Fate of elemental sulfur in an intertidal sediment

Abstract: Sediment from a tidal flat at Wedderwarden, near the mouth of the Weser estuary, northern Germany, was amended with elemental sulfur, and concentrations of metabolic end products were monitored. The production of both sulfate and sulfide was consistent with disproportionation as the most important fate of the added elemental sulfur. A population of bacteria conducting active elemental sulfur disproportionation was also enriched from the sediment. In the enrichments, containing both elemental sulfur and Fe oxides as a sulfide 'scrub', sulfide and sulfate were produced in a ratio of     , somewhat lower than the predicted ratio of     . The mismatch between predicted and observed production ratios is explained by the channelling of electrons into autotrophic or mixotrophic CO₂ fixation rather than sulfide formation. The production of organic carbon, in the correct amount to explain the observed sulfide to sulfate production ratio, was verified by organic carbon analysis. Finally, rates of sulfate reduction were identical in the elemental sulfur amended sediment, and in control sediment with no added sulfur. Hence, the heterotrophic bacterial community was completely unaffected by an active metabolism conducting elemental sulfur disproportionation.     

Gas-phase disproportionation of nitric oxide at elevated pressures

T.P. Melia's chemical kinetics study of the disproportionation of nitric oxide (NO), 3NO →NO₂ + N₂O , (Melia, T.P. (1965) J. Inorg. Nucl. Chem., 27, 95-98), which is the most quoted quantitative investigation presently available, revealed a rather strong dependence of the effective rate constant, Kk ', of Melia's third-order rate law,- d[NO]/d t = Kk'[NO]₃, on the initial pressure of NO. In order to estimate extent of accumulation of NO₂ and N₂O as a function of time by integration of the rate law, we have evaluated the dependence of the effective rate constant as a function of pressure and thus as a function of time on the basis of the non-ideality of NO gas. Although our approach is crude in that the non-idealities of NO₂ and N₂O and other NO_x products and a probable deviation of the gas mixture from the Dalton's law have not been considered, it provides a means for approximately estimating the rate of accumulation of NO₂ and N₂O based on Melia's data. According to these calculations, the extent of the disproportionation is generally negligible at low initial pressures, e.g. 5 atm or less, while at 200 atm, the mole fractions of NO₂ and N₂O can become as high as 12-13% only after 10 days. These values are alarmingly high for handling pressured NO- in N₂-mixture in either research or clinical settings. This information must be borne in mind when compressed NO in commercial cylinders is employed in high precision experiments. Disproportionation of NO under pressure also deserves attention in inhalation of low doses of NO in the treatment of diseases characterized by pulmonary hypertension and hypoxemia.     

Hypotaurine disproportionation reaction: Identification of products

Summary Hypotaurine, concentrated under reduced pressure in HCl solution, partially (30–40%) degrades into taurine (about 30%), 2-aminoethyl-2-aminoethanethiolsulfonate (about 10%) and ethanolamine. The degradation products were identified using LC/APCI-MS, NMR, amino acid analysis and various chromatographics. The identities were confirmed by comparing the HPLS, MS and NMR characteristics of authentic compounds. One of the degradation processes during concentration of HCl solution of hypotaurine is therefore a disproportionation reaction which can interfere with the experimental results, when studying hypotaurine in biological systems.Abbreviations LC/APCI-MS liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry - TLC thin layer chromatography - DTNB 5,5-dithiobis(2-nitrobenzoic acid)     

A dithiocarbamate-stabilized copper(I) cube

The disproportionation reaction between the copper(II) complexes, Cu(ClO₄)₂ · 6H₂O and [Cu(S₂CNR₂)₂] is a well-established route to copper(III) complexes [Cu(S₂CNR₂)₂][ClO₄] but to date the nature of the copper(I) species generated has remained a mystery. We now show that with [Cu(S₂CNPr₂)₂] this is the copper(I) cluster, [Cu₈(μ²,μ²-S₂CNPr₂)₆][ClO₄]₂, which contains a cubic array of copper atoms, each face cube being capped by a dithiocarbamate ligand such that the sulfur atoms define an icosahedron and the backbone carbons an octahedron around the cube centroid. A crystal structure of [Cu₄(μ²,η¹-S₂CNBu₂)₄] is also presented for comparison.     

Trans-pyridine tetraammine complexes of RuII and RuIII with N7-coordninated purine nucleosides

 The synthesis, spectroscopic, and electrochemical properties of trans-[L(Pyr)(NH₃)₄Ru^II/III] (Pyr=py, 3-phpy, 4-phpy, 3-bnpy, or 4-bnpy; L=H₂O, Guo, dGuo, 1MeGuo, Gua, Ino, or G⁷-DNA) are reported. As expected, the Pyr ligand slows DNA binding by trans-[(H₂O)(Pyr)(NH₃)₄Ru^II]²⁺ relative to [(H₂O)(NH₃)₅Ru^II]²⁺ and favors reduction of Ru^III by about 150 mV. The pyridine ligand also promotes the disproportionation of Ru^III to afford the corresponding complexes of Ru^II and, presumably, Ru^IV. For L=Ino, disproportionation follows the rate law: d[Ru^II]/dt=k ₀[Ru^III]+k ₁[OH^–][Ru^III], k ₀=(2.7±0.7)×10^–4s^–1 and k ₁=70±1 M^–1s^–1. Received: 11 May 1998 / Accepted: 3 March 1999     

Homonuclear and heteronuclear transition metal complexes by syn-proportionation reactions

Redox processes consisting of disproportionation and syn-proportionation are reviewed with special attention to metal complexes containing carbon-based ligands, i.e. carbon monoxide or unsaturated hydrocarbons. An introduction and a survey of reactions aimed to show the large applicability of syn-proportionation reactions in the field of coordination chemistry, is followed by examples of the use of these redox processes for the preparation of catalytic precursors. The latter studies derive from the idea that if a syn-proportionation reaction can be carried out between two complexes containing different metals in different oxidation states, inter-metallic systems could be formed which may act as active catalysts, e.g. for polymerization reactions.     

Cyclodextrin glucanotransferase (CGTase) catalyzed synthesis of dodecyl glucooligosides by transglycosylation using α-cyclodextrin or starch

Dodecyl glucooligosides, a class of interesting non ionic surfactant molecules were synthesized by cyclodextrin glucanotransferase from Bacillus macerans using either α-cyclodextrin (α-CD) or soluble starch as glycosyl donor and dodecyl β-d-glucoside (C₁₂G₁) or dodecyl β-d-maltoside (C₁₂G₂) as acceptor substrates. The primary coupling products obtained in the respective reactions were identified as dodecyl glucoheptaoside and dodecyl maltooctaoside by mass spectrometry. Higher yields of coupling products were obtained using α-CD as donor, while more dispoportionation occurred with starch. Nearly 78% conversion of the acceptor substrate C₁₂G₁ into dodecyl glucooligosides could be achieved at 132 μg/ml of CGTase in 20 min, while 93% of C₁₂G₂ could be transformed into products at 17.6 μg/ml of enzyme in 120 min using soluble starch as donor substrate. For applications requiring pure compounds like C₁₂G₇, synthesis using α-CD is advantageous. However, for applications in which a mixture of elongated alkyl glycosides is needed, reactions employing starch are clearly competitive.     

Synthesis of Malto-Oligosaccharides Via the Acceptor Reaction Catalyzed by Cyclodextrin Glycosyltransferases

The disproportionation activity (intermolecular transglycosylation) of cyclomaltodextrin glycosyltransferases (CGTases) from Thermoanaerobacter sp. and Bacillus circulans strain 251 was studied. Using soluble starch as donor, the CGTase from Thermoanaerobacter sp. showed the highest transglycosylation activity with all the malto-oligosaccharides tested as acceptors. At ratios of starch: D-glucose from 2:1 to 1:2 (w/w), the formation of cyclodextrins was completely inhibited, and a homologous series of malto-oligosaccharides (G_n) was produced. The conversion of starch into acceptor products was in the range of 63-79% in 48 h. The degree of polymerisation of malto-oligosaccharides formed could be modulated by the ratio of starch: D-glucose provided; at a ratio of 1:2 (w/w), the reaction was quite selective for the formation of G2-G3.     

The unusual redox properties of flavocytochrome P450 BM3 flavodoxin domain

Flavocytochrome P450 BM3 FMN domain is unique among the family of flavodoxins and homologues, in not forming a stable neutral blue FMN semiquinone radical. Anaerobic, one-electron reduction of the isolated domain over the pH 7-9.5 range showed that it forms an anionic red semiquinone that disproportionates slowly (0.014s(-1) at pH 7). The rate of disproportionation decreased at higher pH, indicating that protonation of the anionic semiquinone is an important feature of the mechanism. The reduction potential for the oxidised-semiquinone couple was determined to be -240mV and was largely independent of pH. The semiquinone appears, therefore, to be kinetically trapped by a slow protonation event, enabling it to act as a low-potential electron donor to the P450 heme.     

Effects of trans-pyridine ligands on the interactions of RuII and RuIII ammine complexes N7-coordinated to purine nucleosides and DNA

 DNA binding by trans-[(H₂O)(Pyr)(NH₃)₄Ru^II]²⁺ (Pyr=py, 3-phpy, 4-phpy, 3-bnpy, 4-bnpy) is highly selective for G⁷ with K _G=1.1×10⁴ to 2.8×10⁴, with the more hydrophobic Pyr ligands exhibiting slightly higher binding. A strong dependence on ionic strength indicates that ion-pairing with DNA occurs prior to binding. At μ=0.05, d[Ru^II-DNA]/dt=k[Ru^II][DNA], where k=0.17–0.21 M^–1s^–1 with the various Pyr ligands. The air oxidation of [(py)(NH₃)₄Ru^II] _n -DNA to [(py)(NH₃)₄Ru^III] _n -DNA at pH 6 occurs with a pseudo-first-order rate constant of k _obs=5.6×10^–4s^–1 at μ=0.1, T=25  °C. Strand cleavage of plasmid DNA appears to occur by both Fenton/Haber-Weiss chemistry and by base-catalyzed routes, some of which are independent of oxygen. Base-catalyzed cleavage is more efficient than O₂ activation at neutral pH and involves the disproportionation of covalently bound Ru^III and, in the presence of O₂, Ru-facilitated autoxidation to 8-oxoguanine. Disproportionation of [py(NH₃)₄Ru^III] _n -DNA occurs according to the rate law: d[Ru^II–G_DNA]/dt=k ₀[Ru^III–G_DNA]+k ₁[Ru^III–G_DNA][OH^–], where k ₀=5.4×10^–4s^–1 and k ₁=8.8 M^–1s^–1 at 25  °C, μ=0.1. The appearance of [(Gua)(py)(NH₃)₄Ru^III] under argon, which occurs according to the rate law: d[Ru^III–G]/dt=k ₀[Ru^III–G_DNA]+k ₁[OH^–][Ru^III–G_DNA] (k ₀=5.74×10^–5 s^–1, k ₁=1.93×10^–2M^–1s^–1 at T=25  °C, μ=0.1), is consistent with lysis of the N-glycosidic bond by Ru^IV-induced general acid hydrolysis. In air, the ratio of [Ru-8-OG]/[Ru-G] and their net rates of appearance are 1.7 at pH 11, 25  °C. Small amounts of phosphate glycolate indicate a minor oxidative pathway involving C4′ of the sugar. In air, a dynamic steady-state system arises in which reduction of Ru^IV produces additional Ru^II. Received: 11 November 1998 / Accepted: 3 March 1999