Protein engineering a PhotoRNR chimera based on a unifying evolutionary apparatus among the natural classes of ribonucleotide reductases.

Ribonucleotide reductases (RNRs) are essential enzymes that catalyze the de novo transformation of nucleoside 5'-di(tri)phosphates [ND(T)Ps, where N is A, U, C, or G] to their corresponding deoxynucleotides. Despite the diversity of factors required for function and the low sequence conservation across RNRs, a unifying apparatus consolidating RNR activity is explored. We combine aspects of the protein subunit simplicity of class II RNR with a modified version of Escherichia coli class la photoRNRs that initiate radical chemistry with light to engineer a mimic of a class II enzyme. The design of this RNR involves fusing a truncated form of the active site containing α subunit with the functionally important C-terminal tail of the radical-generating ß subunit to render a chimeric RNR. Inspired by a recent cryo-EM structure, a [Re] photooxidant is located adjacent to Y356[ß], which is an essential component of the radical transport pathway in class I RNRs. Combination of this RNR photochimera with cytidine diphosphate (CDP), adenosine triphosphate (ATP), and light resulted in the generation of Y356• along with production of deoxycytidine diphosphate (dCDP) and cytosine. The photoproducts reflect an active site chemistry consistent with both the consensus mechanism of RNR and chemistry observed when RNR is inactivated by mechanism-based inhibitors in the active site. The enzymatic activity of the RNR photochimera in the absence of any ß metallocofactor highlights the adaptability of the 10-stranded αß barrel finger loop to support deoxynucleotide formation and accommodate the design of engineered RNRs.

Electrophotocatalytic perfluoroalkylation by LMCT excitation of Ag(II) perfluoroalkyl carboxylates.

Molecular Ag(II) complexes are superoxidizing photoredox catalysts capable of generating radicals from redox-reticent substrates. In this work, we exploited the electrophilicity of Ag(II) centers in [Ag(bpy)2(TFA)][OTf] and Ag(bpy)(TFA)2 (bpy, 2,2'-bipyridine; OTf, CF3SO3-) complexes to activate trifluoroacetate (TFA) by visible light-induced homolysis. The resulting trifluoromethyl radicals may react with a variety of arenes to forge C(sp2)-CF3 bonds. This methodology is general and extends to other perfluoroalkyl carboxylates of higher chain length (RFCO2-; RF, CF2CF3 or CF2CF2CF3). The photoredox reaction may be rendered electrophotocatalytic by regenerating the Ag(II) complexes electrochemically during irradiation. Electrophotocatalytic perfluoroalkylation of arenes at turnover numbers exceeding 20 was accomplished by photoexciting the Ag(II)-TFA ligand-to-metal charge transfer (LMCT) state, followed by electrochemical reoxidation of the Ag(I) photoproduct back to the Ag(II) photoreactant.

Chemoselective bond activation by unidirectional and asynchronous PCET using ketone photoredox catalysts.

The triplet excited states of ketones are found to effect selective H-atom abstraction from strong amide N-H bonds in the presence of weaker C-H bonds through a proton-coupled electron transfer (PCET) pathway. This chemoselectivity, which results from differences in ionization energies (IEs) between functional groups rather than bond dissociation energies (BDEs) arises from the asynchronicity between electron and proton transfer in the PCET process. We show how this strategy may be leveraged to achieve the intramolecular anti-Markovnikov hydroamidation of alkenes to form lactams using camphorquinone as an inexpensive and sustainable photocatalyst.

Reversible CO2 Capture and On-Demand Release by an Acidity-Matched Organic Photoswitch.

Separation of carbon dioxide (CO2) from point sources or directly from the atmosphere can contribute crucially to climate change mitigation plans in the coming decades. A fundamental practical limitation for the current strategies is the considerable energy cost required to regenerate the sorbent and release the captured CO2 for storage or utilization. A directly photochemically driven system that demonstrates efficient passive capture and on-demand CO2 release triggered by sunlight as the sole external stimulus would provide an attractive alternative. However, little is known about the thermodynamic requirements for such a process or mechanisms for modulating the stability of CO2-derived dissolved species by using photoinduced metastable states. Here, we show that an organic photoswitchable molecule of precisely tuned effective acidity can repeatedly capture and release a near-stoichiometric quantity of CO2 according to dark-light cycles. The CO2-derived species rests as a solvent-separated ion pair, and key aspects of its excited-state dynamics that regulate the photorelease efficiency are characterized by transient absorption spectroscopy. The thermodynamic and kinetic concepts established herein will serve as guiding principles for the development of viable solar-powered negative emission technologies.

Substrate-Mediator Duality of 1,4-Dicyanobenzene in Electrochemical C(sp2 )-C(sp3 ) Bond Formation with Alkyl Bromides.

Electrochemical approaches to form C(sp2 )-C(sp3 ) bonds have focused on coupling C(sp3 ) electrophiles that form stabilized carbon-centered radicals upon reduction or oxidation. Whereas alkyl bromides are desirable C(sp3 ) coupling partners owing to their availability and cost-effectiveness, their tendency to undergo radical-radical homocoupling makes them challenging substrates for electroreductive cross-coupling. Herein, we disclose a metal-free regioselective cross-coupling of 1,4-dicyanobenzene, a useful precursor to aromatic nitriles, and alkyl bromides. Alkyl bromide reduction is mediated directly by 1,4-dicyanobenzene radical anions, leading to negligible homocoupling and high cross-selectivity to form 1,4-alkyl cyanobenzenes. The cross-coupling scheme is compatible with oxidatively sensitive and acidic functional groups such as amines and alcohols, which have proven difficult to incorporate in alternative electrochemical approaches using carboxylic acids as C(sp3 ) precursors.

Oxidation Chemistry of Bicarbonate and Peroxybicarbonate: Implications for Carbonate Management in Energy Storage.

Carbonate formation presents a major challenge to energy storage applications based on low-temperature CO2 electrolysis and recyclable metal-air batteries. While direct electrochemical oxidation of (bi)carbonate represents a straightforward route for carbonate management, knowledge of the feasibility and mechanisms of direct oxidation is presently lacking. Herein, we report the isolation and characterization of the bis(triphenylphosphine)iminium salts of bicarbonate and peroxybicarbonate, thus enabling the examination of their oxidation chemistry. Infrared spectroelectrochemistry combined with time-resolved infrared spectroscopy reveals that the photoinduced oxidation of HCO3- by an Ir(III) photoreagent results in the generation of the short-lived bicarbonate radical in less than 50 ns. The highly acidic bicarbonate radical undergoes proton transfer with HCO3- to furnish the carbonate radical anion and H2CO3, leading to the eventual release of CO2 and H2O, thus accounting for the appearance of H2O and CO2 in both electrochemical and photochemical oxidation experiments. The back reaction of the carbonate radical subsequently oxidizes the Ir(II) photoreagent, leading to carbonate. In the absence of this back reaction, dimerization of the carbonate radical provides entry into peroxybicarbonate, which we show undergoes facile oxidation to O2 and CO2. Together, the results reported identify tangible pathways for the design of catalysts for the management of carbonate in energy storage applications.

Disulfide radical anion as a super-reductant in biology and photoredox chemistry.

Disulfides are involved in a broad range of radical-based synthetic organic and biochemical transformations. In particular, the reduction of a disulfide to the corresponding radical anion, followed by S-S bond cleavage to yield a thiyl radical and a thiolate anion plays critical roles in radical-based photoredox transformations and the disulfide radical anion in conjunction with a proton donor, mediates the enzymatic synthesis of deoxynucleotides from nucleotides within the active site of the enzyme, ribonucleotide reductase (RNR). To gain fundamental thermodynamic insight into these reactions, we have performed experimental measurements to furnish the transfer coefficient from which the standard E0(RSSR/RSSR˙-) reduction potential has been determined for a homologous series of disulfides. The electrochemical potentials are found to be strongly dependent on the structures and electronic properties of the substituents of the disulfides. In the case of cysteine, a standard potential of E0(RSSR/RSSR˙-) = -1.38 V vs. NHE is determined, making the disulfide radical anion of cysteine one of the most reducing cofactors in biology.

Generation of Pure Oxygen from Briny Water by Binary Catalysis.

Whereas the emphasis of water splitting is typically on hydrogen generation, there is value in the oxygen produced, especially in the undersea environment and for medicinal applications in the developing world. The generation of pure and breathable oxygen from abundant and accessible sources of water, such as brine and seawater, is challenging owing to the prevalence of the competing halide oxidation reaction to produce halogen and hypohalous acids. We show here that pure O2 may be generated from briny water by using an oxygen evolution catalyst with an overlayer that fulfills the criteria of (i) possessing a point of zero charge that results in halide anion rejection and (ii) promoting the disproportionation of hypohalous acids.

Radical Transport Facilitated by a Proton Transfer Network at the Subunit Interface of Ribonucleotide Reductase.

Ribonucleotide reductases (RNRs) play an essential role in the conversion of nucleotides to deoxynucleotides in all organisms. The Escherichia coli class Ia RNR requires two homodimeric subunits, α and ß. The active form is an asymmetric αα'ßß' complex. The α subunit houses the site for nucleotide reduction initiated by a thiyl radical (C439•), and the ß subunit houses the diferric-tyrosyl radical (Y122•) that is essential for C439• formation. The reactions require a highly regulated and reversible long-range proton-coupled electron transfer pathway involving Y122•[ß] ↔ W48?[ß] ↔ Y356[ß] ↔ Y731[α] ↔ Y730[α] ↔ C439[α]. In a recent cryo-EM structure, Y356[ß] was revealed for the first time and it, along with Y731[α], spans the asymmetric α/ß interface. An E52[ß] residue, which is essential for Y356 oxidation, allows access to the interface and resides at the head of a polar region comprising R331[α], E326[α], and E326[α'] residues. Mutagenesis studies with canonical and unnatural amino acid substitutions now suggest that these ionizable residues are important in enzyme activity. To gain further insights into the roles of these residues, Y356• was photochemically generated using a photosensitizer covalently attached adjacent to Y356[ß]. Mutagenesis studies, transient absorption spectroscopy, and photochemical assays monitoring deoxynucleotide formation collectively indicate that the E52[ß], R331[α], E326[α], and E326[α'] network plays the essential role of shuttling protons associated with Y356 oxidation from the interface to bulk solvent.

Ag(III)···Ag(III) Argentophilic Interaction in a Cofacial Corrole Dyad.

Metallophilic interactions between closed-shell metal centers are exemplified by d10 ions, with Au(I) aurophilic interactions as the archetype. Such an interaction extends to d8 species, and examples involving Au(III) are prevalent. Conversely, Ag(III) argentophilic interactions are uncommon. Here, we identify argentophilic interactions in silver corroles, which are authentic Ag(III) species. The crystal structure of a monomeric silver corrole is a dimer in the solid state, and the macrocycle exhibits an atypical domed conformation. In order to evaluate whether this represents an authentic metallophilic interaction or a crystal-packing artifact, the analogous cofacial or "pacman" corrole was prepared. The conformation of the monomer was recapitulated in the silver pacman corrole, exhibiting a short 3.67 Å distance between metal centers and a significant compression of the xanthene backbone. Theoretical calculations support the presence of a rare Ag(III)···Ag(III) argentophilic interaction in the pacman complex.

Solvent-Induced Spin-State Change in Copper Corroles.

The electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. The ground state of these compounds is best described as an antiferromagnetically coupled Cu(II) corrole radical cation. In coordinating solvents, these molecules become paramagnetic, and this is often accompanied by a color change. The underlying chemistry of these solvent-induced properties is currently unknown. Here, we show that a coordinating solvent, such as pyridine, induces a change in the ground spin state from an antiferromagnetically coupled Cu(II) corrole radical cation to a ferromagnetically coupled triplet. Over time, the triplet reacts to produce a species with spectral signatures that are characteristic of the one-electron-reduced Cu(II) corrole. These observations account for the solvent-induced paramagnetism and the associated color changes that have been observed for copper corroles in coordinating solvents.

Riboflavin synthesis from gaseous nitrogen and carbon dioxide by a hybrid inorganic-biological system.

Microbes can provide a more sustainable and energy-efficient method of food and nutrient production compared to plant and animal sources, but energy-intensive carbon (e.g., sugars) and nitrogen (e.g., ammonia) inputs are required. Gas-fixing microorganisms that can grow on H2 from renewable water splitting and gaseous CO2 and N2 offer a renewable path to overcoming these limitations but confront challenges owing to the scarcity of genetic engineering in such organisms. Here, we demonstrate that the hydrogen-oxidizing carbon- and nitrogen-fixing microorganism Xanthobacter autotrophicus grown on a CO2/N2/H2 gas mixture can overproduce the vitamin riboflavin (vitamin B2). We identify plasmids and promoters for use in this bacterium and employ a constitutive promoter to overexpress riboflavin pathway enzymes. Riboflavin production is quantified at 15 times that of the wild-type organism. We demonstrate that riboflavin overproduction is maintained when the bacterium is grown under hybrid inorganic-biological conditions, in which H2 from water splitting, along with CO2 and N2, is fed to the bacterium, establishing the viability of the approach to sustainably produce food and nutrients.

Syntheses and Properties of Metalated Tetradehydrocorrins.

The monoanionic tetrapyrrolic macrocycle B,C-tetradehydrocorrin (TDC) resides chemically between corroles and corrins. This chemical space remains largely unexplored due to a lack of reliable synthetic strategies. We now report the preparation and characterization of Co(II)- and Ni(II)-metalated TDC derivatives ([Co-TDC]+ and [Ni-TDC]+, respectively) with a combination of crystallographic, electrochemical, computational, and spectroscopic techniques. [Ni-TDC]+ was found to undergo primarily ligand-centered electrochemical reduction, leading to hydrogenation of the macrocycle under cathodic electrolysis in the presence of acid. Transient absorption (TA) spectroscopy reveals that [Ni-TDC]+ and the two-electron-reduced [Ni-TDC]- possess long-lived excited states, whereas the excited state of singly reduced [Ni-TDC] exhibits picosecond dynamics. The Co(I) compound [Co-TDC] is air stable, highlighting the notable property of the TDC ligand to stabilize low-valent metal centers in contradistinction to other tetrapyrroles such as corroles, which typically stabilize metals in higher oxidation states.

Probing the Halide Effect in the δ-Bond with One- and Two-Photon Spectroscopy.

Two electrons in two orbitals give rise to four states. When the orbitals are weakly coupled as in the case for the dxy orbitals of quadruple bond species, two of the states are diradical in character with electrons residing in separate orbitals and two of the states are zwitterionic with electrons paired in one orbital or the other. By measuring one-and two-photon spectra, the one-electron (ΔW) and two-electron (K) energies may be calculated, which are the determinants of the state energies of the four-state model for the two-electron bond. The K energy is thus especially sensitive to the size of the orbital as K is dependent on the distance between electrons. To this end, one- and two-photon spectra of Mo2X4(PMe3)4 are sensitive to secondary bonding interactions of the δ-orbital manifold with the halide orbitals, as reflected in decreasing K energies along the series Cl > Br > I. Additionally, the calculated one-electron energies have been verified with the spectroelectrochemical preparation of the Mo2X4(PMe3)4+ complexes, where the δ bond is a one-electron bond, and K is thus absent. The δ → δ* transition shifts over 10,000 cm-1 upon oxidation of Mo2X4(PMe3)4 to Mo2X4(PMe3)4+, establishing that transitions within the two-electron δ bond are heavily governed by the two-electron exchange energy.

Proton-coupled electron transfer of macrocyclic ring hydrogenation: The chlorinphlorin.

SignificanceThe chemical reduction of unsaturated bonds occurs by hydrogenation with H2 as the reductant. Conversely, in biology, the unavailability of H2 engenders the typical reduction of unsaturated bonds with electrons and protons from different cofactors, requiring olefin hydrogenation to occur by proton-coupled electron transfer (PCET). Moreover, the redox noninnocence of tetrapyrrole macrocycles furnishes unusual PCET intermediates, including the phlorin, which is an intermediate in tetrapyrrole ring reductions. Whereas the phlorin of a porphyrin is well established, the phlorin of a chlorin is enigmatic. By controlling the PCET reactivity of a chlorin, including the use of a hangman functionality to manage the proton transfer, the formation of a chlorinphlorin by PCET is realized, and the mechanism for its formation is defined.

Self-healing oxygen evolution catalysts.

Electrochemical and photoelectrochemical water splitting offers a scalable approach to producing hydrogen from renewable sources for sustainable energy storage. Depending on the applications, oxygen evolution catalysts (OECs) may perform water splitting under a variety of conditions. However, low stability and/or activity present challenges to the design of OECs, prompting the design of self-healing OECs composed of earth-abundant first-row transition metal oxides. The concept of self-healing catalysis offers a new tool to be employed in the design of stable and functionally active OECs under operating conditions ranging from acidic to basic solutions and from a variety of water sources.

Synthesis, Characterization, and Hydrogen Evolution Activity of Metallo-meso-(4-fluoro-2,6-dimethylphenyl)porphyrin Derivatives.

Zn(II), Cu(II), and Ni(II) 5,10,15,20-tetrakis(4-fluoro-2,6-dimethylphenyl)porphyrins (TFPs) have been synthesized and characterized. The electronic spectroscopy and cyclic voltammetry of these compounds, along with the free-base macrocycle (2H-TFP), have been determined; 2H-TFP was also structurally characterized by X-ray crystallography. The Cu(II)TFP exhibits catalytic activity for the hydrogen evolution reaction (HER). The analysis of linear sweep voltammograms shows that the HER reaction of Cu(II)TFP with benzoic acid is first-order in proton concentration with an average apparent rate constant for HER catalysis of k app = 5.79 ± 0.47 × 103 M-1 s-1.

Proton-Coupled Electron Transfer: The Engine of Energy Conversion and Storage.

Proton-coupled electron transfer (PCET) underpins energy conversion in chemistry and biology. Four energy systems are described whose discoveries are based on PCET: the water splitting chemistry of the Artificial Leaf, the carbon fixation chemistry of the Bionic Leaf-C, the nitrogen fixation chemistry of the Bionic Leaf-N and the Coordination Chemistry Flow Battery (CCFB). Whereas the Artificial Leaf, Bionic Leaf-C, and Bionic Leaf-N require strong coupling between electron and proton to reduce energetic barriers to enable high energy efficiencies, the CCFB requires complete decoupling of the electron and proton so as to avoid parasitic energy-wasting reactions. The proper design of PCET in these systems facilitates their implementation in the areas of (i) centralized large scale grid storage of electricity and (ii) decentralized energy storage/conversion using only sunlight, air and any water source to produce fuel and food within a sustainable cycle for the biogenic elements of C, N and P.

Taming the Chlorine Radical: Enforcing Steric Control over Chlorine-Radical-Mediated C-H Activation.

Chlorine radicals readily activate C-H bonds, but the high reactivity of these intermediates precludes their use in regioselective C-H functionalization reactions. We demonstrate that the secondary coordination sphere of a metal complex can confine photoeliminated chlorine radicals and afford steric control over their reactivity. Specifically, a series of iron(III) chloride pyridinediimine complexes exhibit activity for photochemical C(sp3)-H chlorination and bromination with selectivity for primary and secondary C-H bonds, overriding thermodynamic preference for weaker tertiary C-H bonds. Transient absorption spectroscopy reveals that Cl· remains confined through formation of a Cl·|arene complex with aromatic groups on the pyridinediimine ligand. Furthermore, photocrystallography confirms that this selectivity arises from the generation of Cl· within the steric environment defined by the iron secondary coordination sphere.

Polypyrrole-Silicon Nanowire Arrays for Controlled Intracellular Cargo Delivery.

Intracellular cargo delivery is a critical and challenging step in controlling cell states. Silicon nanowire (NW) arrays have emerged as a powerful platform for accessing the intracellular space through a combination of their nanoscale dimensions and electrical properties. Here, we develop and characterize a conductive polypyrrole (PPy)-NW device for temporally controlled intracellular delivery. Fluorescent cargos, doped in electroresponsive PPy matrices at wire tips as well as entire NW arrays, are released with an applied reducing potential. Intracellular delivery into endothelial cells from PPy-Si substrates demonstrated comparable kinetics to solution-based delivery methods while requiring an order of magnitude less cargo loading. This hybrid polymer-semiconductor platform extends methods available for intracellular delivery and links electrical signaling from artificial systems with living molecular transduction.