Developing photoactivated artificial enzymes for sustainable fuel production.

Enzymes catalyze molecular reactions with remarkable efficiency and selectivity under mild conditions. Photoactivated enzymes make use of a light-absorbing chromophore to drive chemical transformations, ideally using sunlight as an energy source. The direct attachment of a chromophore to native enzymes is advantageous, as information on the underlying catalytic mechanisms can be obtained.

Targeted Modulation of Photocatalytic Hydrogen Evolution Activity by Nickel-Substituted Rubredoxin through Functionalized Ruthenium Phototriggers.

Light-driven hydrogen evolution is a promising means of sustainable energy production to meet global energy demand. This study investigates the photocatalytic hydrogen evolution activity of nickel-substituted rubredoxin (NiRd), an artificial hydrogenase mimic, covalently attached to a ruthenium phototrigger (RuNiRd). By systematically modifying the para-substituents on Ru(II) polypyridyl complexes, we sought to optimize the intramolecular electron transfer processes within the RuNiRd system.

Secondary Sphere Lewis Acid Activated Heme Superoxo Adducts Mimic Crucial Non-Covalent Interactions in IDO/TDO Heme Dioxygenases.

Heme enzymes play a central role in a medley of reactivities within a wide variety of crucial biological systems. Their active sites are highly decorated with pivotal evolutionarily optimized non-covalent interactions that precisely choreograph their biological functionalities with specific regio-, stereo-, and chemo-selectivities. Gaining a clear comprehension of how such weak interactions within the active sites control reactivity offers powerful information to be implemented into the design of future therapeutic agents that target these heme enzymes.

Electronic isomerism in a heterometallic nickel-iron-sulfur cluster models substrate binding and cyanide inhibition of carbon monoxide dehydrogenase.

The nickel-iron carbon monoxide dehydrogenase (CODH) enzyme uses a heterometallic nickel-iron-sulfur ([NiFeS]) cluster to catalyze the reversible interconversion of carbon dioxide (CO) and carbon monoxide (CO). These reactions are essential for maintaining the global carbon cycle and offer a route towards sustainable greenhouse gas conversion but have not been successfully replicated in synthetic models, in part due to a poor understanding of the natural system. Though the general protein architecture of CODH is known, the electronic structure of the active site is not well-understood, and the mechanism of catalysis remains unresolved.

Evaluating Diazene to N Interconversion at Iron-Sulfur Complexes.

Biological N reduction occurs at sulfur-rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N through PCET. Here, we test the feasibility of using synthetic sulfur-supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ-η : η-diazene (HN=NH) is the microscopic reverse of the proposed N fixation pathway, revealing the energetics of the process.

Effectiveness of Distal Sodium Channel Block in Managing Lumbosacral Radicular Syndrome: A Pilot Study.

Objective: To find the effectiveness of distal sodium channel blocks in managing lumbosacral radicular syndrome.

Study Design: Open-labelled, non-randomised, single-group, prospective, pilot study. Place and Duration of the Study: Pain Clinic of Armed Forces Institute of Rehabilitation Medicine (AFIRM) Rawalpindi, Pakistan, from January to June 2022.

Rubredoxin Protein Scaffolds Sourced from Diverse Environmental Niches as an Artificial Hydrogenase Platform.

Nickel-substituted rubredoxin (NiRd) from has previously been shown to act as both a structural and functional mimic of the [NiFe] hydrogenase. However, improvements both in turnover frequency and overpotential are needed to rival the native [NiFe] hydrogenase enzymes. Characterization of a library of NiRd mutants with variations in the secondary coordination sphere suggested that protein dynamics played a substantial role in modulating activity.

How to Build a Metalloenzyme: Lessons from a Protein-Based Model of Acetyl Coenzyme A Synthase.

"What I cannot create, I do not understand"─Richard Feynman. This sentiment motivates the entire field of artificial metalloenzymes. Naturally occurring enzymes catalyze reactions with efficiencies, rates, and selectivity that generally cannot be achieved in synthetic systems.

Reductive NO Coupling at Dicopper Center via a [Cu(NO)] Diamond-Core Intermediate.

Treatment of a dicopper(I,I) complex with excess amounts of NO leads to the formation of a dicopper dinitrosyl [Cu(NO)] complex capable of (i) releasing two equivalents of NO reversibly in 90% yield and (ii) reacting with another equivalent of NO to afford NO and dicopper nitrosyl oxo species [Cu(NO)(O)]. Resonance Raman characterization of the [Cu(NO)] complex shows a N-sensitive N═O stretch at 1527.6 cm and two Cu-N stretches at 390.

Thioester synthesis by a designed nickel enzyme models prebiotic energy conversion.

The formation of carbon-carbon bonds from prebiotic precursors such as carbon dioxide represents the foundation of all primordial life processes. In extant organisms, this reaction is carried out by the carbon monoxide dehydrogenase (CODH)/acetyl coenzyme A synthase (ACS) enzyme, which performs the cornerstone reaction in the ancient Wood-Ljungdahl metabolic pathway to synthesize the key biological metabolite, acetyl-CoA. Despite its significance, a fundamental understanding of this transformation is lacking, hampering efforts to harness analogous chemistry.

Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins.

Traditionally, the ferritin-like superfamily of proteins was thought to exclusively use a diiron active site in catalyzing a diverse array of oxygen-dependent reactions. In recent years, novel redox-active cofactors featuring heterobimetallic Mn/Fe active sites have been discovered in both the radical-generating R2 subunit of class Ic (R2c) ribonucleotide reductases (RNRs) and the related R2-like ligand-binding oxidases (R2lox). However, the protein-specific factors that differentiate the radical reactivity of R2c from the C-H activation reactions of R2lox remain unknown.

Controlling the Direction of -Nitrosation versus Denitrosation: Reversible Cleavage and Formation of an S-N Bond within a Dicopper Center.

Iron and copper enzymes are known to promote reversible -nitrosation/denitrosation in biology. However, it is unclear how the direction of S-N bond formation/scission is controlled. Herein, we demonstrate the interconversion of metal--nitrosothiol adduct M(RSNO) and metal nitrosyl thiolate complex M(NO)(SR), which may regulate the direction of reversible -(de)nitrosation.

Protein-based models offer mechanistic insight into complex nickel metalloenzymes.

There are ten nickel enzymes found across biological systems, each with a distinct active site and reactivity that spans reductive, oxidative, and redox-neutral processes. We focus on the reductive enzymes, which catalyze reactions that are highly germane to the modern-day climate crisis: [NiFe] hydrogenase, carbon monoxide dehydrogenase, acetyl coenzyme A synthase, and methyl coenzyme M reductase. The current mechanistic understanding of each enzyme system is reviewed along with existing knowledge gaps, which are addressed through the development of protein-derived models, as described here.

Enzyme Control Over Ferric Iron Magnetostructural Properties.

Fe complexes in aqueous solution can exist as discrete mononuclear species or multinuclear magnetically coupled species. Stimuli-driven change to Fe speciation represents a powerful mechanistic basis for magnetic resonance sensor technology, but ligand design strategies to exert precision control of aqueous Fe magnetostructural properties are entirely underexplored. In pursuit of this objective, we rationally designed a ligand to strongly favor a dinuclear μ-oxo-bridged and antiferromagnetically coupled complex, but which undergoes carboxylesterase mediated transformation to a mononuclear high-spin Fe chelate resulting in substantial T -relaxivity increase.

Uncovering Redox Non-innocent Hydrogen-Bonding in Cu(I)-Diazene Complexes.

The life-sustaining reduction of N to NH is thermoneutral yet kinetically challenged by high-energy intermediates such as NH. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [TpCu](μ-NH) complexes that exhibit H-bonding between pendant aromatic N-heterocycles (Het) such as pyridine and a bridging -NH ligand at copper(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [TpCu](μ-NH) while low-temperature H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs.

Reversible Electron Transfer and Substrate Binding Support [NiFeS] Ferredoxin as a Protein-Based Model for [NiFe] Carbon Monoxide Dehydrogenase.

The nickel-iron carbon monoxide dehydrogenase (CODH) enzyme catalyzes the reversible and selective interconversion of carbon dioxide (CO) to carbon monoxide (CO) with high rates and negligible overpotential. Despite decades of research, many questions remain about this complex metalloenzyme system. A simplified model enzyme could provide substantial insight into biological carbon cycling.

Pendent Relay Enhances HO Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co-NO Complexes.

Generally, cobalt-NO complexes show selectivity for hydrogen peroxide during electrochemical dioxygen (O) reduction. We recently reported a Co(III)-NO complex with a 2,2'-bipyridine-based ligand backbone which showed alternative selectivity: HO was observed as the primary reduction product from O (71 ± 5%) with decamethylferrocene as a chemical reductant and acetic acid as a proton donor in methanol solution. We hypothesized that the key selectivity difference in this case arises in part from increased favorability of protonation at the distal O position of the key intermediate Co(III)-hydroperoxide species.

Assembly of a Genetically Encoded Artificial Metalloenzyme for Hydrogen Production.

The genetic encoding of artificial enzymes represents a substantial advantage relative to traditional molecular catalyst optimization, as laboratory-based directed evolution coupled with high-throughput screening methods can provide rapid development and functional characterization of enzyme libraries. However, these techniques have been of limited utility in the field of artificial metalloenzymes due to the need for cofactor metalation. Here, we report the development of methodology for production of nickel-substituted rubredoxin, an artificial metalloenzyme that is a structural, functional, and mechanistic mimic of the [NiFe] hydrogenases.

Ligand Field Inversion as a Mechanism to Gate Bioorganometallic Reactivity: Investigating a Biochemical Model of Acetyl CoA Synthase Using Spectroscopy and Computation.

The biological global carbon cycle is largely regulated through microbial nickel enzymes, including carbon monoxide dehydrogenase (CODH), acetyl coenzyme A synthase (ACS), and methyl coenzyme M reductase (MCR). These systems are suggested to utilize organometallic intermediates during catalysis, though characterization of these species has remained challenging. We have established a mutant of nickel-substituted azurin as a scaffold upon which to develop protein-based models of enzymatic intermediates, including the organometallic states of ACS.

Rational Ligand Design Enables pH Control over Aqueous Iron Magnetostructural Dynamics and Relaxometric Properties.

Complexes of Fe engage in rich aqueous solution speciation chemistry in which discrete molecules can react with solvent water to form multinuclear μ-oxo and μ-hydroxide bridged species. Here we demonstrate how pH- and concentration-dependent equilibration between monomeric and μ-oxo-bridged dimeric Fe complexes can be controlled through judicious ligand design. We purposed this chemistry to develop a first-in-class Fe-based MR imaging probe, Fe-PyCy2AI, that undergoes relaxivity change via pH-mediated control of monomer vs dimer speciation.