The radical S-adenosylmethionine (SAM) protein PqqE is predicted to function in the pyrroloquinoline quinone (PQQ) biosynthetic pathway via catalysis of carbon-carbon bond formation between a glutamate and tyrosine side chain within the small peptide substrate PqqA. We report here that PqqE activity is dependent on the accessory protein PqqD, which was recently shown to bind PqqA tightly. In addition, PqqE activity in vitro requires the presence of a flavodoxin-and flavodoxin reductasebased reduction system, with other reductants leading to an uncoupled cleavage of the co-substrate SAM. These results indicate that PqqE, in conjunction with PqqD, carries out the first step in PQQ biosynthesis: a radical-mediated formation of a new carbon-carbon bond between two amino acid side chains on PqqA.Pyrroloquinoline quinone (PQQ) 2 is employed by a wide variety of bacteria, where it functions as a redox cofactor in substrate oxidation via an alternate (non-glycolytic) pathway for production of cellular ATP (1). Hundreds of bacterial species are thought to produce PQQ, based on the presence in their genomes of the strongly conserved pqq operon (2) (Fig. 1A). Although the existence of this important redox cofactor has been known for decades, knowledge of the biosynthetic pathway for PQQ has remained scant (3, 4). The PQQ molecule derives from the evolutionarily conserved glutamate and tyrosine side chains within a ribosomally produced peptide substrate PqqA (Fig. 1, B and C). To produce PQQ, a large number of chemical modifications are required: (i) the formation of a new carbon-carbon bond between the ␥-carbon of glutamate and the 3-position of tyrosine (labeled in green in Fig. 1C); (ii) the addition of two additional hydroxyl groups to the tyrosine ring; (iii) a condensation between the 4-OH of tyrosine and the backbone amine of glutamate to produce a heterocyclic ring; and (iv) an 8-electron oxidation catalyzed by PqqC, a cofactorless enzyme that converts 3a-(2-amino-2-carboxyethyl)-4,5-dioxo-4,5,6,7,8,9-hexahydroquinoline-7,9-dicarboxylic acid (AHQQ) to PQQ in the final step of the pathway (5-7).The radical SAM enzyme PqqE has been the primary candidate as the catalyst for the formation of the new carbon-carbon bond between glutamate and tyrosine. Radical SAM proteins use the reductive cleavage of S-adenosyl methionine to initiate free radical chemistry, and can accomplish a wide variety of reactions, including carbon-carbon bond formation (8, 9). PqqE is a founding member of the SPASM domain-containing radical SAM proteins, which contain one or more auxiliary clusters in their C-terminal regions, and of which several are known to modify small peptides or proteins (10, 11). Recently, a small (ϳ10 kDa) protein, PqqD, has been shown to form a strong, sub-micromolar K D complex with the peptide substrate PqqA. This complex was further demonstrated to associate with PqqE (12). These findings suggested that PqqD would serve as a chaperone to deliver PqqA to PqqE, functioning as a necessary and heretofore missing componen...

CONSPECTUS: Climate change, rising global energy demand, and energy security concerns motivate research into alternative, sustainable energy sources. In principle, solar energy can meet the world's energy needs, but the intermittent nature of solar illumination means that it is temporally and spatially separated from its consumption. Developing systems that promote solar-to-fuel conversion, such as via reduction of protons to hydrogen, could bridge this production−consumption gap, but this effort requires invention of catalysts that are cheap, robust, and efficient and that use earth-abundant elements. In this context, catalysts that utilize water as both an earthabundant, environmentally benign substrate and a solvent for proton reduction are highly desirable. This Account summarizes our studies of molecular metal−polypyridyl catalysts for electrochemical and photochemical reduction of protons to hydrogen. Inspired by concept transfer from biological and materials catalysts, these scaffolds are remarkably resistant to decomposition in water, with fast and selective electrocatalytic and photocatalytic conversions that are sustainable for several days. Their modular nature offers a broad range of opportunities for tuning reactivity by molecular design, including altering ancillary ligand electronics, denticity, and/or incorporating redox-active elements. Our first-generation complex, [(PY4)Co(CH 3 CN) 2 ] 2+, catalyzes the reduction of protons from a strong organic acid to hydrogen in 50% water. Subsequent investigations with the pentapyridyl ligand PY5Me 2 furnished molybdenum and cobalt complexes capable of catalyzing the reduction of water in fully aqueous electrolyte with 100% Faradaic efficiency. Of particular note, the complex [(PY5Me 2 )MoO] 2+ possesses extremely high activity and durability in neutral water, with turnover frequencies at least 8500 mol of H 2 per mole of catalyst per hour and turnover numbers over 600 000 mol of H 2 per mole of catalyst over 3 days at an overpotential of 1.0 V, without apparent loss in activity. Replacing the oxo moiety with a disulfide affords [(PY5Me 2 )MoS 2 ] 2+ , which bears a molecular MoS 2 triangle that structurally and functionally mimics bulk molybdenum disulfide, improving the catalytic activity for water reduction. In water buffered to pH 3, catalysis by [(PY5Me 2 )MoS 2 ] 2+ onsets at 400 mV of overpotential, whereas [(PY5Me 2 )MoO] 2+ requires an additional 300 mV of driving force to operate at the same current density. Metalation of the PY5Me 2 ligand with an appropriate Co(II) source also furnishes electrocatalysts that are active in water. Importantly, the onset of catalysis by the [(PY5Me 2 )Co(H 2 O)] 2+ series is anodically shifted by introducing electron-withdrawing functional groups on the ligand. With the [(bpy2PYMe)Co(CF 3 SO 3 )] 1+ system, we showed that introducing a redox-active moiety can facilitate the electro-and photochemical reduction of protons from weak acids such as acetic acid or water. Using a high-throughput photochemical reactor, we...

3M catalyzes aerobic cyclohexene oxidation at mild potentials.

Mn N Schiff-base complexes incorporating a Na (1Na), K (1K), Ba (1Ba), or Sr (1Sr) ion in the ligand framework are reported. The Mn reduction potentials for 1Na, 1K, 1Ba, and 1Sr are 0.591, 0.616, 0.805, and 0.880 V vs. Fe(C H ) , respectively, exhibiting significant positive shifts compared to a MnN Schiff-base complex in the same primary coordination environment but with no associated alkali or alkaline earth metal ion (A, E =0.427 V vs. Fe(C H ) ). One-electron oxidation of the Mn N complexes results in bimolecular coupling to form N with rates (k ) at 20 °C of 2166, 684, 857, and 99.7, an 87 m s for A, 1Na, 1K, 1Ba, and 1Sr respectively, following an inverse linear free energy relationship. Thus, increasing charge through proximal cations results in Mn N complexes that are both more oxidizing and more stable to bimolecular coupling, a trend diametrically opposed to when complexes were modified by ligand substituents through inductive effects.

Iron is an essential metal for living organisms, but misregulation of its homeostasis at the cellular level can trigger detrimental oxidative and/or nitrosative stress and damage events. Motivated to help study the physiological and pathological consequences of biological iron regulation, we now report a reaction-based strategy for monitoring labile Fe2+ pools in aqueous solution and in living cells. Iron Probe 1 (IP1) exploits a bioinspired, iron-mediated oxidative C–O bond cleavage reaction to achieve a selective turn-on response to Fe2+ over a range of cellular metal ions in their bioavailable forms. We show that this first-generation chemical tool for fluorescence Fe2+ detection can visualize changes in exchangeable iron stores in living cells upon iron supplementation or depletion, including labile iron pools at endogenous, basal levels. Moreover, IP1 can be used to identify reversible expansion of labile iron pools by stimulation with vitamin C or the iron regulatory hormone hepcidin, providing a starting point for further investigations of iron signaling and stress events in living systems as well as future probe development.

Electric fields underlie all reactions and impact reactivity by interacting with the dipoles and net charges of transition states, products, and reactants to modify the free energy landscape. However, they are rarely given deliberate consideration in synthetic design to rationally control reactivity. This Perspective discusses the commonalities of electric field effects across multiple platforms, from enzymes to molecular catalysts, and identifies practical challenges to applying them in synthetic molecular systems to mediate reactivity.

The synthesis of the first heteroleptic, two-coordinate Fe(I) complex IPr-Fe-N(SiMe3)DIPP (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; DIPP = 2,6-(i)Pr2-C6H3) is reported. Protonation of the Fe(II) bis(amido) complex Fe[N(SiMe3)DIPP]2 followed by addition of IPr and reduction by potassium graphite in a one-pot reaction results in good yields of 1. The redox activity of 1 and comparison between 1 and its reduction product by (57)Fe Mössbauer spectroscopy are discussed, and the reduction was found to be metal-based rather than ligand-based. The activity of 1 toward the catalytic cyclotrimerization of terminal and internal alkynes is described.

We report the photochemical generation and study of a family of water-soluble iron­(IV)-oxo complexes supported by pentapyridine PY5Me2-X ligands (PY5Me2 = 2,6-bis­(1,1-bis­(2-pyridyl)­ethyl)­pyridine; X = CF3, H, Me, or NMe2), in which the oxidative reactivity of these ferryl species correlates with the electronic properties of the axial pyridine ligand. Synthesis of a systematic series of [FeII(L)­(PY5Me2-X)]2+ complexes, where L = CH3CN or H2O, and characterizations by several methods, including X-ray crystallography, cyclic voltammetry, and Mössbauer spectroscopy, show that increasing the electron-donating ability of the axial pyridine ligand tracks with less positive Fe­(III)/Fe­(II) reduction potentials and quadrupole splitting parameters. The FeII precursors are readily oxidized to their Fe­(IV)-oxo counterparts using either chemical outer-sphere oxidants such as CAN (ceric ammonium nitrate) or flash-quench photochemical oxidation with [Ru­(bpy)3]2+ as a photosensitizer and K2S2O8 as a quencher. The Fe­(IV)-oxo complexes are capable of oxidizing the C–H bonds of alkane (4-ethylbenzene­sulfonate) and alcohol (benzyl alcohol) substrates via hydrogen atom transfer (HAT) and an olefin (4-styrene­sulfonate) substrate by oxygen atom transfer (OAT). The [FeIV(O)­(PY5Me2-X)]2+ derivatives with electron-poor axial ligands show faster rates of HAT and OAT compared to their counterparts supported by electron-rich axial donors, but the magnitudes of these differences are relatively modest.