Weiwei Gu

Functional copper at the acetyl-CoA synthase active site

The bifunctional CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) plays a central role in the Wood–Ljungdahl pathway of autotrophic CO2 fixation. A recent structure of the Moorella thermoacetica enzyme revealed that the ACS active site contains a [4Fe-4S] cluster bridged to a binuclear Cu-Ni site. Here, biochemical and x-ray absorption spectroscopic (XAS) evidence is presented that the copper ion at the M. thermoacetica ACS active site is essential. Depletion of copper correlates with reduction in ACS activity and in intensity of the “NiFeC” EPR signal without affecting either the activity or the EPR spectroscopic properties associated with CODH. In contrast, Zn content is negatively correlated with ACS activity without any apparent relationship to CODH activity. Cu is also found in the methanogenic CODH/ACS from Methanosarcina thermophila. XAS studies are consistent with a distorted Cu(I)–S3 site in the fully active enzyme in solution. Cu extended x-ray absorption fine structure analysis indicates an average Cu–S bond length of 2.25 Å and a metal neighbor at 2.65 Å, consistent with the Cu–Ni distance observed in the crystal structure. XAS experiments in the presence of seleno-CoA reveal a Cu–S3Se environment with a 2.4-Å Se–Cu bond, strongly implicating a Cu–SCoA intermediate in the mechanism of acetyl-CoA synthesis. These results indicate an essential and functional role for copper in the CODH/ACS from acetogenic and methanogenic organisms.

L-edge X-ray absorption spectroscopy of some Ni enzymes: Probe of Ni electronic structure

Abstract L-edge X-ray absorption spectroscopy has been used to study the electronic structure of Ni in the Ni–Fe hydrogenases and CO-dehydrogenases under a variety of conditions. The L-edge spectra are interpreted by comparison with the spectra of Ni model complexes and by ligand field multiplet simulations to examine the Ni oxidation and electronic spin states. The spectra for Ni in oxidized Desulfovibrio gigas and Pyrococcus furiosus enzymes are consistent with a covalent Ni III species. All of the reduced hydrogenases in this study exhibit a high spin Ni II spectrum, and no Ni I has been observed. In contrast to hydrogenases, the native Clostridium thermoaceticum CO-dehydrogenase has a low spin Ni II and exhibits a clearly different spectral multiplet. Spectroscopy of Ni enzymes using a 15-eV resolution STJ detector and using the new ALS beamline 4.0.2 with a 0.2 eV energy resolution show great promises for future biological L-edge spectroscopy.

Origin of chemical shift of manganese in lithium battery electrode materials—a comparison of hard and soft X-ray techniques

Abstract L-edge X-ray absorption near edge spectroscopy and K β emission spectroscopy were applied to monitor ex situ chemical valence changes of manganese in battery electrode materials as they appear during electrode processing and battery operation. We found that significant chemical shifts of the spectra occur already during electrode fabrication, prior to any electrochemical treatment. Employment of these two different techniques allows for the qualitative separation of contributions originating from the surface and from the bulk of the electrode particles.

Nickel–iron–sulfur complexes: approaching structural analogues of the active sites of NiFe-hydrogenase and carbon monoxide dehydrogenase/acetyl-CoA synthase

The NiFe-dinuclear complexes [{Fe(NS3)(CO)2-S,S′}NiCl(dppe)] 1, [{Fe(NS3)(CO)-S,S′}NiCl(dppe)] 2, [{Fe(NS3)(NO)-S,S′}NiCl(dppe)] 3, [{Fe(NS3)(NO)-S,S′}Ni(CH3)(dppe)] 4, [Ni{Fe(NS3)(CO)-S,S′}2] 5 have been prepared using the anions [Fe(NS3)(CO)]− and [Fe(NS3)(NO)]− (NS3 = N(CH2CH2S)33−) as chelate ligands to nickel. Crystal structure characterisation has been carried out on 1, 3 and 5. The spectroscopic properties of the complexes have been measured. Complex 1 is a good structural analogue of the active form of the active site of NiFe-hydrogenase. Complexes 1–5 also show features related to the active sites of the nickel enzymes carbon monoxide dehydrogenase and acetyl-CoA synthase.

Soft X-ray absorption spectroscopy and resonant inelastic X-ray scattering spectroscopy below 100 eV: probing first-row transition-metal M-edges in chemical complexes

X-ray absorption and scattering spectroscopies involving the 3d transition-metal K- and L-edges have a long history in studying inorganic and bioinorganic molecules. However, there have been very few studies using the M-edges, which are below 100 eV. Synchrotron-based X-ray sources can have higher energy resolution at M-edges. M-edge X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) could therefore provide complementary information to K- and L-edge spectroscopies. In this study, M2,3-edge XAS on several Co, Ni and Cu complexes are measured and their spectral information, such as chemical shifts and covalency effects, are analyzed and discussed. In addition, M2,3-edge RIXS on NiO, NiF2 and two other covalent complexes have been performed and different d-d transition patterns have been observed. Although still preliminary, this work on 3d metal complexes demonstrates the potential to use M-edge XAS and RIXS on more complicated 3d metal complexes in the future. The potential for using high-sensitivity and high-resolution superconducting tunnel junction X-ray detectors below 100 eV is also illustrated and discussed.