Apparent Reduction Potentials Research Articles

Identification and Characterization of a Novel Vitamin B12 (Cobalamin) Biosynthetic Enzyme (CobZ) from Rhodobacter capsulatus, Containing Flavin, Heme, and Fe-S Cofactors

One of the most intriguing steps during cobalamin (vitamin B12) biosynthesis is the ring contraction process that leads to the extrusion of one of the integral macrocyclic carbon atoms from the tetrapyrrole-derived framework. The aerobic cobalamin pathway requires the action of a monooxygenase called CobG (precorrin-3B synthase), which generates a hydroxylactone intermediate that is subsequently ring-contracted by CobJ. However, in the photosynthetic bacterium Rhodobacter capsulatus, which harbors an aerobic-like pathway, there is no cobG in the main cobalamin biosynthetic operon although it does contain an additional uncharacterized gene called orf663. To demonstrate the involvement of Orf663 in cobalamin synthesis, the first dedicated 10 genes of the B12 pathway (including orf663), encoding enzymes for the transformation of uroporphyrinogen III into hydrogenobyrinic acid (HBA), were sequentially cloned into a plasmid to generate an artificial operon, which, when transformed into Escherichia coli, endowed the host with the ability to make HBA. Deletion of orf663 from this operon prevented HBA synthesis, demonstrating that it was essential for corrin construction. HBA synthesis was restored to this recombinant strain either by returning orf663 or by substituting it with cobG. Recombinant overproduction of Orf663, now renamed CobZ, allowed the characterization of a novel cofactor-rich protein, housing two Fe-S centers, a flavin, and a heme group, which like B12 itself is a modified tetrapyrrole. A mechanism for Orf663 (CobZ) in cobalamin biosynthesis is proposed.

Thermodynamics of the mechanism of the nitrogenase reaction

The fixation of molecular nitrogen by nitrogenase requires a lot of energy because 16 mol of ATP are hydrolyzed per mole of nitrogen converted to ammonia. Kim and Dees determined the crystallograpic structure of nitrogenase and this has led to a three-step mechanism that involves Feprotein and MoFeprotein in addition to ferredoxin. Each of these steps can be interpreted in terms of two half reactions that are connected through their transfer of electrons. Estimates can be made of the standard apparent reduction potentials of these three steps and their dependencies on pH and ionic strength. This mechanism is compared with the same type of analysis of an alternative three-step mechanism in which the hydrolysis of ATP is coupled with the reduction of molecular nitrogen, rather than the reduction of Feprotein. The problem with the first mechanism is that the second step produces 12 mol of hydrogen ions per mole of nitrogen fixed and the third step consumes 10 mol of hydrogen ions per mole of nitrogen fixed. The alternative mechanism does not have this problem.

Thermodynamic properties of nucleotide reductase reactions.

Recent thermodynamic measurements have made it possible to calculate the apparent equilibrium constants of the ribonucleoside diphosphate reductase reaction and the ribonucleoside triphosphate reductase reaction with various reducing agents. Third law heat capacity measurements on crystals of d-ribose and other calorimetric measurements make it possible to calculate Delta(f)G degrees for D-ribose and two species of D-ribose 5-phosphate. The experimental value of the apparent equilibrium constant K' for the deoxyribose-phosphate aldolase reaction makes it possible to calculate the standard Gibbs energies of formation Delta(f)G degrees for two protonation states of 2'-deoxy-D-ribose 5-phosphate. This shows that Delta(f)G degrees (2'-deoxy-D-ribose 5-phosphate(2)(-)) - Delta(f)G degrees (D-ribose 5-phosphate(2)(-)) = 147.86 kJ mol(-1) at 298.15 K and zero ionic strength in dilute aqueous solutions. This difference between reduced and oxidized forms is expected to apply to D-ribose, D-ribose 1-phosphate, ribonucleosides, and ribonucleotides in general. This expectation is supported by two other enzyme-catalyzed reactions for which apparent equilibrium constants have been determined. The availability of Delta(f)G degrees values for the species of 2'-deoxy-D-ribose and its derivatives makes it possible to calculate standard transformed Gibbs energies of formation of these reactants, apparent equilibrium constants for their reactions, changes in the binding of hydrogen ions in these reactions, and standard apparent reduction potentials of the half reactions involved as a function of pH and ionic strength at 298.15 K. The apparent equilibrium constant for ADP + thioredoxin(red) = 2'-deoxyADP + H(2)O + thioredoxin(ox) is 1.4 x 10(11) at 298.15 K, pH 7, and 0.25 M ionic strength.

Structure of a novel c7-type three-heme cytochrome domain from a multidomain cytochrome c polymer.

The structure of a novel c(7)-type cytochrome domain that has two bishistidine coordinated hemes and one heme with histidine, methionine coordination (where the sixth ligand is a methionine residue) was determined at 1.7 A resolution. This domain is a representative of domains that form three polymers encoded by the Geobacter sulfurreducens genome. Two of these polymers consist of four and one protein of nine c(7)-type domains with a total of 12 and 27 hemes, respectively. Four individual domains (termed A, B, C, and D) from one such multiheme cytochrome c (ORF03300) were cloned and expressed in Escherichia coli. The domain C produced diffraction quality crystals from 2.4 M sodium malonate (pH 7). The structure was solved by MAD method and refined to an R-factor of 19.5% and R-free of 21.8%. Unlike the two c(7) molecules with known structures, one from G. sulfurreducens (PpcA) and one from Desulfuromonas acetoxidans where all three hemes are bishistidine coordinated, this domain contains a heme which is coordinated by a methionine and a histidine residue. As a result, the corresponding heme could have a higher potential than the other two hemes. The apparent midpoint reduction potential, E(app), of domain C is -105 mV, 50 mV higher than that of PpcA.

Standard apparent reduction potentials of biochemical half reactions and thermodynamic data on the species involved

Standard apparent reduction potentials E′° of half reactions of enzyme-catalyzed reactions are useful because they provide a global view of the apparent equilibrium constants of redox reactions. A table of E′° at a specified pH shows at a glance whether a given half reaction will drive another half reaction or be driven by it. This table can be used to calculate apparent equilibrium constants. Standard Gibbs energies of formation of species in a half reaction can be used to calculate E′° values at pHs in the range 5–9 and ionic strengths in the range of 0–0.35 M. My previously published values of E′° values for 42 half reactions has been extended by 22 new E′° values in this paper. When Δ f G° and Δ f H° are both known for all the species in an enzyme-catalyzed reaction at 298.15 K, it is possible to calculate all the standard transformed thermodynamic properties of the reaction over a range of pHs, ionic strengths, and temperatures.

Decoupling optical and potentiometric band gaps in pi-conjugated materials.

Syntheses, optical spectroscopy, potentiometric studies, and electronic structural calculations are reported for two classes of conjugated (porphinato)metal oligomers that feature a meso-to-meso ethyne-bridged linkage topology. One set of these systems, bis[(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD), 5,15-bis[[5'-10',20'-bis[3,5-di(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DDD), and 5,15-bis[[15' '-(5'-10',20'-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5' '-10' ',20' '-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II)]ethyne]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DDDDD), constitute highly soluble analogues of previously studied examples of this structural motif having simple 10,20-diaryl substituents, while a corresponding set of conjugated oligomers, [(5-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5'-15'-ethynyl-10',20'-bis[10,20-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), 5,15-bis[[5'-10',20'-bis[3,5-di(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethynyl]-10,20-bis(heptafluoropropyl)porphinato]zinc(II) (DAD), and 5,15-bis[[15' '-(5'-10',20'-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5' '-(10' ',20' '-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DADAD), features alternating electron-rich and electron-poor (porphinato)zinc(II) units. Electrooptic and computational data for these species demonstrate that it is possible to engineer conjugated oligomeric structures that possess highly delocalized singlet (S1) excited states yet manifest apparent one-electron oxidation and reduction potentials (E1/20/+ and E1/2-/0 values) that are essentially invariant with respect to those elucidated for their constituent monomeric precursors.

The quinol:fumarate oxidoreductase from the sulphate reducing bacterium Desulfovibrio gigas: spectroscopic and redox studies.

The membrane bound fumarate reductase (FRD) from the sulphate-reducer Desulfovibrio gigas was purified from cells grown on a fumarate/sulphate medium and extensively characterized. The FRD is isolated with three subunits of apparent molecular masses of 71, 31, and 22 kDa. The enzyme is capable of both fumarate reduction and succinate oxidation, exhibiting a higher specificity toward fumarate (Km for fumarate is 0.42 and for succinate 2 mM) and a reduction rate 30 times faster than that for oxidation. Studies by Visible and EPR spectroscopies allowed the identification of two B-type haems and the three iron-sulplur clusters usually found in FRDs and succinate dehydrogenases: [2Fe-2S]2+/1+ (S1), [4Fe-4S]2+/1+ (S2), and [3Fe-4S]1+/0 (S3). The apparent macroscopic reduction potentials for the metal centers, at pH 7.6, were determined by redox titrations: -45 and -175 mV for the two haems, and +20 and -140 mV for the S3 and SI clusters, respectively. The reduction potentials of the haem groups are pH dependent, supporting the proposal that fumarate reduction is associated with formation of the membrane proton gradient. Furthermore, co-reconstitution in liposomes of D. gigas FRD, duroquinone, and D. gigas cytochrome bd shows that this system is capable of coupling succinate oxidation with oxygen reduction to water.

A membrane-bound cytochrome c3: a type II cytochrome c3 from Desulfovibrio vulgaris Hildenborough.

A new tetraheme cytochrome c3 was isolated from the membranes of Desulfovibrio vulgaris Hildenborough (DvH). This cytochrome has a molecular mass of 13.4 kDa and a pI of 5.5 and contains four heme c groups with apparent reduction potentials of -170 mV, -235 mV, -260 mV and -325 mV at pH 7.6. The complete sequence of the new cytochrome, retrieved from the preliminary data of the DvH genome, shows that this cytochrome is homologous to the "acidic" cytochrome c3 from Desulfovibrio africanus (Da). A model for the structure of the DvH cytochrome was built based on the structure of the Da cytochrome. Both cytochromes share structural features that distinguish them from other cytochrome c3 proteins, such as a solvent-exposed heme 1 surrounded by an acidic surface area, and a heme 4 which lacks most of the surface lysine patch proposed to be the site of hydrogenase interaction in other cytochrome c3 proteins. Furthermore, in contrast to previously discovered cytochrome c3 proteins, the genes coding for these two cytochromes are adjacent to genes coding for two membrane-associated FeS proteins, which indicates that they may be part of membrane-bound oxidoreductase complexes. Altogether these observations suggest that the DvH and Da cytochromes are a new type of cytochrome c3 proteins (Type II: TpII-c3) with different redox partners and physiological function than the other cytochrome c3 proteins (Type I: TpI-c3). The DvH TpII-c3 is reduced at considerable rates by the two membrane-bound [NiFe] and [NiFeSe] hydrogenases, but catalytic amounts of TpI-c3 increase these rates two- and fourfold, respectively. With the periplasmic [Fe] hydrogenase TpII-c3 is reduced much slower than TpI-c3, and no catalytic effect of TpI-c3 is observed.

Electrochemical Origin of Hysteresis in the Electron-Transfer Reactions of Adsorbed Proteins: Contrasting Behavior of the “Blue” Copper Protein, Azurin, Adsorbed on Pyrolytic Graphite and Modified Gold Electrodes

Azurin and other small redox proteins exhibit fast electron transfer when adsorbed on a pyrolytic graphite “edge” electrode, but close examination reveals unusual electrochemical behavior that is not predicted by simple models. Cyclic voltammetry over a wide range of scan rates (up to 1000 V s-1) shows that the apparent reduction potential depends on the scan rate and initial polarization potential, and that a small finite peak separation persists in the slowest experiments (1 mV s-1). To determine the origin of these effects, the voltammetric behavior of azurin adsorbed at PGE has been compared with results obtained using gold electrodes modified with a self-assembled monolayer (SAM) of hexanethiol or decanethiol. The contrastingly simple results that are obtained with the SAM electrodes show that the complexities stem from properties of the graphite surface or its interface with the protein. Fast scan cyclic voltammetry, initiated after prepolarizing the graphite electrode over a range of potentials, re...

Standard Apparent Reduction Potentials for Biochemical Half Reactions as a Function of pH and Ionic Strength

Standard apparent reduction potentials are important because they give a more global view of the driving forces for redox reactions than do the standard transformed Gibbs energies of formation of the reactants. This paper emphasizes the effects of pH on biochemical half reactions in the range pH 5 to 9, but it also shows the effect of ionic strength. These effects can be calculated if the pKs of acid groups in the reactants are known in the range pH 4 to 10. Raising the pH decreases the standard apparent reduction potentials of half reactions when it has an effect, and the slope is proportional to minus one times the ratio of the change in binding of hydrogen ions in the half reaction to the number of electrons transferred. These effects are discussed for 19 biochemical reactions. This effect is most striking for the nitrogenase reaction, where the apparent equilibrium constant is proportional to 10−10pH and is unfavorable for nitrogen fixation above pH 8.

The succinate dehydrogenase from the thermohalophilic bacterium Rhodothermus marinus: redox-Bohr effect on heme bL.

The succinate dehydrogenase from the thermohalophilic bacterium Rhodothermus marinus is a member of the succinate:menaquinone oxidoreductases family. It is constituted by three subunits with apparent molecular masses of 70, 32, and 18 kDa. The optimum temperature for succinate dehydrogenase activity is 80 degrees C, higher than the optimum growth temperature of R. marinus, 65 degrees C. The enzyme shows a high affinity for both succinate (Km = 0.165 mM) and fumarate (Km = 0.10 mM). It contains the canonical iron-sulfur centers S1, S2, and S3, as well as two B-type hemes. In contrast to other succinate dehydrogenases, the S3 center has an unusually high reduction potential of +130 mV and is present in two different conformations, one of which presents an unusual EPR signal with g values at 2.035, 2.009, and 2.001. The apparent midpoint reduction potentials of the hemes, +75 and -65 mV at pH 7.5, are also higher than those reported for other enzymes. The heme with the lower potential (heme bL) presents a considerable dependence of the reduction potential with pH (redox-Bohr effect), having a pKa(OX) = 6.5 and a pKa(red) = 8.7. This behavior is consistent with the proposal that in these enzymes menaquinone reduction occurs close to heme bL, near to the periplasmic side of the membrane, and involving dissipation of the proton transmembrane gradient.

Calculating apparent equilibrium constants of enzyme-catalyzed reactions at pH 7

Apparent equilibrium constants K′ of biochemical reactions at pH 7 and standard apparent reduction potentials of half reactions at pH 7 can be calculated using a table of standard transformed Gibbs energies of formation Δ f G′ 0 at pH 7. A table is provided for 136 reactants at 25°C, pH 7, and ionic strengths of 0, 0.10, and 0.25 M. Examples are given to illustrate the use of the table.

Calculating apparent equilibrium constants of enzyme-catalyzed reactions at pH 7

Apparent equilibrium constants K' of biochemical reactions at pH 7 and standard apparent reduction potentials of half reactions at pH 7 can be calculated using a table of standard transformed Gibbs energies of formation Delta(f)G'(0) at pH 7. A table is provided for 136 reactants at 25 degrees C, pH 7, and ionic strengths of 0, 0.10, and 0.25 M. Examples are given to illustrate the use of the table.

Calculation of Standard Transformed Formation Properties of Biochemical Reactants and Standard Apparent Reduction Potentials of Half Reactions

The standard Gibbs energies of formation and standard enthalpies of formation of species involved in biochemical reactions are used to calculate standard transformed Gibbs energies of formation and standard transformed enthalpies of formation of 62 biochemical reactants (sums of species) at 298.15 K, pH 7, and ionic strengths of 0, 0.10, and 0.25 M. It has been possible to put the oxidized and reduced forms of some reactants in this table because their standard apparent reduction potentials are known at pH 7. This paper emphasizes redox reactions. Two applications have been made of these 62 new values of standard transformed Gibbs energies of formation at pH 7: (1) They have been used to calculate standard transformed Gibbs energies of formation of 16 more biochemical reactants from measurements of apparent equilibrium constants of redox reactions.(2) They have been used to calculate standard apparent reduction potentials at pH 7 for half reactions involving reactants discussed in this article and the previous one. This table of standard apparent reduction potentials can be extended considerably from known apparent equilibrium constants for enzyme-catalyzed redox reactions. This brings the total number of reactants for which the standard transformed Gibbs energy of formation at 298K, pH 7, and ionic strengths of 0, 0.10, and 0.25 M have been calculated to 142.

The membrane-bound high-molecular-mass cytochromes c from Desulfovibrio gigas and Desulfovibrio vulgaris Hildenborough; EPR and Mössbauer studies

The high-molecular-mass cytochromes c (Hmcs) from the sulfate-reducing bacteria Desulfovibrio gigas and Desulfovibrio vulgaris (Hildenborough) were found to be strongly bound to the cytoplasmic membrane. After detergent solubilization they were shown to be water soluble and to be similar to those previously isolated from the soluble fractions in terms of N-terminal sequence, molecular mass, UV-visible and EPR spectroscopies. In D. gigas, higher amounts of Hmc can be obtained from the membranes than from the soluble fraction. This enabled further characterization of both cytochromes. The apparent heme reduction potentials of both Hmcs, determined at pH 7.5 through visible and EPR redox titrations, span a large range of redox potentials, approximately between 0 and –280 mV, and can be roughly divided into three groups: four to five hemes have E0s of –30 mV to –100 mV, three to four hemes have E0s around –170 mV, and seven to eight hemes have a lower E0 of –250 to –280 mV. Several of these redox potentials are strongly pH dependent. Mossbauer studies of oxidized and reduced D. vulgaris Hmc show that this protein contains two high-spin hemes in both oxidation states. The rate of reduction of both Hmcs with the periplasmic hydrogenases from the corresponding organisms is extremely slow.

S-Glutathiolated Hepatocyte Proteins and Insulin Disulfides as Substrates for Reduction by Glutaredoxin, Thioredoxin, Protein Disulfide Isomerase, and Glutathione

The disulfide-reducing activities of glutaredoxin, thioredoxin, protein disulfide isomerase, glutathione, and cysteine were directly compared with a mixture of hepatocyte35S-glutathiolated proteins as the substrate. Dethiolation of individual35S-labeled protein bands from the mixture of hepatocyte proteins was analyzed by SDS–PAGE. All of the35S-labeled protein bands could be completely dethiolated by glutaredoxin, thioredoxin, protein disulfide isomerase, glutathione, or cysteine. On a molar basis glutaredoxin was over 10 times more effective than either thioredoxin or protein disulfide isomerase. Dethiolation rates of individual proteins varied in minor ways. For example, glutaredoxin dethiolated the 15-, 30-, and 48-kDa protein bands 3 to 4 times faster than the 27-, 28-, and 77-kDa bands. Glutaredoxins from pig liver or from bovine heart had the same specificity and similar activity. The rate of dethiolation by glutathione alone was low compared to the glutaredoxin-catalyzed process, but all35S-labeled protein bands could be reduced by glutathione, cysteine, or dithiothreitol. Glutathione was clearly more effective than cysteine when these two thiols were compared on the basis of the concentration of thiolate anion available at neutral pH. Therefore, glutathione is a more specific reductant of S-glutathiolated proteins than is cysteine but it is much less effective than glutaredoxin. Since glutaredoxin activity in cells is 10 times higher than the concentration used in these experiments, ample activity is available to account for substantial rates of dethiolationin vivo.Thioredoxin is quite inefficient as a reductant of S-glutathiolated proteins, but it was reasoned that it might first reduce glutaredoxin, which then could reduce the S-glutathiolated protein. A combination of thioredoxin and glutaredoxin was effective. It is proposed that glutaredoxin is the principal agent responsible for protein dethiolationin vivo.The effectiveness of glutaredoxin, thioredoxin, and protein disulfide isomerase as reductants for protein disulfide bonds was examined with insulin as the substrate. Protein disulfide isomerase was very effective and thioredoxin was nearly as effective. Human thioredoxin was similar toEscherichia colithioredoxin in reactivity and specificity. Glutaredoxin did not facilitate insulin reduction at equal concentrations. Thus, protein disulfide isomerase and thioredoxin are more effective than glutaredoxin as reductants of insulin protein disulfides. The apparent reduction potential of pig liver glutaredoxin (−0.159 ± 0.004 V) was determined by measuring the amount of reduced glutaredoxin in equilibrium with mixtures of glutathione and glutathione disulfide. Glutaredoxin was a weaker reductant thanE. colithioredoxin (−0.260 V) and was similar to protein disulfide isomerase (−0.11 to −0.19 V). The role of these proteins as disulfide reductants is not determined solely by thermodynamic considerations. A glutathione binding site at the dithiol region of glutaredoxin may be of primary importance for its function in protein dethiolation, while a different specific peptide binding site in thioredoxin may be more suited to certain protein disulfide structures.

A Consideration of the Indirect Electrooxidation of Alkylbenzenes by Metal-Ion Mediators

Abstract Selective oxidation of substituted alkylbenzenes to the corresponding aromatic aldehydes by metal-ion redox mediators has been investigated. The apparent oxidation potential of the substrate alkylbenzene (Es,ox) measured by voltammetry in an organic medium was used as a measure of the substrate reactivity. The oxidizing power of the mediator was also evaluated based on the apparent reduction potential in the voltammogram (Ered). The yields of the experimentally determined reaction products were explained by the mutual relation between Es,ox of the substrate and Ered of the mediator.

Elementary reconstitution of the water splitting light reaction in photosynthesis. 3. Photooxidative properties of chlorophyll dihydrate on metal as catalyst for water photolysis

The photooxidative properties of chlorophyll a dihydrate as photocatalyst for the water splitting reaction are examined using Pt and other metals as nonbiological catalysts for reaction product formation. The photoelectrodeposition of (Chl a 2H/sub 2/O)/sub n/ microcrystals from n-pentane suspensions on Pt electrodes is described. The formation of a contiguous multilayer of (Chl a 2H/sub 2/O)/sub n/ on Pt is examined by means of electron microscopy. The effect of an applied voltage on the electrodeposition process is delineated in terms of the amount of Chl a plated and the photocathodic activity of the resulting Pt/Chl a electrode. A quantum efficiency of 0.88% was obtained at the red maximum, 735 nm, of the photogalvanic action spectrum. The catalytic function of the metal surface is examined by determining the activity of a Pt/Chl a H/sub 2/O photocathode against a series of different counterelectrodes (anodes), viz., Pt, Cu, Fe, and Ni. The order Cu > Ni > Fe > platinized Pt > shiny Pt is established for the relative effectiveness of these metals as counterelectrodes at photocurrent densities greater than or equal to 10/sup -1/ ..mu..A/cm/sup 2/ in oxygen-depleted aqueous solutions. The observation of light-induced decay and dark restoration of the photogalvanic activitymore » is numerically fitted to a kinetic model for gaseous product evolution and removal at the electrodes. The apparent reduction potential for a 1:3 mixture of (Chl a 2H/sub 2/Oreverse arrow/sub 2//sup +/. and (Chl a-2H/sub 2/O)/sub 6//sup +/ . responsible for the observed oxygen evolution from water splitting is determined by potentiostatic and electron spin resonance measurements to be 1.05 V. This observation is attributable to the reduction potential of (Chl a-2H/sub 2/O)/sub n//sup +/. 11 figures.« less

Role of polymer ligands in the oxygenation of metal chlorophyllins

AbstractIn order to obtain information about the characteristics of polymer‐(metal chlorophyllins), their redox and oxygenation reactions were studied. Complexes composed of metal chlorophyllins (MChn) and partially quaternized poly(4‐vinylpyridine) (QPVP) exhibited an increased stability due to electrostatic interaction between polymer cation and carboxylate anions of side groups in the MChn. The reduction of these polymer complexes by S2O42‐ occurred easily with an decrease in their apparent reduction potentials. The resulting low‐valence complexes had an ability to absorb molecular oxygen. The oxygenation of these polymer complexes was not completely reversible; however the rate of the irreversible reoxidation was less by a factor of 1010 for the polymeric complex than that for the monomeric MChn‐pyridine complex. The stability of these oxygen complexes is discussed in relation to the hydrophobicity of QPVPs.

Monodispersed tantalum disulfide and adsorption complexes with cations

Cathodic electrolysis of 2H⋅⋅⋅TaS2 in a dilute sulfuric acid electrolyte results in intercalation of hydrogen and water. The water content increases as the ionic strength of the electrolyte is decreased. Addition of neutral surfactants (1%−10% Triton X−705 or glycerol) followed by high−speed stirring gave TaS2 layers monodispersed in the aqueous surfactant. Electronic absorption spectra of the dispersions agree well with spectra of cleaved solids. Dyes were used to study adsorption on the layers. Only cationic dyes were adsorbed and the adsorption was determined to be due to an ion exchange reaction with protons. Solid adsorption complexes were prepared by ultracentrifugation of the dispersion or by adding dyes to undispersed, water swollen TaS2. The solid adsorption complexes gave no x−ray powder patterns, and were deficient in sulfur. Ac susceptibility measurements did not indicate bulk superconductivity in any of the complexes. Electron transfer from the TaS2 layers to reducible dyes was observed, the apparent reduction potential being 0.25 V vs NHE.