Formation of Hydroxysulphate Green Rust 2 as a Single Iron(II-III) Mineral in Microbial Culture

Although GR2(SO₄ ^2-) can be easily formed by abiotic synthesis, the biotic formation of hydroxysulphate as a single iron(II-III) mineral in microbial culture and its characterization was not achieved. This study was carried out to investigate the sole formation of GR2(SO₄ ^2-) during the reduction of γ-FeOOH by a dissimilatory iron-respiring bacterium, Shewanella putrefaciens CIP 8040^T. Reduction experiments were performed in a non-buffered medium devoid of organic compounds, with 25 mM of sulphate and with a range of lepidocrocite concentrations with H₂ as the electron donor under nongrowth conditions. The resulting biogenic solids, after iron-respiring activity, were characterized by X-ray diffraction (XRD), transmission Mössbauer spectroscopy (TMS) and electron microscopy (SEM and TEM). The sulphate has been identified as the intercalated anion by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In addition, the structure of this sulphate anion was discussed. Our experimental study demonstrated that, under H₂ atmosphere, the biogenic solid was a GR2(SO₄ ^2-), as the sole iron(II-III) bearing mineral, whatever the initial lepidocrocite concentration. The crystals of the biotically formed GR2(SO₄ ^2-) are significantly larger than those observed for GR2(SO₄ ^2-) obtained through abiotic preparation, < 15 μ m diameter as against 0.5–4 μm, respectively.     

Calculated vibrational properties of semiquinones in the A1 binding site in photosystem I

Time-resolved (P700⁺A₁^? – P700A₁) FTIR difference spectra have been obtained using photosystem I (PSI) particles with several different quinones incorporated into the A₁ protein binding site. Difference spectra were obtained for PSI with unlabeled and ¹⁸O labeled phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone) and 2-methyl-1,4-naphthaquinone (2MNQ) incorporated, and for PSI with unlabeled 2,3-dimethyl-1,4-naphthoquinone (DMNQ) incorporated. (¹⁸O – ¹⁶O), (2MNQ – PhQ) and (DMNQ – PhQ) FTIR double difference spectra were constructed from the difference spectra. These double difference spectra allow one to more easily distinguish protein and pigment bands in convoluted difference spectra. To further aid in the interpretation of the difference spectra, particularly the spectra associated with the semiquinones, we have used two-layer ONIOM methods to calculate corresponding difference and double difference spectra. In all cases, the experimental and calculated double difference spectra are in excellent agreement. In previous two and three-layer ONIOM calculations it was not possible to adequately simulate multiple difference and double difference spectra. So, the computational approach outlined here is an improvement over previous calculations. It is shown that the calculated spectra can vary depending on the details of the molecular model that is used. Specifically, a molecular model that includes several water molecules that are near the incorporated semiquinones is required.