Maurice van Gastel

A single-crystal ENDOR and density functional theory study of the oxidized states of the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F.

The catalytic center of the [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F in the oxidized states was investigated by electron paramagnetic resonance and electron–nuclear double resonance spectroscopy applied to single crystals of the enzyme. The experimental results were compared with density functional theory (DFT) calculations. For the Ni-B state, three hyperfine tensors could be determined. Two tensors have large isotropic hyperfine coupling constants and are assigned to the β-CH2 protons of the Cys-549 that provides one of the bridging sulfur ligands between Ni and Fe in the active center. From a comparison of the orientation of the third hyperfine tensor with the tensor obtained from DFT calculations an OH− bridging ligand has been identified in the Ni-B state. For the Ni-A state broader signals were observed. The signals of the third proton, as observed for the “ready” state Ni-B, were not observed at the same spectral position for Ni-A, confirming a structural difference involving the bridging ligand in the “unready” state of the enzyme.

A Metal–Metal Bond in the Light-Induced State of [NiFe] Hydrogenases with Relevance to Hydrogen Evolution

The light-induced Ni-L state of [NiFe] hydrogenases is well suited to investigate the identity of the amino acid base that functions as a proton acceptor in the hydrogen turnover cycle in this important class of enzymes. Density functional theory calculations have been performed on large models that include the complete [NiFe] center and parts of the second coordination sphere. Combined with experimental data, in particular from electron paramagnetic resonance and Fourier transform infrared (FTIR) spectroscopy, the calculations indicate that the hydride ion, which is located in the bridging position between nickel and iron in the Ni-C state, dissociates upon illumination as a proton and binds to a nearby thiolate base. Moreover, the formation of a functionally relevant nickel-iron bond upon dissociation of the hydride is unequivocally observed and is in full agreement with the observed g values, ligand hyperfine coupling constants, and FTIR stretching frequencies. This metal-metal bond can be protonated and thus functions like a base. The nickel-iron bond is important for all intermediates with an empty bridge in the catalytic cycle, and the electron pair that constitutes this bond thus plays a crucial role in the hydrogen evolution catalyzed by the enzyme.

Theoretical spectroscopy of the Ni(II) intermediate states in the catalytic cycle and the activation of [NiFe] hydrogenases.

[NiFe] hydrogenases catalyze the reversible oxidation of dihydrogen. The corresponding catalytic cycle involves a formidable number of redox states of the Ni‐Fe active site; these can be distinguished experimentally by the IR stretching frequencies of their CN and CO ligands coordinated to iron. These spectroscopic fingerprints serve as sensitive probes for the intrinsic electronic structure of the metal core and, indirectly, for the structural composition of the active site. In this study, density functional theory (DFT) was used to calculate vibrational frequencies, by focusing on the EPR‐silent intermediate states that contain divalent metal centers. By using the well‐characterized Ni‐C and Ni‐B states as references, we identified candidates for the Ni‐SIr, Ni‐SIa, and Ni‐R states by matching the predicted relative frequency shifts with experimental results. The Ni‐SIr and Ni‐SIa states feature a water molecule loosely bound to nickel and a formally vacant bridge. Both states are connected to each other through protonation equilibria; that is, in the Ni‐SIa state one of the terminal thiolates is protonated, whereas in Ni‐SIr this thiolate is unprotonated. For the reduced Ni‐R state two feasible models emerged: in one, H2 coordinates side‐on to nickel, and the second features a hydride bridge and a protonated thiolate. The Ni‐SU state remains elusive as no unequivocal correspondence between the experimental data and calculated frequencies of the models was found, thus indicating that a larger structural rearrangement might occur upon reduction from Ni‐A to Ni‐SU and that the bridging ligand might dissociate.

An Intermediate Cobalt(IV) Nitrido Complex and its N-Migratory Insertion Product

Low-temperature photolysis experiments (T = 10 K) on the tripodal azido complex [(BIMPN(Mes,Ad,Me))Co(II)(N3)] (1) were monitored by EPR spectroscopy and support the formation of an exceedingly reactive, high-valent Co nitrido species [(BIMPN(Mes,Ad,Me))Co(IV)(N)] (2). Density functional theory calculations suggest a low-spin d(5), S = 1/2, electronic configuration of the central cobalt ion in 2 and, thus, are in line with the formulation of complex 2 as a genuine, low-spin Co(IV) nitride species. Although the reactivity of this species precludes handling above 50 K or isolation in the solid state, the N-migratory insertion product [(NH-BIMPN(Mes,Ad,Me))Co(II)](BPh4) (3) is isolable and was reproducibly synthesized as well as fully characterized, including CHN elemental analysis, paramagnetic (1)H NMR, IR, UV-vis, and EPR spectroscopy as well as SQUID magnetization and single-crystal X-ray crystallography studies. A computational analysis of the reaction pathway 2 → 3 indicates that the reaction readily occurs via N-migratory insertion into the Co-C bond (activation barrier of 2.2 kcal mol(-1)). In addition to the unusual reactivity of the nitride 2, the resulting divalent cobalt complex 3 is a rare example of a trigonal pyramidal complex with four different donor ligands of a tetradentate chelate-an N-heterocyclic carbene, a phenolate, an imine, and an amine-binding to a high-spin Co(II) ion. This renders complex 3 chiral-at-metal.

Electron paramagnetic resonance and electron nuclear double resonance investigation of the diradical bis(α-iminopyridinato)zinc complex

The neutral complex Zn(L*)(2) and its monocationic analogue [ZnL(L*).THF](1+) have been previously reported to contain two and one monoanionic alpha-iminopyridinate((1-)) pi radical ligands, respectively [Lu, C.C.; Bill, E.; Weyhermüller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc. 2008, 130, 3181-3197] (L represents the neutral alpha-iminopyridine form and THF is tetrahydrofuran). The electronic structures of these complexes have now been studied by Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) spectroscopy in conjunction with density-functional theory (DFT) calculations. The ENDOR spectra of the triplet Zn(L*)(2) are characteristic of a localized diradical: these two ligand radicals exhibit dipolar exchange interactions, but no superexchange mechanism is operative, which would be consistent with its nearly tetrahedral coordination geometry. The monocationic species [ZnL(L*).THF](1+) (S = 1/2) has also been investigated by pulse EPR spectroscopy using large interpulse separations. It is shown that no radical hopping takes places on the time scale of the EPR experiment. The results obtained here, in particular the lack of asymmetry in the charge distribution between the two ligands in the triplet state, may be relevant for a better understanding of the electronic structure of naturally occurring diradicals and triplet states.

Wavelength dependence of the photo-induced conversion of the Ni–C to the Ni–L redox state in the [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F

The conversion process of the Ni–C to the Ni–L redox state of the [NiFe] center in Desulfovibrio vulgaris Miyazaki F hydrogenase is investigated by EPR spectroscopy following laser excitation at distinct wavelengths. The Ni–L state is characterized by a rhombic g tensor with principal values of 2.30, 2.12 and 2.05. The action spectrum associated with the Ni–C to Ni–L photoconversion spans the complete visible range and local maxima are present at 590 and 700 nm and a shoulder around 850 nm. In the UV/VIS spectrum of the Ni–C state, bands of low intensity can also be observed at 590 and 700 nm. It is proposed that these bands are associated with the [NiFe] center and correspond to electronic transitions that trigger the conversion process. On the basis of the small molar absorption coefficients (1100 M−1 cm−1 and 1400 M−1 cm−1), these transitions contain at most a small amount of ligand-to-metal charge transfer character. A light-induced back conversion, starting from the Ni–L redox state, has not been observed. Based on comparison with data available from X-ray crystallography and from ENDOR and pulse EPR spectroscopy of the Ni–C and Ni–L states, we propose that the Ni–C to Ni–L conversion process is associated with a dissociation of the Ni–H bond and translocation of the released proton to the sulfur of one of the terminal cysteines.

Open‐Shell Complexes Containing Metal–Germanium Triple Bonds

The chemistry of compounds featuring triple bonds of the Group 14 elements Si–Pb with transition metals is a research area combining modern molecular main-group-element with transition-metal chemistry. Following the first report of a molybdenum germylidyne complex by Power et al. , a series of uncommon complexes of the general formula trans[XL4M E R] (M = Mo, W; E = Ge–Pb; X = halogen; L = phosphane; R = bulky alkyl or aryl group), have become accessible in our group by taking advantage of a very efficient N2/PMe3 elimination method. [2] Recently, also the first silylidyne complex, [Cp(CO)2Mo Si-R] (Cp = C5H5; R = C6H3-2,6-Trip2; Trip = 2,4,6-triisopropylphenyl), has been isolated. All these compounds are closed-shell 18 valence electron (VE) complexes, which contain linear-coordinated, triply bonded Si–Pb atoms. In comparison, openshell congeners of these intriguing compounds are presently unknown. This situation is not surprising as even openshell alkylidyne complexes are very rare and quite reactive species. Quantumchemical calculations of the germylidyne, stannylidyne, and plumbylidyne complexes trans-[XL4M E R] revealed that these compounds have a similar electronic structure with Fischer-type alkylidyne complexes, and contain a d configured metal center and a metal-centered HOMO, that is non-bonding with respect to the metal–tetrel triple bond. d,e,f,6] The theoretical results implied that the compounds trans[XL4M E R] should be prone to one-electron oxidation providing access to unprecedented Si–Pb analogues of openshell alkylidyne complexes. Experimental verification of this prediction is presented herein with the synthesis and full characterization of the first open-shell 17 VE germylidyne complexes trans-[Cl(PMe3)4M Ge-C6H3-2,6-Mes2][B(C6F5)4] (M = Mo, W; Mes = 2,4,6-trimethylphenyl). The entry into this chemistry provided the 18 VE germylidyne complexes trans-[Cl(PMe3)4M Ge-C6H3-2,6-Mes2] (1Mo: M = Mo; 1-W: M = W), which were obtained selectively upon heating the dinitrogen complexes cis-[M(N2)2(PMe3)4] [7] (M = Mo, W) with 0.5 equivalents of the organogermanium(II) chloride [{GeCl(C6H3-2,6-Mes2)}2] [8] in toluene at 100 8C [Eq. (1)]. The products were isolated as air-sensitive, wine-red (1-Mo) and dark brown-red (1-W) solids in good

Zinc-bacteriochlorophyllide dimers in de novo designed four-helix bundle proteins. A model system for natural light energy harvesting and dissipation.

Photosynthetic organisms utilize interacting pairs of chlorophylls and bacteriochlorophylls as excitation energy donors and acceptors in light harvesting complexes, as photosensitizers of charge separation in reaction centers, and maybe as photoprotective quenching centers that dissipate excess excitation energy under high light intensities. To better understand how the pigments local environment and spatial organization within the protein tune its ground- and excited-state properties to perform different functions, we prepared and characterized the simplest possible system of interacting bacteriochlorophylls within a protein scaffold. Using HP7, a high-affinity heme-binding protein of the HP class of de novo designed four-helix bundles, we incorporated 13(2)-OH-zinc-bacteriochlorophyllide-a (ZnBChlide), a water-soluble bacteriochlorophyll derivative, into specific binding sites within the four-helix bundle protein core. We capitalized on the rich and informative optical spectrum of ZnBChlide to rigorously characterize its complexes with HP7 and two variants, in which a single heme-binding site is eliminated by replacing histidine residues at positions 7 or 42 by phenylalanine. Surprisingly, we found the ZnBChlide binding capacity of HP7 and its variants to be higher than for heme: up to three ZnBChlide pigments bind per HP7, or two per each single histidine variant. The formation of dimers within HP7 results in dramatic quenching of ZnBChlide fluorescence, reducing its quantum yield by about 80%, and the singlet excited-state lifetime by 2 orders of magnitudes compared to the monomer. Thus, HP7 and its variants are the first examples of a simple protein environment that can isolate a self-quenching pair of photosynthetic pigments in pure form. Unlike its complicated natural analogues, this system can be constructed from the ground up, starting with the simplest functional element, increasing the complexity as needed.

Electron-electron double resonance-detected NMR to measure metal hyperfine interactions: 61Ni in the Ni-B state of the [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F.

Electron double resonance-detected NMR (EDNMR) is introduced as a powerful technique to directly measure metal hyperfine interactions in the active sites of metalloproteins. Measurement of these quantities by electron nuclear double resonance is usually difficult because of fast relaxation times and large anisotropic (dipolar) hyperfine couplings. In EDNMR, electron paramagnetic resonance (EPR) “forbidden” transitions are excited, which become EPR allowed to some extent because of the presence of a large dipolar hyperfine and possibly quadrupole interaction. The usefulness of EDNMR is demonstrated with measurements on 61Ni-enriched hydrogenase of D. vulgaris Miyazaki F in the Ni−B state.

Structural and spectroscopic investigation of an anilinosalen cobalt complex with relevance to hydrogen production.

A Co(II) anilinosalen catalyst containing proton relays in the first coordination sphere has been synthesized that catalyzes the electrochemical production of hydrogen from acid in dichloromethane and acetonitrile solutions. The complex has been spectroscopically and theoretically characterized in different protonation and redox states. We show that both coordinated anilido groups of the neutral Co(II) complex can be protonated into aniline form. Protonation induces an anodic shift of more than 1 V of the reduction wave, which concomitantly becomes irreversible. Hydrogen evolution that originates from the aniline protons located in the first coordination sphere is observed upon bulk electrolysis at -1.5 V of the protonated complex in absence of external acid. Structures for intermediates in the catalytic reaction have been identified based on this data.