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Photocatalysis provides an eco-friendly route for the hydrogenation of aromatic carbonyls to O-free aromatics, which is an important refining process in the chemical industry that is generally carried out under high pressure of hydrogen at elevated temperatures. However, aromatic carbonyls are often only partially hydrogenated to alcohols, which readily desorbs and are hardly further deoxygenated under ambient conditions. Here, we show that by constructing an oxide surface over the Pd cocatalyst supported on graphitic carbon nitride, an alternative hydrogenation path of aromatic carbonyls becomes available via a step-wise acetalization and hydrogenation, thus allowing efficient and selective production of O-free aromatics. The PdO surface allows for optimum adsorption of reactants and intermediates and rapid abstraction of hydrogen from the alcohol donor, favoring fast acetalization of the carbonyls and their consecutive hydrogenation to O-free hydrocarbons. The photocatalytic hydrogenation of benzaldehyde into toluene shows a high selectivity of >90% and a quantum efficiency of ∼10.2% under 410 nm irradiation. By adding trace amounts of HCl to the reaction solution, the PdO surface remains stable and active for long-term operation at high concentrations, offering perspective for practical applications.

Mo-dependent nitrogenase is a major contributor to global biological N(2) reduction, which sustains life on Earth. Its multi-metallic active-site FeMo-cofactor (Fe(7)MoS(9)C-homocitrate) contains a carbide (C(4−)) centered within a trigonal prismatic CFe(6) core resembling the structural motif of the iron carbide, cementite. The role of the carbide in FeMo-cofactor binding and activation of substrates and inhibitors is unknown. To explore this role, the carbide has been in effect selectively enriched with (13)C, which enables its detailed examination by ENDOR/ESEEM spectroscopies. (13)C-carbide ENDOR of the S = 3/2 resting state (E(0)) is remarkable, with an extremely small isotropic hyperfine coupling constant, (C)a = +0.86 MHz. Turnover under high CO partial pressure generates the S = 1/2 hi-CO state, with two CO molecules bound to FeMo-cofactor. This conversion surprisingly leaves the small magnitude of the (13)C carbide isotropic hyperfine-coupling constant essentially unchanged, (C)a = −1.30 MHz. This indicates that both the E(0) and hi-CO states exhibit an exchange-coupling scheme with nearly cancelling contributions to (C)a from three spin-up and three spin-down carbide-bound Fe ions. In contrast, the anisotropic hyperfine coupling constant undergoes a symmetry change upon conversion of E(0) to hi-CO that may be associated with bonding and coordination changes at Fe ions. In combination with the negligible difference between CFe(6) core structures of E(0) and hi-CO, these results suggest that in CO-inhibited hi-CO the dominant role of the FeMo-cofactor carbide is to maintain the core structure, rather than to facilitate inhibitor binding through changes in Fe-carbide covalency or stretching/breaking of carbide–Fe bonds.

Nitrogen fixation by nitrogenase begins with the accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co), generating a state (denoted E4(4H)) with two [Fe-H-Fe] bridging hydrides. Recently, photolytic reductive elimination (re) of the E4(4H) hydrides showed that enzymatic re of E4(4H) hydride yields an H2-bound complex (E4(H2,2H)), in a process corresponding to a formal 2-electron reduction of the metal-ion core of FeMo-co. The resulting electron-density redistribution from Fe-H bonds to the metal ions themselves enables N2 to bind with concomitant H2 release, a process illuminated here by QM/MM molecular dynamics simulations. What is the nature of this redistribution? Although E4(H2,2H) has not been trapped, cryogenic photolysis of E4(4H) provides a means to address this question. Photolysis of E4(4H) causes hydride-re with release of H2, generating doubly reduced FeMo-co (denoted E4(2H)*), the extreme limit of the electron-density redistribution upon formation of E4(H2,2H). Here we examine the doubly reduced FeMo-co core of the E4(2H)* limiting-state by 1H, 57Fe, and 95Mo ENDOR to illuminate the partial electron-density redistribution upon E4(H2,2H) formation during catalysis, complementing these results with corresponding DFT computations. Inferences from the E4(2H)* ENDOR results as extended by DFT computations include (i) the Mo-site participates negligibly, and overall it is unlikely that Mo changes valency throughout the catalytic cycle; and (ii) two distinctive E4(4H) 57Fe signals are suggested as associated with structurally identified "anchors" of one bridging hydride, two others with identified anchors of the second, with NBO-analysis further identifying one anchor of each hydride as a major recipient of electrons released upon breaking Fe-H bonds.

Constructing single-crystal inorganic helical structures is a fascinating subject for a large variety of research fields. However, the driving force of self-coiling, particularly in helical architectures, still remains a major challenge. Here, using MoO3-x sub-nanometric wires (SNWs) as an example, we identified that spontaneous helical architecture with different dimensional features is closely related with their surface asymmetrical defects. Specifically, the surface defects of SNWs are critical to produce the self-coiling process, thereby achieving the ordered helical conformations. Theoretical calculations further suggest that the formation of in-plane and out-of-plane coiling structures is determined by the asymmetrical distribution of the surface defects, and the inhomogeneous charge separation with strong Coulomb attraction dominates the different structural configurations. The resulting MoO3-x SNW exhibits excellent photothermal behaviors in both aqueous solutions and hydrogel matrixes. Our study provides a novel protocol to achieve helical structure design for their future applications.

Investigation of chirality in on-surface synthesis is of significance not only for fabricating atomically precise covalently bonded chiral species but also for unveiling chiral phenomena involving chemical reactions. In this contribution, we present the growth of single-layered homochiral 2D covalent organic frameworks (COFs) on surfaces based on a steric hindrance strategy, by which both the chiral expression of the prochiral precursor and the newly formed C═N bonds are successfully steered. When coupling a tritopic monomer with the prochiral ditopic molecule with phenyl substituents, two enantiomers of the precursor are randomly integrated in the product via variable C═N linkages, resulting in distorted hexagonal frameworks without chiral expression. After equipping the prochiral precursor with more hindered bulky substituents, highly regular homochiral 2D COFs are fabricated, in which only one of the enantiomers of the prochiral precursor is integrated, and all C═N linkages possess the same configuration. Structural analysis based on high resolution scanning tunneling microscopy images and theoretical simulations indicate that the homochiral 2D COFs are generated through an enantioselective on-surface polymerization driven by the steric hindrance effect. This result not only benefits understanding and controlling chirality in on-surface synthesis but also provides a new approach for the growth of highly regular COFs on surfaces.

Nitrogenase enzymes are the only biological catalysts able to convert N2 to NH3. Molybdenum-dependent nitrogenase consists of two proteins and three metallocofactors that sequentially shuttle eight electrons between three distinct metallocofactors during the turnover of one molecule of N2. While the kinetics of isolated nitrogenase has been extensively studied, little is known about the thermodynamics of its cofactors under catalytically relevant conditions. Here, we employ a recently described pyrene-modified linear poly(ethylenimine) hydrogel to immobilize the catalytic protein of nitrogenase onto an electrode surface. The resulting electroenzymatic interface enabled direct measurement of reduction potentials associated with each metallocofactor of the nitrogenase complex, illuminating the role of nitrogenase reductase in altering the potential landscape in the active site of nitrogenase and revealing the endergonic nature of electron-transfer steps associated with the conversion of N2 to NH3 under physiological conditions.

Iron–nitrogen on carbon (Fe–N/C) catalysts have emerged as promising nonprecious metal catalysts (NPMCs) for oxygen reduction reaction (ORR) in energy conversion and storage devices. It has been widely suggested that an active site structure for Fe–N/C catalysts contains Fe–Nx coordination. However, the preparation of high-performance Fe–N/C catalysts mostly involves a high-temperature pyrolysis step, which generates not only catalytically active Fe–Nx sites, but also less active large iron-based particles. Herein, we report a general “silica-protective-layer-assisted” approach that can preferentially generate the catalytically active Fe–Nx sites in Fe–N/C catalysts while suppressing the formation of large Fe-based particles. The catalyst preparation consisted of an adsorption of iron porphyrin precursor on carbon nanotube (CNT), silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. Temperature-co...

We have proposed a reductive elimination/oxidative addition (re/oa) mechanism for reduction of N2 to 2NH3 by nitrogenase, based on identification of a freeze-trapped intermediate of the α-70Val→Ile substituted MoFe protein as the Janus intermediate that stores four reducing equivalents on FeMo-co as two [Fe-H-Fe] bridging hydrides (denoted E4(4H)). The mechanism postulates that obligatory re of the hydrides as H2 drives reduction of N2 to a state (denoted E4(2N2H)) with a moiety at the diazene (HN=NH) reduction level bound to the catalytic FeMo-cofactor. In the present work, EPR/ENDOR and photophysical measurements show that a state freeze-trapped during N2 reduction by wild type (WT) MoFe protein is the same Janus intermediate, thereby establishing the α-70Val→Ile intermediate as a reliable guide to mechanism, and enabling new experimental tests of the re/oa mechanism with WT enzyme. These allow us to show that the re/oa mechanism accounts for the longstanding Key Constraints on mechanism. Monitoring the S = ½ FeMo-co EPR signal of Janus in WT MoFe during N2 reduction under mixed-isotope condition, H2O buffer/D2, and the converse, establishes that the bridging hydrides/deuterides do not exchange with solvent during enzymatic turnover, thereby explaining earlier observations and verifying the re/oa mechanism. Relaxation of E4(2N2H) to the WT resting-state is shown to occur via oa of H2 and release of N2 to form Janus, followed by sequential release of two H2, demonstrating the kinetic reversibility of the re/oa equilibrium. The relative populations of E4(2N2H) and E4(4H) freeze-trapped during WT turnover furthermore show that the rapidly reversible re/oa equilibrium between [E4(4H) + N2] and [E4(2N2H) + H2] is roughly thermoneutral (ΔreG0 ~ −2 kcal/mol), whereas hydrogenation of gas-phase N2 would be highly endergonic. These findings establish (i) that re/oa satisfies all key constraints on mechanism, (ii) that Janus is the key to N2 reduction by WT enzyme, which (iii) indeed occurs via the re/oa mechanism. Thus emerges a picture of the central mechanistic steps by which the nitrogenase MoFe protein carries out one of the most challenging chemical transformation in biology, the reduction of the N≡N triple bond.

We recently demonstrated that N2 reduction by nitrogenase involves the obligatory release of one H2 per N2 reduced. These studies focused on the E4(4H) ‘Janus intermediate’, which has accumulated four reducing equivalents as two [Fe-H-Fe] bridging hydrides. E4(4H) is poised to bind and reduce N2 through reductive elimination (re) of the two hydrides as H2, coupled to the binding/reduction of N2. To obtain atomic-level details of the re activation process, we carried out in situ 450 nm photolysis of E4(4H) in an EPR cavity at temperatures below 20 K. ENDOR and EPR measurements show that photolysis generates a new FeMo-co state, denoted E4(2H)*, through the photoinduced re of the two bridging hydrides of E4(4H) as H2. During cryoannealing at temperatures above 175 K, E4(2H)* reverts to E4(4H) through the oxidative addition (oa) of the H2. The photolysis quantum yield is temperature invariant at liquid helium temperatures and shows a rather large kinetic isotope effect, KIE = 10. These observations imply that photoinduced release of H2 involves a barrier to the combination of the two nascent H atoms, in contrast to a barrierless process for mono-metallic inorganic complexes, and further suggest that H2 formation involves nuclear tunneling through that barrier. The oa recombination of E4(2H)* with the liberated H2 offers compelling evidence for the Janus intermediate as the point at which H2 is necessarily lost during N2 reduction; this mechanistically coupled loss must be gated by N2 addition that drives the re/oa equilibrium toward reductive elimination of H2 with N2 binding/reduction.

Freeze-quenching nitrogenase during turnover with N2 traps an S = ½ intermediate that was shown by ENDOR and EPR spectroscopy to contain N2 or a reduction product bound to the active-site molybdenum-iron cofactor (FeMo-co). To identify this intermediate (termed here EG), we turned to a quench-cryoannealing relaxation protocol. The trapped state is allowed to relax to the resting E0 state in frozen medium at a temperature below the melting temperature; relaxation is monitored by periodically cooling the sample to cryogenic temperature for EPR analysis. During -50 °C cryoannealing of EG prepared under turnover conditions in which the concentrations of N2 and H2 ([H2], [N2]) are systematically and independently varied, the rate of decay of EG is accelerated by increasing [H2] and slowed by increasing [N2] in the frozen reaction mixture; correspondingly, the accumulation of EG is greater with low [H2] and/or high [N2]. The influence of these diatomics identifies EG as the key catalytic intermediate formed by reductive elimination of H2 with concomitant N2 binding, a state in which FeMo-co binds the components of diazene (an N-N moiety, perhaps N2 and two [e(-)/H(+)] or diazene itself). This identification combines with an earlier study to demonstrate that nitrogenase is activated for N2 binding and reduction through the thermodynamically and kinetically reversible reductive-elimination/oxidative-addition exchange of N2 and H2, with an implied limiting stoichiometry of eight electrons/protons for the reduction of N2 to two NH3.

Iron-nitrogen on carbon (Fe-N/C) catalysts have emerged as promising nonprecious metal catalysts (NPMCs) for oxygen reduction reaction (ORR) in energy conversion and storage devices. It has been widely suggested that an active site structure for Fe-N/C catalysts contains Fe-N

Enzymatic N(2) reduction proceeds along a reaction pathway composed of a sequence of intermediate states generated as a dinitrogen bound to the active-site iron-molybdenum cofactor (FeMo-co) of the nitrogenase MoFe protein undergoes six steps of hydrogenation (e(-)/H(+) delivery). There are two competing proposals for the reaction pathway, and they invoke different intermediates. In the 'Distal' (D) pathway, a single N of N(2) is hydrogenated in three steps until the first NH(3) is liberated, and then the remaining nitrido-N is hydrogenated three more times to yield the second NH(3). In the 'Alternating' (A) pathway, the two N's instead are hydrogenated alternately, with a hydrazine-bound intermediate formed after four steps of hydrogenation and the first NH(3) liberated only during the fifth step. A recent combination of X/Q-band EPR and (15)N, (1,2)H ENDOR measurements suggested that states trapped during turnover of the α-70(Ala)/α-195(Gln) MoFe protein with diazene or hydrazine as substrate correspond to a common intermediate (here denoted I) in which FeMo-co binds a substrate-derived [N(x)H(y)] moiety, and measurements reported here show that turnover with methyldiazene generates the same intermediate. In the present report we describe X/Q-band EPR and (14/15)N, (1,2)H ENDOR/HYSCORE/ESEEM measurements that characterize the N-atom(s) and proton(s) associated with this moiety. The experiments establish that turnover with N(2)H(2), CH(3)N(2)H, and N(2)H(4) in fact generates a common intermediate, I, and show that the N-N bond of substrate has been cleaved in I. Analysis of this finding leads us to conclude that nitrogenase reduces N(2)H(2), CH(3)N(2)H, and N(2)H(4) via a common A reaction pathway, and that the same is true for N(2) itself, with Fe ion(s) providing the site of reaction.

Double-stranded DNA cleavage of light-activated lysine conjugates is strongly enhanced at the slightly acidic pH (7) suitable for selective targeting of cancer cells. This enhancement stems from the presence of two amino groups of different basicities. The first amino group plays an auxiliary role by enhancing solubility and affinity to DNA, whereas the second amino group, which is positioned next to the light-activated DNA cleaver, undergoes protonation at the desired pH threshold. This protonation results in two synergetic effects which account for the increased DNA-cleaving ability at the lower pH. First, lysine conjugates show tighter binding to DNA at the lower pH, which is consistent with the anticipated higher degree of interaction between two positively charged ammonium groups with the negatively charged phosphate backbone of DNA. Second, the unproductive pathway which quenches the excited state of the photocleaver through intramolecular electron transfer is eliminated once the donor amino group next to the chromophore is protonated. Experiments in the presence of traps for diffusing radicals show that reactive oxygen species do not contribute significantly to the mechanism of DNA cleavage at the lower pH, which is indicative of tighter binding to DNA under these conditions. This feature is valuable not only because many solid tumors are hypoxic but also because cleavage which does not depend on diffusing species is more localized and efficient. Sequence-selectivity experiments suggest combination of PET and base alkylation as the chemical basis for the observed DNA damage. The utility of these molecules for phototherapy of cancer is confirmed by the drastic increase in toxicity of five conjugates against cancer cell lines upon photoactivation.

One major class of disease-causing RNAs is expanded repeating transcripts. These RNAs cause diseases via multiple mechanisms, including: (i) gain-of-function, in which repeating RNAs bind and sequester proteins involved in RNA biogenesis and (ii) repeat associated non-ATG (RAN) translation, in which repeating transcripts are translated into toxic proteins without use of a canonical, AUG, start codon. Herein, we develop and study chemical probes that bind and react with an expanded r(CGG) repeat (r(CGG)(exp)) present in a 5' untranslated region that causes fragile X-associated tremor/ataxia syndrome (FXTAS). Reactive compounds bind to r(CGG)(exp) in cellulo as shown with Chem-CLIP-Map, an approach to map small molecule binding sites within RNAs in cells. Compounds also potently improve FXTAS-associated pre-mRNA splicing and RAN translational defects, while not affecting translation of the downstream open reading frame. In contrast, oligonucleotides affect both RAN and canonical translation when they bind to r(CGG)(exp), which is mechanistically traced to a decrease in polysome loading. Thus, designer small molecules that react with RNA targets can be used to profile the RNAs to which they bind in cells, including identification of binding sites, and can modulate several aspects of RNA-mediated disease pathology in a manner that may be more beneficial than oligonucleotides.

Calculations utilizing anionic substituted derivates of the cationic N(wedge)N--Ni(II) and Pd(II) diimine Brookhart complex have been carried out on the barriers of ethylene and acrylonitrile insertion into a M- methyl, propyl and CH(CN)Et bond for M = Ni, Pd. The possibility of side reactions such as chelate formation with the polar functionality and oligomerization of the active species after acrylonitrile insertion are explored. The diimine ring system N--N = -NR' 'CR(1)CR(2)NR' ' with R' ' = 2,6-C(6)H(3)(i-Pr)(2) and R(1),R(2) = Me was functionalized by adding one or two anionic groups (BF(3)(-), etc.) in place of i-Pr on the aryl rings or by replacing one Me diimine backbone group (R(1)) with BH(3)(-). The objective of this investigation is computationally to design catalysts for ethylene/acrylonitrile copolymerization that have activities that are comparable to that of the cationic Ni(II) diimine or at least the Pd(II) diimine Brookhart system for ethylene homopolymerization. Complexes that might meet this objective are discussed.

By using the in situ thermal analysis-mass spectroscopic technique, combined with transmission electron microscopic characterization of the carbon nanotube (CNT) product, we have studied the chemical vapor deposition (CVD) growth of CNTs with Fe-Co/gamma-Al2O3 catalyst and benzene precursor in the range of room temperature to 700 degrees C. The growth process has been clearly illuminated, which starts from the reduction of catalyst around 645 degrees C followed by the dissociation of carbon-hydrogen bonds of benzene and the sequential growth of CNTs. A surprising fact is that no possible hydrocarbon species derived from benzene was detected, indicating that the carbon-carbon bond was not broken under our experimental conditions. All of the experimental results strongly reinforce the six-membered-ring-based growth model, and a schematic elucidation is presented accordingly. This in situ study not only reveals the unique and convincing information directly related to the growth mechanism from the involved chemistry, but also provides a powerful way to clarify the mechanism of CVD synthesis of CNTs with other precursors.

Biomacromolecules such as proteins, although extremely complex in microstructure, can crystallize into macro-sized crystals after self-adjusting their shapes, based on which the structure of biology is built. Inorganic nanowires/nanoribbons with a similar one-dimensional topology but much simpler structures can hardly be as flexible as macromolecules when constructing superlattice structures because of their inherent rigidity. Here we report the synthesis of crystalline indium sulfide nanoribbon-based nanocoils that are formed by spontaneous self-coiling of ultrathin nanoribbons. The nanostructures are flexible and appear as relatively random coils because of their ultrathin ribbon structures (~0.9 nm in thickness) with high aspect ratios. Moreover, the nanocoils can self-adjust their shapes and assemble into two-dimensional superlattices and three-dimensional supercrystals in solution. The ultrathin nanocoils are expected to bring new insights into the use of flexible nanocrystals as building blocks for constructing superstructures.

A comprehensive study was conducted to investigate the stability of a wide variety of mesoporous silica-supported amine-containing adsorbents in the presence of carbon dioxide under dry conditions. CO(2)-induced degradation of grafted primary and secondary monoamines (pMono, sMono), diamines with one primary and one secondary amines (Diamine) and triamine with one primary and two secondary amines (TRI) as well as different impregnated polyamines such as branched and linear polyethylenimine (BPEI and LPEI) and polyallylamine (PALL) was investigated using extensive CO(2) adsorption-desorption cycling as well as diffuse reflectance infrared Fourier transform (DRIFT) and (13)C CP MAS NMR measurements. Except for sMono, all other supported amines underwent significant deactivation in the presence of dry CO(2) under mild conditions. In all cases, the decrease in CO(2) uptake was associated with the formation of urea linkages at the expense of amine groups. The urea-containing species were identified, and the deactivation pathways were delineated.

This article describes the synthesis and functions of a porous catalytic framework based on conjugated micro- and mesoporous polymers with metalloporphyrin building blocks (FeP-CMP). FeP-CMP was newly synthesized via a Suzuki polycondensation reaction and was developed as a heterogeneous catalyst for the activation of molecular oxygen to convert sulfide to sulfoxide under ambient temperature and pressure. FeP-CMP is intriguing because the polymer skeleton itself is built from catalytic moieties and serves as built-in catalysts, bears inherent open nanometer-scale pores that are accessible for substrates, and possesses large surface areas (1270 m(2) g(-1)) that facilitate the transformation reaction. It is highly efficient with high conversion (up to 99%) and a large turnover number (TON = 97,320), is widely applicable to various sulfides covering from aromatic to alkyl and cyclic substrates, displays high selectivity (up to 99%) to form corresponding sulfoxides, and is highly chemoselective for the oxidation of a sulfide group even in the coexistence of other oxidative functionalities. Owing to the covalent linkages between catalytic sites in the frameworks, FeP-CMP can be recycled with good retention of its porous structure and allows for large-scale transformation. These unique characteristics clearly originate from the covalent porous catalytic framework structure and demonstrate the usefulness of CMPs in the exploration of built-in heterogeneous catalysts, a new potential of these materials that have thus far been reported to exhibit noteworthy gas adsorption functions.

A simple, flexible, reproducible, high-yielding (ca. 100%) synthesis of hexagonally facetted SBA-15 straight rods with uniform sizes (ca. 1.5 x 0.4 mum) under static conditions without using inorganic salts is provided. This method affords rodlike particles with temperature-dependent pore sizes ranging from 5.8 to 12.5 nm but with similar external dimensions. This work shows that in order to prepare short monodispersed SBA-15 rods, (i) the absence of stirring is essential, (ii) the temperature of the first stage should not exceed 60 degrees C, and (iii) addition of inorganic salts is not required.

We here report the first direct evidence addressing the possible involvement of Mo in substrate interactions during catalytic turnover. When the alpha-70(Ile) MoFe protein is freeze-trapped during H(+) reduction under Ar, the majority of the resting state EPR signal from the molybdenum-iron cofactor (FeMo-co) disappears and is replaced by the S = 1/2 signal of an intermediate that has been shown to be the E(4) MoFe state, which is activated for N(2) binding and reduction through the accumulation of 4 electrons/protons by FeMo-co. ENDOR studies of E(4) showed that it contains two hydrides bound to FeMo-co. We calculate that Mo involvement in hydride binding would require a vector-coupling coefficient for Mo of |K(Mo)| approximately0.2 and determine K(Mo) for the E(4) intermediate state through 35 GHz ENDOR measurements of a (95)Mo enriched MoFe protein, further comparing the results with those for the E(0) resting state. The experiments show that Mo of the resting-state FeMo-co is perturbed by the alpha-70(Ile) substitution and that the isotropic (95)Mo hyperfine coupling in E(4) is a(iso) approximately 4 MHz, less than that for the resting state. The decrease in a(iso) for (95)Mo of E(4) from the already small value in the resting state MoFe protein strongly suggests that the resting Mo(IV) is not one-electron reduced during the accumulation of the four electrons of E(4). In any case, the effective K for Mo is very small; |K(Mo)| approximately0.04, at least 5-fold less than the lower bound required for Mo to be involved in forming a Mo-H-Fe, hydride. As the hydride couplings also are both far too small and of the wrong symmetry to be associated with a terminal hydride on Mo, we may thus conclude that Mo does not participate in binding a hydride of the catalytically central E(4) intermediate and that only Fe ions are involved. Nonetheless, the response of the Mo coupling to subtle conformational changes in E(0) and to the formation of E(4) suggests that Mo is intimately involved in tuning the geometric and electronic properties of FeMo-co in these states.