The effect of pH on the inorganic carbon source for photosynthesis in the seagrass Zostera muelleri irmisch ex aschers

The ability of the seagrass Zostera muelleri Irmisch ex Aschers. to use HCO₃⁻ as well as CO₂ for photosynthesis was investigated by measuring photosynthetic O₂ evolution over a range of pH values. It was found that the apparent K_m CO₂ fell from 0.128 mM at pH 7.9 to 0.016 mM at pH 9.1 indicating that HCO₃⁻ as well as CO₂ may act as a substrate for photosynthesis.The true K_m CO₂ could not be determined due to inhibition of photosynthesis at pHs less than 7.8 K_m CO₂ must be at least 0.128 mM, the apparent K_m at pH 7.9, and is probably of the order of 0.200 mM CO₂, the same as that reported for other marine plants. K_m HCO₃⁻¹ is about 20 mM when CO₂-dependent photosynthesis is minimal. Such a high K_m HCO₃⁻ resembles values reported for freshwater, rather than marine plants.Photosynthetic O₂ evolution is not saturated with respect to total inorganic carbon in natural seawater (pH 8.2). It is suggested that the distinctive shoulder from pH 8.1 to 8.5 in the pH profile of photosynthetic O₂ evolution at a constant concentration of inorganic carbon is caused by an effect of pH on HCO₃⁻ uptake. The effect of pH on HCO₃⁻ uptake was determined by constructing a pH profile of photosynthesis at constant HCO₃⁻ concentration, and subtracting the estimated contribution of CO₂ to photosynthesis from this rate. The resultant curve has a maximum at pH 8.4 and declines sharply at pHs less than 8.     

Resveratrol and para-coumarate serve as ring precursors for coenzyme Q biosynthesis

Coenzyme Q (Q or ubiquinone) is a redox-active polyisoprenylated benzoquinone lipid essential for electron and proton transport in the mitochondrial respiratory chain. The aromatic ring 4-hydroxybenzoic acid (4HB) is commonly depicted as the sole aromatic ring precursor in Q biosynthesis despite the recent finding that para-aminobenzoic acid (pABA) also serves as a ring precursor in Saccharomyces cerevisiae Q biosynthesis. In this study, we employed aromatic ¹³C₆-ring-labeled compounds including ¹³C₆-4HB, ¹³C₆-pABA, ¹³C₆-resveratrol, and ¹³C₆-coumarate to investigate the role of these small molecules as aromatic ring precursors in Q biosynthesis in Escherichia coli, S. cerevisiae, and human and mouse cells. In contrast to S. cerevisiae, neither E. coli nor the mammalian cells tested were able to form ¹³C₆-Q when cultured in the presence of ¹³C₆-pABA. However, E. coli cells treated with ¹³C₆-pABA generated ¹³C₆-ring-labeled forms of 3-octaprenyl-4-aminobenzoic acid, 2-octaprenyl-aniline, and 3-octaprenyl-2-aminophenol, suggesting UbiA, UbiD, UbiX, and UbiI are capable of using pABA or pABA-derived intermediates as substrates. E. coli, S. cerevisiae, and human and mouse cells cultured in the presence of ¹³C₆-resveratrol or ¹³C₆-coumarate were able to synthesize ¹³C₆-Q. Future evaluation of the physiological and pharmacological responses to dietary polyphenols should consider their metabolism to Q.     

Thermodynamics of Formate-Oxidizing Metabolism and Implications for H2 Production

Formate-dependent proton reduction to H₂ (HCOO⁻ + H₂O → HCO₃⁻ + H₂) has been reported for hyperthermophilic Thermococcus strains. In this study, a hyperthermophilic archaeon, Thermococcus onnurineus strain NA1, yielded H₂ accumulation to a partial pressure of 1 × 10⁵ to 7 × 10⁵ Pa until the values of Gibbs free energy change (ΔG) reached near thermodynamic equilibrium (−1 to −3 kJ mol⁻¹). The bioenergetic requirement for the metabolism to conserve energy was demonstrated by ΔG values as small as −5 kJ mol⁻¹, which are less than the biological minimum energy quantum, −20 kJ mol⁻¹, as calculated by Schink (B. Schink, Microbiol. Mol. Biol. Rev. 61:262-280, 1997). Considering formate as a possible H₂ storage material, the H₂ production potential of the strain was assessed. The volumetric H₂ production rate increased linearly with increasing cell density, leading to 2,820 mmol liter⁻¹ h⁻¹ at an optical density at 600 nm (OD₆₀₀) of 18.6, and resulted in the high specific H₂ production rates of 404 ± 6 mmol g⁻¹ h⁻¹. The H₂ productivity indicates the great potential of T. onnurineus strain NA1 for practical application in comparison with H₂-producing microbes. Our result demonstrates that T. onnurineus strain NA1 has a highly efficient metabolic system to thrive on formate in hydrothermal systems.     

The effects of external electric field: creating non-zero first hyperpolarizability for centrosymmetric benzene and strongly enhancing first hyperpolarizability for non-centrosymmetric edge-modified graphene ribbon H2N-(3,3)ZGNR-NO2

How to generate a non-zero first hyperpolarizability for a centrosymmetric molecule is a challenging question. In this paper, an external (pump) electric field is used to make a centrosymmetric benzene molecule generate a non-zero value of the electric field induced first hyperpolarizability (β ^F ). This comes from the centrosymmetry breaking of electron cloud. Two interesting rules are exhibited. (1) β ^F is anisotropic for different directional fields (F _i, i?=?X, Y, Z). (2) The field dependence of β ^F is a non-monotonic function, and an optimum external electric field causes the maximum value of β ^F . The largest first hyperpolarizability β ^F reaches the considerable level of 3.9?×?10⁵ a.u. under F _Y?=?330?×?10^?4 a.u. for benzene. The external electric field effects on non-centrosymmetric edge-modified graphene ribbon H₂N-(3,3)ZGNR-NO₂ was also studied in this work. The first hyperpolarizability reaches as much as 2.1?×?10⁷ a.u. under F _X?=?600?×?10^?4 a.u. for H₂N-(3,3)ZGNR-NO₂. We show that the external electric field can not only create a non-zero first hyperpolarizability for centrosymmetric molecule, but also remarkably enhance the first hyperpolarizability for a non-centrosymmetric molecule.     

Enrichment for Hydrogen-Oxidizing Acinetobacter spp. in the Rhizosphere of Hydrogen-Evolving Soybean Root Nodules

Field soybean plants were inoculated with Hup⁺ wild-type or H₂ uptake-negative (Hup⁻) mutants of Bradyrhizobium japonicum. For two consecutive summers we found an enrichment for acinetobacters associated with the surfaces of the H₂-evolving nodules. Soybean root nodules that evolved H₂ had up to 12 times more Acinetobacter spp. bacteria associated with their surfaces than did nodules incapable of evolving H₂. All of the newly isolated strains identified as Acinetobacter obtained from the surfaces of root nodules, as well as known established Acinetobacter strains, were capable of oxidizing H₂, a property not previously described for this alkane-degrading soil bacterium.     

Polyguaiacol: a Useful Model Polymer for Lignin Biodegradation Research

A polymer of ring-labeled [¹⁴C]o-methoxyphenol ([¹⁴C]guaiacol) was prepared by peroxidase-H₂O₂-catalyzed oxidation of the ¹⁴C-labeled monomeric compound. The ring-labeled [¹⁴C]polyguaiacol contained 67.71% carbon, 5.09% hydrogen, 27.49% oxygen, 25.44% methoxyl, and 8.60% phenolic hydroxyl. The polymer had an average molecular weight of between 5,000 and 15,000, as determined by gel chromatography. A schematic representation of the polymer, similar to previously published structures of polyguaiacols, was devised to meet these and other analytical parameters. The polymer is primarily composed of o-o and p-p-linked guaiacol moieties, with an occasional o-p-biphenyl link and some p-diphenoquinone structures. An approximate molecular formula is [C₄₉O₁₄H₃₁]_n, where n 5.8. Its C₆ formula is C₆H_2.3O_0.3^carbonyl (OH)_0.7(OCH₃)_1.0. Polyguaiacol has many of the characteristics of a synthetic lignin. It is easier and less expensive to prepare than standard synthetic lignins (dehydrogenation polymers of coniferyl alcohol). It is degraded ([¹⁴C]polyguaiacol → ¹⁴CO₂) by the lignolytic system of the white-rot fungus Phanaerochaete chrysosporium. It is suggested that [¹⁴C]polyguaiacol may be of value as a substrate for lignin biodegradation research.     

Conformational requirements for the polymerization of hemoglobin S: studies of mixed liganded hybrids

Gelation experiments with artificially formed half-liganded hybrid tetramers of hemoglobin S demonstrate that when either the α chains or the β^s chains are fixed in the cyanmet (CNmet) liganded state, gelation occurs upon deoxygenation of the ferrous chains. The minimum concentration of hemoglobin required for gelation is equivalent for both hybrids (α₂^cnmetβ₂^s and α₂β₂^scnmet), is considerably higher than the concentration required to gel deoxy-Hb S (α₂β₂^s), and can be restored to the lower minimum gelling point of α₂β₂^s by reduction of the CNmet chains with dithionite. These results suggest that the most important conformational determinant of the deoxy state for polymerization of Hb S is the quaternary deoxy structure rather than the tertiary structural effect of the ligand state of the α or the β^s chains, and are furthermore consistent with the notion that asymmetric deoxy-CNmet hybrid tetramers assume a conformation which resembles, but is not identical to that of deoxyhemoglobin.The results of gelation experiments with mixtures of hemoglobins S and A in which selected chains of one or both hemoglobins are in the CNmet form support the concept that certain non-S hemoglobins may participate in the sickling process by forming hybrid tetramers with Hb S (such as α₂β^aβ^s). The conformational requirement for participation of these hybrids in polymers also appears to be a quaternary deoxy-like structure.     

Single-trait and antagonistic index selection for litter size and body weight in mice

Individual selection based on female performance only was conducted in four lines of mice: L⁺ for increased litter size, W⁺ for increased 6-week body weight, L^-W⁺ for a selection index aimed at decreasing litter size and increasing 6-week body weight and L⁺W^- for a selection index aimed at increasing litter size and decreasing 6-week body weight. A fifth line (K) served as an unselected control. All litters were standardized to eight mice at one day of age. Expected heritability was based on twice the regression of offspring on dam (h²_d), which contains additive genetic variance due to direct (σ²_Ao) and maternal (σ²_Am) effects and their covariance (σ_AoAm). Responses and correlated responses were measured either deviated (method 1) or not deviated (method 2) from the control line. Realized heritabilities (h²_R) for litter size were 0.19 ± 0.04 (1) and 0.16 ± 0.03 (2), which were similar to h ²_d of 0.17 ± 0.04. The h² _R for 6-week body weight of 0.55 ± 0.07 (1) and 0.44 ± 0.07 (2) agreed with h²_d of 0.42 ± 0.02. Realized genetic correlations (r*_GR) between litter size and 6-week body weight calculated from the double-selection experiment were 0.52 ± 0.10 (1) and 0.52 ± 0.13 (2), which were not significantly different from the base population estimate of r* _Gd = 0.63 ± 0.14. Divergence (L^-W ⁺ minus L⁺W^-) in the antagonistic index selection lines was 0.21 ± 0.01 index units (I = 0.305 P_W - 0.436 P_L, where P _W and P_L are the phenotypic values for 6-week body weight and litter size, respectively.). The h² _R of index units of 0.14 ± 0.02 calculated from divergence agreed with h²_d of 0.14 ± 0.04. Divergences in litter size (-0.19 ± 0.07) and 6-week body weight (0.46 ± 0.10) were in the expected direction. Antagonistic index selection yielded about one-half the expected divergence in litter size, while divergence in 6-week body weight was only slightly less than expected. Realized genetic correlations indicated that litter size, 6-week body weight and index units each showed positive pleiotropy with 3-week body weight, postweaning gain and weight at vaginal introitus and negative pleiotropy with age at vaginal introitus. Sex ratio and several components of fitness (days from joining to parturition, percent fertile matings and percent perinatal survival) did not change significantly in the selected lines.     

Functional Identification of a Hydroxyproline-O-galactosyltransferase Specific for Arabinogalactan Protein Biosynthesis in Arabidopsis

Although plants contain substantial amounts of arabinogalactan proteins (AGPs), the enzymes responsible for AGP glycosylation are largely unknown. Bioinformatics indicated that AGP galactosyltransferases (GALTs) are members of the carbohydrate-active enzyme glycosyltransferase (GT) 31 family (CAZy GT31) involved in N- and O-glycosylation. Six Arabidopsis GT31 members were expressed in Pichia pastoris and tested for enzyme activity. The At4g21060 gene (named AtGALT2) was found to encode activity for adding galactose (Gal) to hydroxyproline (Hyp) in AGP protein backbones. AtGALT2 specifically catalyzed incorporation of [¹⁴C]Gal from UDP-[¹⁴C]Gal to Hyp of model substrate acceptors having AGP peptide sequences, consisting of non-contiguous Hyp residues, such as (Ala-Hyp) repetitive units exemplified by chemically synthesized (AO)₇ and anhydrous hydrogen fluoride-deglycosylated d(AO)₅₁. Microsomal preparations from Pichia cells expressing AtGALT2 incorporated [¹⁴C]Gal to (AO)₇, and the resulting product co-eluted with (AO)₇ by reverse-phase HPLC. Acid hydrolysis of the [¹⁴C]Gal-(AO)₇ product released ¹⁴C-radiolabel as Gal only. Base hydrolysis of the [¹⁴C]Gal-(AO)₇ product released a ¹⁴C-radiolabeled fragment that co-eluted with a Hyp-Gal standard after high performance anion-exchange chromatography fractionation. AtGALT2 is specific for AGPs because substrates lacking AGP peptide sequences did not act as acceptors. Moreover, AtGALT2 uses only UDP-Gal as the substrate donor and requires Mg²⁺ or Mn²⁺ for high activity. Additional support that AtGALT2 encodes an AGP GALT was provided by two allelic AtGALT2 knock-out mutants, which demonstrated lower GALT activities and reductions in β-Yariv-precipitated AGPs compared with wild type plants. Confocal microscopic analysis of fluorescently tagged AtGALT2 in tobacco epidermal cells indicated that AtGALT2 is probably localized in the endomembrane system consistent with its function.     

Evidence for electrogenic bicarbonate transport in Amphiuma small intestine

Isolated segments of small intestine of Amphiuma were short-circuited in buffer containing bicarbonate. Theophylline (10 mM) increased short circuit current (I_sc) in proportion to the bicarbonate concentration in the bath. The theophylline-stimulated I_sc was rapidly reduced, though not abolished, in the presence of acetazolamide at concentrations as low as 10^?6 M. Unidirectional fluxes of ²²Na and ²⁴Na in paired intestinal segments in Cl-free buffer reveal that the increase in I_sc produced by theophylline is not accounted for by an increase in net sodium flux. These results suggest that theophylline stimulates an electrogenic secretion of bicarbonate.     

Triaryl (Z)-olefins suitable for radiolabeling with carbon-11 or fluorine-18 radionuclides for positron emission tomography imaging of cyclooxygenase-2 expression in pathological disease

A group of (Z)-1,1-diphenyl-2-(4-methylsulfonylphenyl)alk-1-enes were synthesized using methodologies that will allow incorporation of a [¹¹C]OCH₃ substituent at the para-position of the C-1 phenyl ring, a [¹¹C]SO₂CH₃ substituent at the para-position of the C-2 phenyl ring, a [¹⁸F]OCH₂CH₂F substituent at the para-position of the C-1 phenyl ring, and a [¹⁸F]CH₂CH₂F substituent at the C-2 position of the olefinic bond. The [¹¹C] and [¹⁸F] radiotracers are designed as potential radiopharmaceuticals to image cyclooxygenase-2 (COX-2) expression in any organ where COX-2 is upregulated. The COX-1/COX-2 inhibition data acquired suggest that compounds having a [¹¹C]OMe or [¹⁸F]OCH₂CH₂F substituent at the para-position of the C-1 phenyl ring may be more suitable for imaging COX-2 expression in view of their ability to exclusively inhibit the COX-2 isozyme.     

Carbonic acid as the host signal for the development of parasitic stages of nematodes

Petronijevic T., Rogers W. P. and Sommerville R. I. 1985. Carbonic acid as the host signal for the development of parasitic stages of nematodes. International Journal for Parasitology15: 661–667. This paper gives results on which may be based an identification of the component of the system CO₂ + H₂O ai H₂CO₃ ai H⁺ HCO₃^? which acts as the stimulus from the animal host for some nematodes. Using infective juveniles of Nematospiroides dubius and Haemonchus contortus, the effects on exsheathment of (1) low pCO₂ values, (2) the presence of carbonic anhydrase in the stimulating medium, and (3) the inhibition of carbonic anhydrase within the juveniles have been examined. The results lead to the suggestion that it is the “readily available” undissociated H₂CO₃, or H₂CO₃ + HCO₃^? which is the critical factor in the stimulus for development. The wide range of [H⁺]s over which “readily available” H₂CO₃ is present in physiological environments suggests that this host signal may be important for infection with many species.     

Competition by 4-chloronitrosobenzene for the active glycolaldehyde intermediate of transketolase

The incubation of 4-chloronitrosobenzene with yeast transketolase, Mg²⁺, and thiamime pyrophosphate in the presence of excess xylulose-5-phosphate resulted in the formation of N-(4-chlorophenyl)glycolhydroxamic acid. This enzyme-catalyzed C₂ transfer displayed a K_m of 0.92 mM and a V_max of 5.2 × 10^?2 μmol min^?1 unit enzyme^?1. Conversion was inhibited by the normal acceptor sugar, ribose-5-phosphate, with a K_i of 0.35 mM. Kinetic analysis showed inhibition was competitive in nature, reinforcing the proposed theory for similarity in catalytic formation of both the hydroxamic acid and sedoheptulose-7-phosphate. Most interesting about the conversion of this alternative substrate is that even at high concentrations of ribose-5-phosphate, a significant amount of the nitroso compound was converted to the hydroxamic acid, implying that 4-chloronitrosobenzene can successfully compete for active glycoaldehyde. Using the yeast enzyme as a model for transketolase in higher organisms, the adventitious conversion of such xenobiotics in vivo is proposed.     

A new solid-state microelectrode for measuring intra-cellular chloride activities

Solid-state microelectrodes for measuring intracellular Cl^? activity (aⁱCl) were made by sealing the tips of tapered glass capillaries (tip diameter 0.3 μm), coating them under vacuum with a 0.2–0.3 μm thick layer of spectroscopic grade silver, and sealing them (except for the terminal 2–5 μm of the tip) inside tapered glass shields. 106 microelectrodes had an average slope of 55.0 ± 0.6 mV (S.E.) per decade change in α_Cl. Tip resistance was (77.1 ± 3.1 × 10⁹ω (n=30). Electrode response was rapid (10–20 s), was unaffected by HCO₃^?, H₂PO₄^2? or protein, and remained essentially unchanged over a 24-h period. αⁱ_Cl in frog sartorius muscle fibers and epithelial cells of bullfrog small intestine was measured in vitro. In both tissues, αⁱ_Cl significantly exceeded the value corresponding to equilibrium distribution of Cl^? across the cell membrane.     

Heart mass and the maximum cardiac output of birds and mammals: implications for estimating the maximum aerobic power input of flying animals

Empirical studies of cardiovascular variables suggest that relative heart muscle mass (relative M_h) is a good indicator of the degree of adaptive specialization for prolonged locomotor activities, for both birds and mammals. Reasonable predictions for the maximum oxygen consumption of birds during flight can be obtained by assuming that avian heart muscle has the same maximum physiological and biomechanical performance as that of terrestrial mammals. Thus, data on M_h can be used to provide quantitative estimates for the maximum aerobic power input (aerobic P_i,max) available to animals during intense levels of locomotor activity. The maximum cardiac output of birds and mammals is calculated to scale with respect to M_h (g) as 213 M_h^0.88+-0.04 (ml min^-1), while aerobic P_i,max is estimated to scale approximately as 11 M_h^0.88+-0.09 (W). In general, estimated inter-species aerobic P_i,max, based on M_h for all bird species (excluding hummingbirds), is calculated to scale with respect to body mass (M_b in kg) as 81 M_b^0.82+-0.11 (W). Comparison of family means for M_h indicate that there is considerable diversity in aerobic capacity among birds and mammals, for example, among the medium to large species of birds the Tinamidae have the smallest relative M_h (0.25 per cent) while the Otidae have unusually large relative M_h (1.6 per cent). Hummingbirds have extremely large relative M_h (2.28 per cent), but exhibit significant sexual dimorphism in their scaling of M_h and flight muscle mass, so that when considering hummingbird flight performance it may be useful to control for sexual differences in morphology. The estimated scaling of aerobic P_i,max (based on M_h and M_b in g) for male and female hummingbirds is 0.51 M_b^{0.83 +/-0.07} and 0.44 M_b^{0.85+- 0.11} (W), respectively. Locomotory muscles are dynamic structures and it might be anticipated that where additional energetic ''costs'' occur seasonally (e.g. due to migratory fattening or the development of large secondary sexual characteristics) then the relevant cardiac and locomotor musculature might also be regulated seasonally. This is an important consideration, both due to the intrinsic interest of studying muscular adaptation to changes in energy demand, but also as a confounding variable in the practical use of heart rate to estimate the energetics of animals. Haemoglobin concentration (or haematocrit) may also be a confounding variable. Thus, it is concluded that data on the cardiovascular and flight muscle morphology of animals provides essential information regarding the behavioural, ecological and physiological significance of the flight performance of animals.     

99mTc-tricarbonyl labeled agents for cell labeling: Development,biodistribution in normal mice and preliminary in vitro evaluation

We have developed four ^99mTc(CO)₃-labeled lipophilic tracers as potential radiolabeling agents for cells based on a hexadecyl tail. ^99mTc(CO)₃-hexadecylamino-N,N′-diacetic acid (negatively charged), ^99mTc(CO)₃-hexadecylamino-N-α-picolyl-N′-acetic acid (uncharged), ^99mTc(CO)₃-N,N′-dipicolylhexadecylamine (positively charged), ^99mTc(CO)₃-N-hexadecylaminoethyl-N′-aminoethylamine (positively charged) were prepared in a radiolabeling yield: >90%. Preliminary cell uptake studies were performed in mixed blood cells with or without plasma and were compared with ^99mTc-d,l-HMPAO and [¹⁸F]FDG. In plasma-free blood cells, maximum uptake (78%) was obtained for ^99mTc(CO)₃-N-hexadecylaminoethyl-N′-aminoethylamine after 60 min incubation (compared to 55% and 23% for ^99mTc-d,l-HMPAO and [¹⁸F]FDG, respectively) while in plasma-rich medium, ^99mTc(CO)₃-N,N′-dipicolylhexadecylamine was best bound (54%, similar to the binding of ^99mTc-d,l-HMPAO). Biodistribution in normal mice showed mainly hepatobiliary clearance of the agents and initial high lung uptake. The radiolabeled compounds showed good blood clearance with maximally 7.9% injected dose per gram at 60 min post injection. While the least lipophilic agent (^99mTc(CO)₃-N,N′-dipicolylhexadecylamine, log P = 1.3) showed the best cell uptake, there appears to be no direct correlation between lipophilicity and tracer uptake in mixed blood cells. In view of its comparable cell uptake to well known cell labeling agent ^99mTc-d,l-HMPAO, ^99mTc(CO)₃-N,N′-dipicolylhexadecylamine merits further evaluation as a potential cell labeling agent.     

Novel Processes for Anaerobic Sulfate Production from Elemental Sulfur by Sulfate-Reducing Bacteria

Sulfate reducers and related organisms which had previously been found to reduce Fe(III) with H₂ or organic electron donors oxidized S⁰ to sulfate when Mn(IV) was provided as an electron acceptor. Organisms catalyzing this reaction in washed cell suspensions included Desulfovibrio desulfuricans, Desulfomicrobium baculatum, Desulfobacterium autotrophicum, Desulfuromonas acetoxidans, and Geobacter metallireducens. These organisms produced little or no sulfate from S⁰ with Fe(III) as a potential electron acceptor or in the absence of an electron acceptor. In detailed studies with Desulfovibrio desulfuricans, the stoichiometry of sulfate and Mn(II) production was consistent with the reaction S⁰ + 3 MnO₂ + 4H⁺→SO₄^2- + 3Mn(II) + 2H₂O. None of the organisms evaluated could be grown with S⁰ as the sole electron donor and Mn(IV) as the electron acceptor. In contrast to the other sulfate reducers evaluated, Desulfobulbus propionicus produced sulfate from S⁰ in the absence of an electron acceptor and Fe(III) oxide stimulated sulfate production. Sulfide also accumulated in the absence of Mn(IV) or Fe(III). The stoichiometry of sulfate and sulfide production indicated that Desulfobulbus propionicus disproportionates S⁰ as follows: 4S⁰ + 4H₂O→SO₄^2- + 3HS^- + 5 H⁺. Growth of Desulfobulbus propionicus with S⁰ as the electron donor and Fe(III) as a sulfide sink and/or electron acceptor was very slow. The S⁰ oxidation coupled to Mn(IV) reduction described here provides a potential explanation for the Mn(IV)-dependent sulfate production that previous studies have observed in anoxic marine sediments. Desulfobulbus propionicus is the first example of a pure culture known to disproportionate S⁰.     

Is it always correct to use the Marcus cross relation for calculations of electron self-exchange rates?

The rate constant of the electron self-exchange reaction, which proceeds via the outer sphere mechanism, k₁₁, of a redox couple, reflects the basic tendency of the reaction to participate in redox processes. Often k₁₁ is derived from the rate constant of a redox reaction, k₁₂, by applying the Marcus cross relation: k₁₂ = (k₁₁k₂₂K₁₂f₁₂)^1/2W₁₂. This derivation is based on the assumption that the products of the cross reaction are formed in their ground states. However, the k₁₁ values obtained by this method for several redox systems, e.g. for [Co(NH₃)₆]^3+/2+, Cu^2+/+(aq) and Eu^3+/2+(aq), depend strongly on the redox reaction studied for its derivation. It is proposed that these discrepancies are due to the formation, in some of these reactions, of products in vibrationally excited states and/or as isomers of the final products. Thus, the lowest value of k₁₁ obtained experimentally is either correct or an upper limit for the correct value if correct k₂₂ are used.     

Preparation and solution properties of pullulan fractions as standard samples for water-soluble polymers

A series of pullulan fractions with molecular weights in the range 5 × 10³ to 8 × 10⁵ were prepared. The weight-average molecular weight (M_w) of all the samples was determined by sedimentation equilibrium. The hydrodynamic properties of pullulan in aqueous solution were investigated by viscometry and ultracentrifugation. The experimental results indicate that pullulan molecules in water are fairly stable and behave as expanded random coils when M_w is above 2 × 10⁴. The molecular weight distributions of the fractions were measured by gel filtration. The ratio M_w/M_n was close to 1·1, except for a sample with the highest M_w.It is concluded that the pullulan fractions prepared by the present work are well characterized and have a narrow molecular weight distribution. They may be useful as standard samples for studies of water-soluble polymers.     

Development of cesium phosphotungstate salt and chitosan composite membrane for direct methanol fuel cells

A novel composite membrane has been developed by doping cesium phosphotungstate salt (Cs_xH_3−xPW₁₂O₄₀ (0 ≤ x ≤3), Cs_x-PTA) into chitosan (CTS/Cs_x-PTA) for application in direct methanol fuel cells (DMFCs). Uniform distribution of Cs_x-PTA nanoparticles has been achieved in the chitosan matrix. The proton conductivity of the composite membrane is significantly affected by the Cs_x-PTA content in the composite membrane as well as the Cs substitution in PTA. The highest proton conductivity for the CTS/Cs_x-PTA membranes was obtained with x = 2 and Cs₂-PTA content of 5 wt%. The value is 6 × 10⁻³ S cm⁻¹ and 1.75 × 10⁻² S cm⁻¹ at 298 K and 353 K, respectively. The methanol permeability of CTS/Cs₂-PTA membrane is about 5.6 × 10⁻⁷, 90% lower than that of Nafion-212 membrane. The highest selectivity factor (φ) was obtained on CTS/Cs₂-PTA-5 wt% composite membrane, 1.1 × 10⁴/S cm⁻³ s. The present study indicates the promising potential of CTS/Cs_x-PTA composite membrane as alternative proton exchange membranes in direct methanol fuel cells.