Publication highlights

Picks of our best science in 2020!

December 2020: Henrik Land, Alina Sekretareva, Ping Huang, Holly J. Redman, Brigitta Németh, Nakia Polidori, Lívia S. Mészáros, Moritz Senger, Sven T. Stripp and Gustav Berggren published an article in Chemical Science:
Characterization of a putative sensory [FeFe]-hydrogenase provides new insight into the role of the active site architecture

Abstract: [FeFe]-hydrogenases are known for their high rates of hydrogen turnover, and are intensively studied in the context of biotechnological applications. Evolution has generated a plethora of different subclasses with widely different characteristics. The M2e subclass is phylogenetically distinct from previously characterized members of this enzyme family and its biological role is unknown. It features significant differences in domain- and active site architecture, and is most closely related to the putative sensory [FeFe]-hydrogenases. Here we report the first comprehensive biochemical and spectroscopical characterization of an M2e enzyme, derived from Thermoanaerobacter mathranii. As compared to other [FeFe]-hydrogenases characterized to-date, this enzyme displays an increased H2 affinity, higher activation enthalpies for H+/H2 interconversion, and unusual reactivity towards known hydrogenase inhibitors. These properties are related to differences in active site architecture between the M2e [FeFe]-hydrogenase and “prototypical” [FeFe]-hydrogenases. Thus, this study provides new insight into the role of this subclass in hydrogen metabolism and the influence of the active site pocket on the chemistry of the H-cluster.


November 2020: Anna M. Beiler , Brian D. McCarthy , Ben A. Johnson and Sascha Ott published an article in Nature communications:

Enhancing photovoltages at p-type semiconductors through a redox-active metal-organic framework surface coating.

AbstractSurface modification of semiconductors can improve photoelectrochemical performance by promoting efficient interfacial charge transfer. We show that metal-organic frameworks (MOFs) are viable surface coatings for enhancing cathodic photovoltages. Under 1-sun illumination, no photovoltage is observed for p-type Si(111) functionalized with a naphthalene diimide derivative until the monolayer is expanded in three dimensions in a MOF. The surface-grown MOF thin film at Si promotes reduction of the molecular linkers at formal potentials >300 mV positive of their thermodynamic potentials. The photocurrent is governed by charge diffusion through the film, and the MOF film is sufficiently conductive to power reductive transformations. When grown on GaP(100), the reductions of the MOF linkers are shifted anodically by >700 mV compared to those of the same MOF on conductive substrates. 

This photovoltage, among the highest reported for GaP in photoelectrochemical applications, illustrates the power of MOF films to enhance photocathodic operation.

November 2020: Sergey Kosourov, Valéria Nagya, Dmitry Shevela, Martina Jokela, Johannes Messinger and Yagut Allahverdiyeva published an article in The Proceedings of the National Academy of Sciences of the USA

Water oxidation by photosystem II is the primary source of electrons for sustained H2 photoproduction in nutrient-replete green algae

Significance: Photosynthetic H2 production in the green alga Chlamydomonas reinhardtii is catalyzed by O2-sensitive [FeFe]-hydrogenases, which accept electrons from photosynthetically reduced ferredoxin and reduce protons to H2. Since the process occurs downstream of photosystem I, the contribution of photosystem II (PSII) in H2 photoproduction has long been a subject of debate. Indeed, water oxidation by PSII results in O2 accumulation in chloroplasts, which inhibits H2 evolution. Therefore, clear evidence for direct water biophotolysis resulting in simultaneous H2 and O2 releases in algae has never been presented. This paper demonstrates that sustained H2 photoproduction in C. reinhardtii is directly linked to PSII-dependent water oxidation and brings insights into  regulation of PSII activity and H2 production by CO2/HCO3 – under microoxic conditions.

October 2020: Robin Tyburski, Chenyu Wen, Rui Sun, Mohamed Abdellah, Jing Huang, Luca D’Amario, Gerrit Boschloo, Leif Hammarström, and Haining Tian published an article in The Journal of the American Chemical Society:

Understanding the Role of Surface States on Mesoporous NiO Films

Abstract: Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al2O3. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. 

A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO2-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.

June 2020: Astrid Nilsen-Moe, Clorice R. Reinhardt, Starla D. Glover, Li Liang, Sharon Hammes-Schiffer,
Leif Hammarström, and Cecilia Tommos published a paper in Journal of the American Chemical Society:

Proton-Coupled Electron Transfer from Tyrosine in the Interior of a de novo Protein: Mechanisms and Primary Proton Acceptor

Abstract: Proton-coupled electron transfer (PCET) from tyrosine produces a neutral tyrosyl radical (Y•) that is vital to many catalytic redox reactions. To better understand how the protein environment influences the PCET properties of tyrosine, we have studied the radical formation behavior of Y32 in the α3Y model protein. The previously solved α3Y solution NMR structure shows that Y32 is sequestered ∼7.7 ± 0.3 Å below the protein surface without any primary proton acceptors nearby. 

Here we present transient absorption kinetic data and molecular dynamics (MD) simulations to resolve the PCET mechanism associated with Y32 oxidation. Y32• was generated in a bimolecular reaction with [Ru(bpy)3]3+ formed by flash photolysis. At pH > 8, the rate constant of Y32• formation (kPCET) increases by one order of magnitude per pH unit, corresponding to a proton-first mechanism via tyrosinate (PTET). At lower pH < 7.5, the pH dependence is weak and shows a previously measured KIE ≈ 2.5, which best fits a concerted mechanism. kPCET is independent of phosphate buffer concentration at pH 6.5. This provides clear evidence that phosphate buffer is not the primary proton acceptor. MD simulations show that one to two water molecules can enter the hydrophobic cavity of α3Y and hydrogen bond to Y32, as well as the possibility of hydrogen-bonding interactions between Y32 and E13, through structural fluctuations that reorient surrounding side chains. Our results illustrate how protein conformational motions can influence the redox reactivity of a tyrosine residue and how PCET mechanisms can be tuned by changing the pH even when the PCET occurs within the interior of a protein.

June 2020:  Kateřina Holá, Mariia V. Pavliuk, Brigitta Németh, Ping Huang, Lukáš Zdražil, Henrik Land, Gustav Berggren, and Haining Tian published an article in ACS Catalysis:

Carbon Dots and [FeFe] Hydrogenase Biohybrid Assemblies for Efficient Light-Driven Hydrogen Evolution

Abstract: Artificial photosynthesis is seen as a path to convert and store solar energy into chemical energy for our society. In this work, highly fluorescent aspartic acid-based carbon dots (CDs) are synthesized and employed as a photosensitizer to drive photocatalytic hydrogen evolution with an [FeFe] hydrogenase (CrHydA1). The direct interaction in CDs from l-aspartic acid (AspCDs)/CrHydA1 self-assembly systems, which is visualized from native gel electrophoresis, has been systematically investigated to understand the electron-transfer dynamics and its impact on photocatalytic efficiency. The study discloses the significant influence of the electrostatic surrounding generated by sacrificial electron donors on the intimate interplay within the oppositely charged subunits of the biohybrid assembly as well as the overall photocatalytic performance. The system reaches an external quantum efficiency of 1.7% at 420 nm and an initial activity of 1.73 μmol(H2) /mg (hydrogenase) /min under favorable electrostatic conditions. Owing to the ability of the synthesized AspCDs to operate efficiently under visible light, in contrast to other materials that require UV illumination, the stability of the biohybrid assembly in the presence of a redox mediator extends beyond 1 week.

June 2020:  Rui Miao, Hao Xie, Xufeng Liu, Pia Lindberg and Peter Lindblad published an article in Current Opinion in Chemical Biology:

Current processes and future challenges of photoautotrophic production of acetyl-CoA-derived solar fuels and chemicals in cyanobacteria

Abstract: The production of fuels and other valuable chemicals via biological routes has gained significant attention during last decades. Cyanobacteria are prokaryotes that convert solar energy to chemical compounds in vivo in direct processes. Intensive studies have been carried out with the aim of engineering cyanobacteria as microfactories for solar fuel and chemical production. Engineered strains of photosynthetic cyanobacteria can produce different compounds on a proof-of-concept level, but few products show titers comparable with those achieved in heterotrophic organisms. Efficient genetic engineering tools and metabolic modeling can accelerate the development of solar fuel and chemical production in cyanobacteria. 

This review addresses the most recent approaches to produce solar fuels and chemicals in engineered cyanobacteria with a focus on acetyl-CoA-dependent products.

April 2020:  Mohammad Ziaur Rahman,  Haining Tian, and  Tomas Edvinsson published an article in Angewandte Chemie International Edition:

Revisiting the Limiting Factors for Overall Water‐Splitting on Organic Photocatalysts

Abstract: In pursuit of inexpensive and earth abundant photocatalysts for solar hydrogen production from water, conjugated polymers have shown potential to be a viable alternative to widely used inorganic counterparts. The photocatalytic performance of polymeric photocatalysts, however, is very poor in comparison to that of inorganic photocatalysts. Most of the organic photocatalysts are active in hydrogen production only when a sacrificial electron donor (SED) is added into the solution, and their high performances often rely on presence of noble metal co‐catalyst (e.g. Pt). For pursuing a carbon neutral and cost‐effective green hydrogen production, unassisted hydrogen production solely from water is one of the critical requirements to translate a mere bench‐top research interest into the real world applications. Although this is a generic problem for both inorganic and organic types of photocatalysts, organic photocatalysts are mostly investigated in the half‐reaction and have so far shown limited success in hydrogen production from overall water‐splitting. To make progress, this article exclusively discusses critical factors that are limiting the overall water‐splitting in organic photocatalysts. Additionally, we also have extended the discussion to stability issues and the accurate reporting of the hydrogen production, and issues to be resolved to reach 10% STH (solar‐to‐hydrogen) conversion efficiency.

April 2020:  M. Ibrahim,  T. Fransson,  R. Chatterjee,  Mun Hon Cheah,  R. Hussein, L. Lassalle,  K. D. Sutherlin,  I. D. Young,  F. D. Fuller, S. Gul, I.-S. Kim, P.S. Simon,  Casper de Lichtenberg, Petko Chernev, I. Bogacz, C. C. Pham,  A. M. Orville, N. Saichek, T. Northen,  A.r Batyuk, S. Carbajo, R. Alonso-Mori, K. Tono, S. Owada, A. Bhowmick, R. Bolotovsky, D. Mendez,  N. W. Moriarty,  J. M. Holton, H. Dobbek,  A. S. Brewster, P. D. Adams, N. K. Sauter,  U. Bergmann,  A. Zouni,  Johannes Messinger,  J. Kern,  V. K. Yachandra, and  J. Yano published an article in Proceedings of the National Academy of Sciences of the U.S.A.:

Untangling the sequence of events during the S2 → S3 transition in photosystem II and implications for the water oxidation mechanism

Abstract: In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S1, S2, S3, and S0, showing that a water molecule is inserted during the S2 → S3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S2 → S3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, QA and QB, are observed. At the donor site, tyrosine YZ and His190 H-bonded to it move by 50 µs after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of OX(H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a “water wheel”-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (τ of ∼350 µs) during the S2 → S3 transition mirrors the appearance of OX electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated..

March 2020:  Brian D. McCarthy, Anna M. Beiler, Ben A. Johnson, Timofey Liseev, Ashleigh T. Castner, and Sascha Ott published an article in Coordination Chemistry Reviews:

Analysis of Electrocatalytic Metal-organic Frameworks

Abstract: The electrochemical analysis of molecular catalysts for the conversion of bulk feedstocks into energy-rich clean fuels has seen dramatic advances in the last decade. More recently, increased attention has focused on the characterization of metal-organic frameworks (MOFs) containing well-defined redox and catalytically active sites, with the overall goal to develop structurally stable materials that are industrially relevant for large-scale solar fuel syntheses. Successful electrochemical analysis of such materials draws heavily on well-established homogeneous techniques, yet the nature of solid materials presents additional challenges. In this tutorial-style review, we cover the basics of electrochemical analysis of electroactive MOFs, including considerations of bulk stability, methods of attaching MOFs to electrodes, interpreting fundamental electrochemical data, and finally electrocatalytic kinetic characterization. We conclude with a perspective of some of the prospects and challenges in the field of electrocatalytic MOFs.

February 2020: Ouissam El Bakouri, Joshua R. Smith, and Henrik Ottosson published an article in Journal of the American Chemical Society:

Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground-State and Excited-State Aromaticity Rules.

Singlet exciton fission photovoltaic technology requires chromophores with their lowest excited states arranged so that 2E(T1) < E(S1) and E(S1) < E(T2). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4n rule on excited-state aromaticity, combined with Hückel’s 4n + 2 rule for ground-state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the E(T1), E(S1), and E(T2) of benzene and cyclobutadiene (CBD) as excited-state antiaromatic and aromatic archetypes, respectively, and reveal that CBD fulfills the criteria on the state ordering for a singlet fission chromophore. We then look at fulvenes, a class of compounds that can be tuned by choice of substituents from Baird-antiaromatic to Baird-aromatic in T1 and S1 and from Hückel-aromatic to Hückel-antiaromatic in S0. The T1 and S1 states of most substituted fulvenes (159 of 225) are described by singly excited HOMO → LUMO configurations, providing a rational for the simultaneous tuning of E(T1) and E(S1) along an approximate (anti)aromaticity coordinate. Key to the tunability is the exchange integral (KH,L), which ideally is constant throughout the compound class, providing a constant ΔE(S1 – T1). This leads us to a geometric model for the identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated E(T1) values of ∼1 eV or higher are identified among benzannelated 4nπ-electron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird’s 4n and Hückel’s 4n + 2 rules, together with substituent effects (electronic and steric) and benzannelation, can be used to tailor new chromophores with potential use in singlet fission photovoltaics.

January 7, 2020:   Mun Hon Cheah,  Miao Zhang,  Dmitry Shevela,  Fikret Mamedov,  Athina Zouni, and  Johannes Messinger published an article in Proceedings of the National Academy of Sciences of the USA:

Assessment of the manganese cluster’s oxidation state via photoactivation of photosystem II microcrystals

Significance: Photosynthetic water oxidation by the multi-subunit membrane protein complex PSII is an important process that sustains all aerobic life on Earth by producing molecular oxygen from sunlight and water. Understanding the mechanism of this process is crucial toward advancing fundamental knowledge as well as providing a blueprint for the development of solar fuel devices. Important pieces of information required for solving the mechanism of biological water oxidation are the oxidation states of the manganese ions forming the catalytic site of water oxidation in PSII. Here, we resolve a long-standing controversy between 2 competing schools of thought, by providing a clear-cut determination of overall manganese oxidation states using a simple counting experiment.

Fig. 4: (Top) Laser flash spacing’s used for photoactivation of apo-PSII microcrystals suspension by various combinations of tightly spaced preflashes (green line) and monitoring flashes (red line) with 15-s spacing. The dark time of 240 s was employed to allow the back reaction of the S2 and S3 states to S1. (Bottom) Total number of flashes required to observe the first O2 evolution (left axis) during the photoactivation of apo-PSII crystals with various combinations of preflashes and monitoring flashes (see Top). The yields of the first O2 peaks, plotted in blue, are the averages of 2 repeat measurements, and the error bars are SDs.

2019 publications

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 Sepember 1, 2021