019:  Sergii I. Shylin, Mariia V. Pavliuk, Luca D’Amario, Fikret Mamedov, Jacinto Sá, Gustav Berggren and Igor O. Fritsky published an article in Chemical communications: 

Efficient visible light-driven water oxidation catalysed by an iron(IV) clathrochelate complex.

Abstract: A water-stable Fe(IV) clathrochelate complex catalyses fast and homogeneous photochemical oxidation of water to dioxygen with a turnover frequency of 2.27 /s and a maximum turnover number of 365. An Fe(V) intermediate generated under catalytic conditions is trapped and characterised using EPR and Mössbauer spectroscopy.

January 4 2019:  Kamonchanock Eungrasamee, Rui Miao, Aran Incharoensakdi, Peter Lindblad and Saowarath Jantaro published an article in Biotechnology for biofuels: 

Improved lipid production via fatty acid biosynthesis and free fatty acid recycling in engineered Synechocystis sp. PCC 6803.

Abstract: Cyanobacteria are potential sources for third generation biofuels. Their capacity for biofuel production has been widely improved using metabolically engineered strains. In this study, we employed metabolic engineering design with target genes involved in selected processes including the fatty acid synthesis (a cassette of accD, accA, accC and accB encoding acetyl-CoA carboxylase, ACC), phospholipid hydrolysis (lipA encoding lipase A), alkane synthesis (aar encoding acyl-ACP reductase, AAR), and recycling of free fatty acid (FFA) (aas encoding acyl–acyl carrier protein synthetase, AAS) in the unicellular cyanobacterium Synechocystis sp. PCC 6803.

January 3 2019:  Brigitta Németh, Charlène Esmieu,  Holly J. Redman and Gustav Berggren published an article in Dalton transactions: 

Monitoring H-cluster assembly using a semisynthetic HydF protein.

Abstract: The [FeFe] hydrogenase enzyme interconverts protons and molecular hydrogen with remarkable efficiency. The reaction is catalysed by a unique metallo-cofactor denoted as the H-cluster containing an organometallic dinuclear Fe component, the [2Fe] subsite. The HydF protein delivers a precursor of the [2Fe] subsite to the apo-[FeFe] hydrogenase, thus completing the H-cluster and activating the enzyme. Herein we generate a semi-synthetic form of HydF by loading it with a synthetic low valent dinuclear Fe complex. We show that this semi-synthetic protein is practically indistinguishable from the native protein, and utilize this form of HydF to explore the mechanism of H-cluster assembly. More specifically, we show that transfer of the precatalyst from HydF to the hydrogenase enzyme results in the release of CO, underscoring that the pre-catalyst is a four CO species when bound to HydF. Moreover, we propose that an electron transfer reaction occurs during H-cluster assembly, resulting in an oxidation of the [2Fe] subsite with concomitant reduction of the [4Fe4S] cluster present on the HydF protein. 

January 2019:  Bo Xu, Lei Tian, Ahmed S.Etman, Junliang Sun, and Haining Tian published an article in Nano energy:

Solution-processed nanoporous NiO-dye-ZnO photocathodes: Toward efficient and stable solid-state p-type dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells.

Abstract: A solution-processed NiO-dye-ZnO photocathode was developed for applications in both solid-state p-type dye-sensitized solar cells (p-ssDSCs) and p-type dye-sensitized photoelectrosynthesis cells (p-DSPECs). In p-ssDSCs, the solar cell using ZnO as electron transport material showed a short circuit current, up to 680 µA cm−2, which is 60-fold larger than that previously reported device using TiO2 as electron transport material with similar architecture. In the p-DSPECs, a remarkable photocurrent of 100 μA cm−2 was achieved in a pH = 5.0 acetate buffer solution under a bias potential at 0.05 V vs RHE with platinum as the proton reduction catalyst. A Faradaic efficiency approaching 100% for the H2 evolution reaction was obtained after photoelectrolysis for 9 h. Importantly, the solution-processed NiO-dye-ZnO photocathode exhibited excellent long-term stability in both p-ssDSCs and p-DSPECs. To the best of our knowledge, this is the first study where a solution-processable, nanoporous NiO-dye-ZnO photocathode is used for both p-ssDSCs and p-DSPECs having both excellent device performance and stability.

Last updated January 15, 2020