Pauli Kofod

HiPIP oxido-reductase activity in membranes from aerobically grown cells of the facultative phototroph Rhodoferax fermentans

The role of the periplasmically located, water‐soluble, HiPIP (high‐potential iron‐sulfur protein) in the respiratory chain of the facultative phototroph Rhodoferax fermentans has been examined. The oxidized HiPIP is reduced by succinate‐dependent respiration via the bc 1 complex, this reaction being inhibited by myxothiazol and/or stigmatellin. The reduced HiPIP can be oxidized by the membrane‐bound cytochrome oxidase, this reaction being inhibited by 0.1 mM cyanide. We conclude that aerobically grown Rf. fermentans contains a redox chain in which HiPIP mediates electron transfer between the bc 1 complex and the cb‐type cytochrome oxidase.

On the role of cytochrome c8 in photosynthetic electron transfer of the purple non-sulfur bacterium Rhodoferax fermentans

We report on the isolation, purification and functional characterization of a soluble c-type cytochrome from light-grown cells of the purple phototroph Rhodoferax fermentans. This cytochrome is basic (pI = 8), has a molecular mass of 12 kDa, and is characterized by a midpoint reduction potential of +285 mV. Partial analysis of the N-terminus amino-acid sequence shows a high similarity with cytochromes of c8 type (formerly called Pseudomonas cytochrome c-551 type). Time-resolved spectrophotometric studies show that this cytochrome c8 reduces the tetraheme subunit of the photosynthetic reaction center, in a fast (sub-ms) and a slow (ms) phase. Competition experiments in the presence of both cytochrome c8 and high potential iron-sulfur protein (HiPIP), isolated from the same microorganism, show that cytochrome c8 oxidation is decreased upon addition of HiPIP. These observations suggest that cytochrome c8 and HiPIP might play alternative roles in the photosynthetic electron flow of Rhodoferax fermentans.

The Zn-Site in Bovine Copper, Zinc Superoxide Dismutase Studied by 111Cd Pac

The active site in bovine copper, zinc superoxide dismutase (Cu2, Zn2SOD) has been studied by 111Cd time differential Perturbed Angular Correlation (PAC) on enzyme with Zn2+ replaced by excited 111Cd2+. The PAC spectra obtained for both the oxidized and the reduced form of Cu2Cd2SOD show no asymmetry between the two Zn-sites in the dimeric enzyme. The spectra further reveal that a significant change has taken place at the Zn-site in the reduced form compared to the oxidized form. Semi-empirical calculations based on the Angular Overlap Model (AOM) and coordinates from the crystal structure of the native enzyme are in agreement with the experimental PAC data of the oxidized enzyme. The results indicate that Cd2+ coordinates in the same manner as Zn2+ and that the crystal structure of SOD is valid for the enzyme in solution. The PAC spectrum of the reduced enzyme can be explained by extending the AOM calculations to the enzyme in the reduced form and assuming that the imidazol ring of His61 is no longer bridging the copper and cadmium ions in the reduced state.

The methylcobalt(III) complex of a tetrapodal pentadentate amine ligand, 2,6-bis(1′,3′-diamino-2′-methyl-prop-2′-yl)pyridine

The pentaamine methylcobalt(III) compound [Co(pyN 4 )(CH 3 )](NO 3 ) 2 (pyN 4 =2,6-bis(1′,3′-diamino-2′-methyl-prop-2′-yl)pyridine) has been synthesised from [Co(NH 3 ) 5 (CH 3 )](NO 3 ) 2 and pyN 4 by ligand exchange, and characterised by IR, 1 H, 13 C and 59 Co NMR spectroscopy as well as elemental analysis. The structure of the complex has been determined using the dithionate salt [Co(pyN 4 )(CH 3 )]S 2 O 6 . The coordination geometry at cobalt is close to octahedral, the tetrapodal pentadentate amine ligand providing a C 2 -symmetrical coordination cap for the metal centre. One axial position is occupied by the pyridine nitrogen atom, while the four equivalent primary amino groups take the equatorial positions. The other axial position, trans to the pyridine ring, is occupied by the methyl group. The Co–N py bond length of 2.018(2) A is significantly elongated compared with other cobalt(III) complexes of the pyN 4 ligand, demonstrating the strong trans influence of the CH 3 ligand. The Co–C bond length (1.975(4) A) is virtually identical to the value found in the related complex [Co(NH 3 ) 5 (CH 3 )](S 2 O 6 ).

Making a Robust Carbon-Cobalt(III) Bond

The coordination ion with a well-characterized carbon-cobalt(III) bond, the (1,4,7-triazacyclononane)(1,6-diamino-3-thia-4-hexanido)cobalt(III) dication, [Co(tacn)(C-aeaps)](2+) (aeaps, for aminoethylaminopropylsulfide), has been reacted with iodomethane, and the S-methyl thionium derivative has been isolated. The crystal structure of the resulting [Co(tacn)(C-aeaps-SCH(3))]Br(3) x 3 H(2)O at 122 K has been determined by X-ray diffraction techniques to verify the structure. The crystal structure determination shows that the carbon-cobalt bond length is even shorter (2.001(4) A) than in [Co(aeaps)(C-aeaps)](2+) (2.026(3) A), while its trans elongating effect is less pronounced. The (1,4,7-triazacyclononane)(1,6-diamino-3-thia-4-hexanido)cobalt(III) dication [Co(tacn)(C-aeaps)](2+) (aeaps, for aminoethylaminopropylsulfide) reacts relatively fast with acid, for example, with NH(4)(+) to form a sulfur-bound aeaps ligand. The [Co(tacn)(C-aeaps-SCH(3))](3+) ion is remarkably robust in strongly acidic aqueous solution in spite of the supposed high basicity of the carbon anion. However, with a large excess of iodide, the methyl group can be removed as iodomethane. The experimentally obtained distances around cobalt(III) for the three involved coordination ions are compared to those computed from DFT with different standard choices for functionals and basis sets. The agreements range from poor to modest depending of the choice of functionals. It is noteworthy, however, that a sulfur 3p orbital in [Co(tacn)(C-aeaps)](2+) participates in bonding to cobalt(III), having implications for the transformation between the carbon- and sulfur-bound forms of the aeaps ligand.

SULFUR-BONDED AND CARBON-BONDED FORMS OF THE COBALT(III) COMPLEX WITH THE LIGANDS 2-AMINOETHYL 3-AMINOPROPYL SULFIDE AND 1,1,1-TRIS(AMINOMETHYL)ETHANE

The new sulfur-bonded compound [Co(tame)( aeaps )] Cl3.H2O has been synthesized by the reaction of aeaps with Co(tame)Cl3 [tame = 1,1,1-tris( aminomethyl )ethane, and aeaps = 2- aminoethyl 3-aminopropyl sulfide or 3-thiahexane-1,6-diamine]. The Co(tame)( aeaps )3+ ion equilibrates in basic solution with the corresponding carbon-bonded species: Co(tame)( aeaps )3++HO-↔ Co(tame)(C-aeaps )2++H2O A salt of the carbon-bonded species, [Co(tame)(C- aeaps )](S2O6), has been isolated and its structure solved by X-ray diffraction analysis ( C- aeaps = 1,6-diamino-3-thiahexan-4-ide). The crystals are orthorhombic, Pna21, with cell dimensions a 20.455(10), b 9.960(10), c 8.982(10) Ǻ at 122(2) K. Preliminary thermodynamic and kinetic data are similar to the recently reported values for the corresponding coordination ion Co( tacn )( aeaps )3+ ( tacn = 1,4,7-triazacyclononane). In basic solution the Co(tame)( aeaps )3+ species exchanges one of its methylene protons orders of magnitude faster than any other methylene protons and also much faster than it forms the alkyl complex as shown by 13C n.m.r. measurements.

H3O2−, O22− and O2˙− bridging ligands in cobalt(III) complexes of an acyclic phenolate-hinged dinucleating ligand

The dicobalt(III) complex, [Co2(bpbp)(μ-H3O2)2](ClO4)3 (bpbp− = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-tert-butylphenolate), obtained by reaction of cobalt(II) perchlorate with Hbpbp under ambient conditions contains two μ-H3O2− bridging ligands. The H-bonded O⋯O distances in this motif are 2.437(3) and 2.456(4) A, respectively, with a Co⋯Co separation of 3.601(1) A. The structurally precedented peroxo bridged complexes, [Co2(bpbp)(μ-O2)(μ-RCO2)](ClO4)2 (R = CH3 or C6H5), are formed if a carboxylate is present. The X-ray crystal structure showed O–O and Co⋯Co distances of 1.422(3) and 3.168(1) A, respectively, in the case of R = CH3. ESI-MS shows that the bridging peroxo ligand is easily eliminated from [Co2(bpbp)(μ-O2)(μ-CH3CO2)]2+, m/z 390.0, as implicated by the observation of [Co2(bpbp)(μ-CH3CO2)]2+, m/z 374.1, corresponding to loss of the mass equivalent of dioxygen. The superoxo complex [Co2(bpbp)(μ-O2)(μ-CH3CO2)]3+ can be prepared by Ce(IV) oxidation of [Co2(bpbp)(μ-O2)(μ-CH3CO2)]2+. Reaction of [Co2(bpbp)(μ-H3O2)2]3+ with hydrazine in air gives the dicobalt(II) complex [Co2(bpbp)(NH2NHCO2)2]ClO4. In this complex the two exogenous hydrazinecarboxylato ligands are bound to individual metal ions with weak H-bonding between them as shown by X-ray structure analysis.

The CoIII—C bond in (1-thia-4,7-diazacyclodecyl-κ3N4,N7,C10)(1,4,7-triazacyclononane-κ3N1,N4,N7)cobalt(III) dithionate hydrate

In the title compound, [Co(C 6 H 15 N 3 )(C 7 H 15 N 2 S)]S 2 O 6 .H 2 O, the Co-C bond distance is 1.9930 (13) A, which is shorter than for related compounds with the linear 1,6-diamino-3-thiahexan-4-ide anion in place of the macrocyclic 1-thia-4,7-diazacyclodecan-8-ide anion. The coordinated carbanion produces an elongation of 0.102 (7) A of the Co-N bond to the 1,4,7-triazacyclononane N atom in the trans position. This relatively small trans influence is presumably a result of the triamine ligand forming strong bonds to the Co III atom.

A methyl-coordinated RhIII ion in methyl­penta­amminerhodium(III)–chloro­pentaammine­rhodium(III)–di­thionate (0.73/2.27/3)

Some disorder is seen in the crystal structure of [Rh(CH3)(NH3)5]0.73[RhCl(NH3)5]2.27(S2O6)3. It is, however, clear that the methyl group has a pronounced trans influence on the Rh—N distance, with an elongation of 0.11 A. No trans influence is observed for the Rh—N distance due to the Cl− ion. Both observations are in agreement with observations in equivalent cobalt complexes.