Mun Hon Cheah

During photosynthetic water oxidation, the Mn₄Ca cluster in Photosystem II progresses through five intermediate S_i (i = 0–4) states. X-ray crystallography studies have reported the insertion of one new O ligand during the formation of the S₃ state, but recent studies question the presence of this additional ligand based on cryo-EM and earlier room-temperature crystallography data. There is also controversy about whether the O-O bond interaction already occurs in the S₃ state or in the subsequent S₃ to S₀ transition. Here we report conventional high-resolution data for the S₁, S₂, and S₃ states to a resolution of ~1.9 Å, and anomalous diffraction data at two energies (9.5 keV and 7 keV), that was used to model the Mn positions, followed by determination of oxygen positions using the high-resolution maps. We show that the new oxygen atom, O_X (or O6), in the S₃ state is observable as a distinct peak without any restraints, confirming its ligation to Mn1 and Ca. The O_X-O5 distance is ~2.1 Å, supporting no strong interaction between them in the S₃ state, suggesting that if this is the O-O bond formation site, it is formed during the S₃ to S₀ transition initiated by the final oxidation of the cluster.

Iron porphyrin complexes constitute a well‐established and versatile class of molecular electrocatalysts for the reduction of CO ₂ to CO. In both organic and aqueous media, the reaction mechanism is typically proposed to involve the interaction of CO ₂ with a formally defined [(porphyrin)Fe ⁰ ] intermediate. In this work, we performed a mechanistic investigation of CO ₂ reduction using the water‐soluble complex [( p TMA)Fe ^III Cl]Cl ₄ under aqueous conditions. In situ scanning spectroelectrochemistry was employed, enabling the synchronized acquisition of UV–vis or IR spectra during cyclic voltammetry experiments. Our results provide strong evidence for CO ₂ binding to the electrogenerated [( p TMA)Fe ^I ] ³⁺ species, followed by reductive C─O bond cleavage to yield a stable [( p TMA)(Cl)Fe ^II ‐CO] ^{3 +} complex. This process corresponds to an overall two‐electron reduction per iron center. This mechanism, which has not been previously considered for molecular iron porphyrins in CO ₂ reduction, is proposed to be facilitated by the charged porphyrin periphery and the hydrogen‐bonding network of the aqueous medium. These features may open new avenues toward achieving CO ₂ reduction at lower overpotentials in water.

Iron porphyrin complexes constitute a well‐established and versatile class of molecular electrocatalysts for the reduction of CO ₂ to CO. In both organic and aqueous media, the reaction mechanism is typically proposed to involve the interaction of CO ₂ with a formally defined [(porphyrin)Fe ⁰ ] intermediate. In this work, we performed a mechanistic investigation of CO ₂ reduction using the water‐soluble complex [( p TMA)Fe ^III Cl]Cl ₄ under aqueous conditions. In situ scanning spectroelectrochemistry was employed, enabling the synchronized acquisition of UV–vis or IR spectra during cyclic voltammetry experiments. Our results provide strong evidence for CO ₂ binding to the electrogenerated [( p TMA)Fe ^I ] ³⁺ species, followed by reductive C─O bond cleavage to yield a stable [( p TMA)(Cl)Fe ^II ‐CO] ^{3 +} complex. This process corresponds to an overall two‐electron reduction per iron center. This mechanism, which has not been previously considered for molecular iron porphyrins in CO ₂ reduction, is proposed to be facilitated by the charged porphyrin periphery and the hydrogen‐bonding network of the aqueous medium. These features may open new avenues toward achieving CO ₂ reduction at lower overpotentials in water.

Minimal metallopeptide complexes offer simplified yet powerful models of metalloenzyme active sites, enabling the investigation of fundamental structure-function relationships. While α-helical constructs are widely employed for this purpose, β-sheet-based scaffolds remain underexplored. In this work, we leverage the intrinsic symmetry of the trinuclear copper center (TNC) binding pocket in a small laccase to design a β-harpin metallopeptide, LacZip. The construct links β-strands through a tryptophan zipper motif, imparting conformational rigidity to the peptide chain. Spectroscopic analyses, including circular dichroism and fluorescence quenching, demonstrate that LacZip undergoes metal-induced dimerization, reproducing the TNC architecture and binding Cu2+ ions with positive cooperativity. Notably, EPR spectroscopy reveals signals consistent with the formation of a trinuclear Cu3(LacZip)2 complex at high metal concentrations. Overall, our work highlights the potential of β-sheet scaffolds for self-assembly of multinuclear metallopeptide complexes, offering additional tools for the design of stable and functional artificial biomimics.

Metal-dependent redox enzymes are central for microbial processing of gases, as exemplified by hydrogenase, nitrogenase, and carbon monoxide dehydrogenase. Due to their remarkable efficiencies and high biotechnological relevance, such gas-processing enzymes are intensively studied. Nevertheless, many of their mechanistic details remain opaque. We herein report a new method for solution assays under reducing conditions based on europium(II) as a terminal reductant and show how it can be employed to gain new insight into hydrogenase kinetics. Compared with the commonly used reductant sodium dithionite, this work shows that Eu(II) can serve as a robust and relatively easy-to-handle alternative electron donor, also providing a larger potential window for catalytic studies. Further, this work clarifies previous discrepancies in the literature regarding the influence of pH on hydrogenase kinetics in these assays. Our study shows that sodium dithionite, most likely due to its decomposition into SO₂, alters hydrogenase kinetics in solution assays. Using [FeFe]-hydrogenase I from Clostridium pasteurianum (CpI) as a model system, Eu(II)-based solution assays demonstrated a pH optimum of 5-6 and rates greatly exceeding those observed with sodium dithionite assays. The higher turnover frequencies observed at low pH obtained with Eu(II) align more closely with the electrochemical data. Additionally, a strong driving force dependency was identified. A solution potential change of approximately 180 mV resulted in a 35-fold increase in the catalytic rate, yielding activities far surpassing those of earlier reports on CpI turnover frequencies. These findings provide new insight into the pH dependence and overall kinetic performance of [FeFe]-hydrogenases. More broadly, the report outlines alternative assay methods employing Eu(II) to better understand the enzyme kinetics of hydrogenases and related metalloenzymes.

Using small organic molecular redox carriers to reversibly capture CO ₂ and convert it to carbon‐based chemicals is a promising approach to mitigate the ongoing climate crisis. 2,1,3‐benzothiadiazole (BT) is an interesting unit due to its proven interaction with CO ₂ upon reduction and the ease of tuning its structure. In this work, by introducing two CN in BT, the molecule 2,1,3‐benzothiadiazole‐4,7‐dicarbonitrile (BTDN) has multiple reduced states as compared to BT and is found to interact with CO ₂ at multiple reduced states. The work is carried out with a combination of (spectro‐)electrochemical and computational studies. Cyclic voltammetry experiments in the presence of CO ₂ show a clear interaction between BTDN and CO ₂ upon the second reduction of BTDN and a large current increase at the third reduction. Density functional theory calculations prove a large variety of possible CO ₂ ‐bound species that can match the experimental data. The binding of CO ₂ on BTDN is found to be reversible upon the oxidation of the species, especially with low concentrations of CO ₂ . From NMR and IR experiments, certain amount of reduced product – oxalate is detected after bulk electrolysis at the third reduction potential in the presence of CO ₂ , showing the potential toward electrocatalysis after structural tuning and systematical optimization.

Nature-inspired or biomimetic catalysts aim to reach the high catalytic performance and selectivity of natural enzymes while possessing the chemical stability and processability of synthetic catalysts. A promising strategy for designing biomimetic catalysts relies on mimicking the structure of the enzyme active site. This can either entail complicated total synthesis of a synthetic catalyst or design of peptide sequences, able to self-assemble in the presence of metal ions, thus forming metallo-peptide complexes that mimic the active sites of natural enzymes. Using a bioinformatics approach, we designed a minimal peptide made up of eight amino acids (H4pep) to act as a functional mimic of the trinuclear Cu site of the laccase enzyme. Cu(II) binding to H4pep results in the formation of a Cu²⁺(H4pep)₂ complex with a β-sheet secondary structure, able to reduce O₂. Our study demonstrates the viability and potential of using short peptides to mimic the minimal functional site of natural enzymes.

The use of synthetic peptides or metallopeptides are increasingly common in the field of sensors or electrocatalysis. Heterogenization of these synthetic peptides on electrode surfaces are a crucial step in building functional devices. Here we report a strategy to perform direct solid-phase peptide synthesis on graphitic electrodes as a fast and convenient way to build integrated peptide-electrode assemblies.

Atomic layer-deposited (ALD) films are widely used as insulating barriers in (photo)electrochemical systems, yet their stability and charge-transfer behavior under operational conditions remain poorly characterized. Here, we systematically investigate how film thickness and electrolyte composition influence the performance of ALD-grown amorphous Al₂O₃ films on indium tin oxide. Using cyclic voltammetry and electrochemical impedance spectroscopy, we find that a thickness of ∼4-5 nm is required to achieve stable insulation and tunneling-limited electron transfer, which is significantly more than the minimum needed to form a continuous film. Moreover, the extracted tunneling decay constant, 0.30 Å^-1, is lower than values reported for crystalline Al₂O₃, indicating noticeable charge transport through amorphous thin films. On the other hand, a reduction in the effective diffusion of redox active molecules at the electrode surface is suggested for films thicker than 3 nm. We further demonstrate that specific ions strongly influence film lifetime. Unexpectedly, we found that acetate buffers are significantly less detrimental to film stability compared to commonly used phosphate buffers. Moreover, the addition of low concentrations of Al³⁺ ions dramatically delays film degradation. In contrast, pH effects between 4 and 8 are minimal. Notably, film failure shows stochastic behavior while also being broadly consistent with gradual homogeneous dissolution rather than discrete pinhole formation previously reported for TiO₂ and Al₂O₃ insulating films. These results reveal the critical and previously underappreciated role of electrolyte composition in determining the lifetime of insulating oxide films. Our findings offer practical design guidelines and highlight the need for controlled conditions when implementing ALD barriers in electrochemical devices.