Multi-heme cytochromes--new structures, new chemistry

Heme is one of the most pervasive cofactors in nature and the c-type cytochromes represent one of the largest families of heme-containing proteins. Recent progress in bacterial genomic analysis has revealed a vast range of genes encoding novel c-type cytochromes that contain multiple numbers of heme cofactors. The genome sequence of Geobacter sulfurreducens, for example, includes some one hundred genes encoding c-type cytochromes, with around seventy of these containing two, or more, heme groups and with one protein containing an astonishing twenty seven heme groups. This wealth of cytochromes is of great significance in the respiratory flexibility shown by bacteria such as Geobacter. In addition, we are now discovering that many of these multi-heme cytochromes have associated enzymatic activities and in some cases this is revealing new chemistries. The purpose of this perspective is to describe recent progress in the structural and functional analyses of these new multi-heme cytochromes. To illustrate this we have chosen to focus on three of these cytochromes which exhibit catalytic activities; nitrite reductase, hydroxylamine oxidoreductase and tetrathionate reductase. In addition we consider the multi-heme cytochromes from Geobacter and Desulfovibrio species. Finally, we consider and contrast the repeating structural modules found in these multi-heme cytochromes.     

Electrospray mass spectrometry of lanthanide β-diketone complexes. II-Ligand-exchange processes involving acetate ions and Ln(tfc)3 complexes

Electrospray ionization on Ln(tfc)₃ complexes (Ln = Eu, Yb; tfc = D -3-trifluoroacetylcamphorate) was performed with samples dissolved in methanol–water containing acetic acid. The spectra were obtained with a four-sector mass spectrometer. The mass spectra exhibit ions containing tfc ligand in addition to ions with acetato ligand. Electro-spray and tandem mass spectra are presented.     

The preparation and properties of cage polycyclic systems—III: The cleavage in alkaline conditions of acetals derived from pentacyclo[5.3.0.02,5.03,9.04,8]decane and pentacyclo[4.3.0.02,5.03,8.04,7]nonane systems

The synthesis of some acetals derived from pentacyclo[5.3.0.0^2,5.0^3,9.0^4,8]decane is described. The reductive cleavage of an ethylenedioxy group

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and a dimethoxy group

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in pentacyclo[5.3.0.0^2,5.0^3,9.0^4,8]decane and pentacyclo[4.3.0.0^2,5.0^3,8.0^4,7]nonane systems to give the methylene group is shown to occur in alkaline conditions in the presence of hydrazine. Evidence is presented for a mechanism which involves initial cleavage of the acetal by alkali to form the ketone. The substitution of a Br atom in the position neighbouring the CO group of 5-bromo-6,6-ethylenedioxypentacyclo[5.3.0.0^2,5.0^3,9.0^4,8]decan-10-one facilitates the Wolff-Kishner reaction to such an extent that hydrazine hydrate is a sufficiently strong base to induce the decomposition of the hydrazone directly.     

Gas-phase kinetics and activation parameters for thermal 1,5-hydrogen shifts interconverting monodeuterium-labeled 1,3-cyclohexadienes

Thermal equilibrations among the three possible monodeuterium-labeled 1,3-cyclohexadienes have been followed in the gas phase at temperatures from 254 to 284 degrees C. The temperature-dependent rate constants for the 1,5-shift of a single hydrogen lead to the activation parameters E(a) = (40.1 +/- 0.8) kcal/mol, log A = (12.1 +/- 0.3), and DeltaS = -(6.3 +/- 1.3) e.u. These activation parameters are reconciled with experimental values reported earlier for reactions starting with 1,4-d(2)-cyclohexadiene.     

The mass spectra of the trimethylsilyl ethers of some hydroxylated di- and tetracarboxylic porphyrins

The preparation and the mass spectra of the trimethylsilyl ethers of six hydroxyporphyrins are reported. In every case the side chain bearing the ether grouping is the most labile on electron-impact and exhibits characteristic ‘benzylic’ cleavage. The presence of a metastable peak allows a definite identification of this cleavage to be made.     

Improved cesium sensitivity in electrothermal atomic absorption spectrometry

The sensitivity for cesium determination by electrothermal atomic absorption spectrometry is improved four-fold by the addition of a large excess of potassium nitrate. Zeeman background corrections is used to compensate for the large non-specific absorption signal resulting from the potassium. The characteristic concentration and detection limit are 0.44 and 2 μg l^?1, respectively, and the coefficient of variation is 2% at the 50 μg l^?1 level. The procedure is suitable for the rapid determination of cesium in leach solutions from nuclear waste fixation experiments.     

Assignment of coherence features in NMR q-space plots to particular diffusion modes in erythrocyte suspensions.

NMR q-space plots derived from water diffusing inside and around erythrocytes in a suspension display reproducible and characteristic coherence features. The aim of the present work was to determine which water population gives rise to the respective features. The central experimental strategy was to use choline and choline phosphate which are virtually membrane impermeant on the time scale of the experiment; the former was incorporated into erythrocytes by a lysis-resealing method and the latter was simply added to the suspensions. Dimethyl sulfoxide, which readily but more slowly exchanges across the cell membranes than water, also yielded q-space plots which were similar to those of water, but the differences were able to be accounted for on the basis of its slower transmembrane exchange rate. Random walk simulations using a Monte Carlo procedure, together with a model of an array of biconcave discocytes, helped verify the interpretations of the assignment of the features of the plots to molecules diffusing in the two regions. In addition, the simulations revealed how the presence or absence of transmembrane exchange affects the form of q-space plots.     

Uncertainty Analysis of Radionuclide Transport in a Fractured Coastal Aquifer with Geothermal Effects

Groundwater flow and radionuclide transport at the Milrow underground nuclear test site on Amchitka Island are modeled using two-dimensional numerical simulations. A multi-parameter uncertainty analysis is adapted and used to address the effects of uncertainties associated with the definition of the modeled processes and the values of the parameters governing these processes. In particular, we focus on the effects on radioactive transport of uncertainties associated with conduction and convection of heat relative to the uncertainties associated with other flow and transport parameters. These include recharge, hydraulic conductivity, fracture porosity, dispersivity and strength of matrix diffusion. The flow model is conceptualized to address the problem of density-driven flow under conditions of variable salinity and geothermal gradient. The conceptual transport model simulates the advection–dispersion process, the diffusion process from the high-velocity fractures into the porous matrix blocks, and radioactive decay.For this case study, the uncertainty of the recharge-conductivity ratio contributes the most to the output uncertainty (standard deviation of mass flux across the seafloor). The location of the freshwater–saltwater transition zone changes dramatically as this ratio changes with the thickness of the freshwater lens and the location of the seepage face changing as well. In the context of radionuclide transport from the nuclear test cavity that is located in the area where the transition zone is uncertain, travel times of radionuclide mass from the cavity to the seepage face along the seafloor are significantly impacted. The variation in transition zone location changes the velocity magnitude at the cavity location by a large factor (probably an order of magnitude). When this effect is combined with porosity and matrix diffusion uncertainty, the uncertainty of transport results becomes large. Although thermal parameters have an effect on the solution of the flow problem and also on travel times of radionuclides, the effect is relatively small compared to other flow and transport parameters.