The E2 state of FeMoco: Hydride Formation versus Fe Reduction and a Mechanism for H2 Evolution

The iron-molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E₀, reduced states of FeMoco are much less well characterized. The E₂ state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E₂ state can, however, relax back the E₀ state via a H₂ side-reaction, implying a hydride intermediate prior to H₂ formation. This E₂→E₀ pathway is one of the primary mechanisms for H₂ formation under low-electron flux conditions. In this study we present an exploration of the energy surface of the E₂ state. Utilizing both cluster-continuum and QM/MM calculations, we explore various classes of E₂ models: including terminal hydrides, bridging hydrides with a closed or open sulfide-bridge, as well as models without. Importantly, we find the hemilability of a protonated belt-sulfide to strongly influence the stability of hydrides. Surprisingly, non-hydride models are found to be almost equally favorable as hydride models. While the cluster-continuum calculations suggest multiple possibilities, QM/MM suggests only two models as contenders for the E₂ state. These models feature either i) a bridging hydride between Fe₂ and Fe₆ and an open sulfide-bridge with terminal SH on Fe₆ ( E₂-hyd ) or ii) a double belt-sulfide protonated, reduced cofactor without a hydride ( E₂-nonhyd ). We suggest both models as contenders for the E₂ redox state and further calculate a mechanism for H₂ evolution. The changes in electronic structure of FeMoco during the proposed redox-state cycle, E₀→E₁→E₂→E₀, are discussed.     

X-Ray Photoelectron Spectral (XPS) Studies of Molybdenum (0) and Tungsten (0) Dinitrogen Complexes

The X-ray photoelectron spectra of M(N₂)₂ (dppe)₂, M(N₂)₂ (PMePh₂)₄ and M(N₂)₂ (PMe₂Ph)₄; where M=W or Mo, Ph is phenyl, Me is methyl and dppe is (Ph)₂P-CH₂-CH₂-P(Ph)₂; have been investigated. The N(1s), P(2p_3/2), Mo(3d_5/2) and W(4f_7/2) binding energies do not show significant differences between these complexes. The N(1s) signals show the separation of 1.3-1.4 eV.     

Early effects of radiation radical pairs in a single crystal of bartituric acid

Radical pairs are formed and trapped when barbituric acid is irradiated with electrons at 77 K. The radical pairs were identified and the molecular structure determined in single crystal studies. The two unpaired species, constituting the pair, were found to be identical and equal to the isolated radical previously studied. The radical is formed by a hydrogen abstraction from C ₅, and the pair is formed by two such abstractions in neighbour molecules. The pair formation is quite selective and involve exclusively the nearest neighbour molecules.     

Conformational energies of silacyclohexanes C5H10SiHMe,C5H10SiH(CF3) and C5H10SiCl(SiCl3) from variable temperature Raman spectra

From variable temperature vibrational Raman spectra, the axial/equatorial enthalpy differences for the substituted silacyclohexanes C₅H₁₀SiHMe, C₅H₁₀SiH(CF₃) and C₅H₁₀SiCl(SiCl₃) were determined. The pure liquids and solutions in various solvents were investigated. Preferred conformations are equatorial for methylsilacyclohexane and axial for trifluoromethylsilacyclohexane, consistent with earlier results from nuclear magnetic resonance experiments and ab initio calculations. For C₅H₁₀SiCl(SiCl₃) an enthalpy difference close to zero was found, which is supported by high‐level which is supported by high‐level quantum chemical calculations at the second‐order Møller‐Plesset (MP2) and coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) levels, which employed various basis sets. A novel synthesis for C₅H₁₀SiCl(SiCl₃) was developed using ClMg(CH₂)₅MgCl instead of BrMg(CH₂)₅MgBr as a starting material. The procedure avoids the formation of partially brominated products, facilitating the purification of the compound. ¹H, ¹³C and ²⁹Si nuclear magnetic resonance data are reported. Copyright © 2012 John Wiley & Sons, Ltd.     

Siciak–Zahariuta extremal functions,analytic discs and polynomial hulls

We prove two disc formulas for the Siciak–Zahariuta extremal function of an arbitrary open subset of complex affine space. We use these formulas to characterize the polynomial hull of an arbitrary compact subset of complex affine space in terms of analytic discs. Similar results in previous work of ours required the subsets to be connected.     

The charge distribution of iron complexes studied by XPS,Mössbauer spectroscopy,and SCC-MO calculations

A series of iron complexes has been studied by XPS and Mössbauer spectroscopy. Atomic charges and iron atom populations have been estimated from the experimental results and compared with iterative extended Hückel (IEH) calculations as well as with calculations based on a modified form of this method (MIEH).     

Preconditioned Iterative Methods for Scattered Data Interpolation

The scattered data interpolation problem in two space dimensions is formulated as a partial differential equation with interpolating side conditions. The system is discretized by the Morley finite element space. The focus of this paper is to study preconditioned iterative methods for the corresponding discrete systems. We introduce block diagonal preconditioners, where a multigrid operator is used for the differential equation part of the system, while we propose an operator constructed from thin plate radial basis functions for the equations corresponding to the interpolation conditions. The effect of the preconditioners are documented by numerical experiments.     

Auger spectra for elements of low atomic number

KLL Auger spectra for the light elements sodium (Z=11) and magnesium (Z=12) have been studied. All five lines predicted by the extremeLS coupling theory are observed. A comparison is made with intensities predicted by the inter-configuration interaction theory ofAsaad. Agreement is obtained for the transition intensities to the three differentKLL configurations; however, theory does not give correct relative intensities within the 2s ² 2p ⁴ configuration. The measured Auger energies are compared with those calculated from the semi-empirical expression ofAsaad andBurhop and modified byHörnfeldt et al. Good agreement is obtained when we use electron binding energies measured by the ESCA method.     

The masking effect of iron(III) on the interference from nickel and copper in the determination of tellurium by hydride generation/atomic absorption spectrometry

Iron(III) can minimize the serious interferences from copper(II) and nickel(Il) on the determination of tellurium by hydride generation/atomic absorption spectrometry. The optimal concentrations were found to be 1 g l^?1 and 2 g l^?1 Fe (III) in 4.0 mol l^?1 HCl in presence of nickel (II) and copper (II), respectively. The signals were only 25 % lower in a solution of 1.6 g 1^?1 Ni(II) than for a nickel-free solution. For copper (II), reasonable sensitivity was retained in the presence of 100 mg l^?1 Cu(II).