The Effectiveness of Cryoprotectants in Inhibiting Multiple Freeze-Thaw-Induced Functional and Rheological Changes in the Myofibrillar Proteins of Common Carp (Cyprinus carpio) Surimi

The purpose of the present study was to investigate the effects of cryoprotectants (4 % sucrose?+?4 % sorbitol) on functional and rheological changes, including thermal stability, dynamic rheological and emulsifying properties, gel texture profile, gel whiteness, gel water-binding capacity, and gel microstructure, in the myofibrillar proteins (MP) of common carp (Cyprinus carpio) surimi subjected to multiple freeze-thaw cycles (FT, 0, 1, 3 and 5 times). Increased numbers of FT cycles were accompanied by reduced thermal transition temperature (T _max), reduced enthalpy of denaturation (ΔH) (P?<?0.05), and enhanced susceptibility to thermal aggregation. The observation of the gel microstructure indicated that the FT processes produced empty spaces and changed the aggregate gel structure into a coarser-stranded network. The addition of cryoprotectants significantly prevented the reduction of MP thermal stability (P?<?0.05). The protective effect on protein functionality was demonstrated by the cryoprotectant’s efficacy in maintaining the three-dimensional gel network by small-strain oscillatory rheological testing and increasing the emulsifying capacity and emulsion stability of the MP. The FT process caused a decrease (P?<?0.05) in whiteness, water-binding capacity, and texture (hardness, springiness, and gumminess) of the MP gels, which were reduced by the addition of cryoprotectants. Overall, the FT-induced loss of the abovementioned functional and rheological properties was significantly reduced by the presence of cryoprotectants.     

Structural changes of the myofibrillar proteins in common carp (Cyprinus carpio) muscle exposed to a hydroxyl radical-generating system

Oxidation is a leading cause for quality deterioration during processing and storage of food. The objective of the present study was to examine the sensitivity of common carp (Cyprinus carpio) myofibrillar protein (MP) to oxidising radicals produced by a hydroxyl radical-generating system. Both structural and functional changes of common carp MP were evaluated. With increasing H₂O₂ concentrations and oxidation time, the protein carbonyl content, surface hydrophobicity and turbidity of MP increased (P < 0.05), while total sulfhydryl groups decreased (P < 0.05). Sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed protein polymerisation in oxidised MP. The oxidative process destroyed (P < 0.05) the texture (springiness and hardness) of MP gels and decreased their water-binding capacity and whiteness. The thermal gelation profile analysis indicated that oxidation led to a great reduction in the elasticity of samples. Taken together, proteins are susceptible to free radical attack, and oxidative stress had a detrimental effect on protein structure and the general functionality of MP.     

Property of Fish Gelatin gel as Affected by the Incorporation of Gellan and Calcium Chloride

Properties of fish gelatin (FG) gel as affected by gellan (GL) at different levels (2.5–7.5% FG substitution) in combination with calcium chloride (CaCl₂) at various concentrations (3–9 mM) were studied. Gel strength and hardness of FG/GL mixed gel increased as the levels of GL increased (P < 0.05). Increasing CaCl₂ concentration also resulted in the increases in both gel strength and hardness of mixed gel when GL at the same level was incorporated (P < 0.05). Conversely, the increasing GL and CaCl₂ levels caused a decrease in springiness but an increase in syneresis of mixed gels (P < 0.05). Gelling and melting temperatures were increased in the mixed gel as levels of GL and CaCl₂ increased. L*- and b*-values of mixed gels decreased, whereas ?E*-value increased with increasing GL and CaCl₂ levels (P < 0.05). Microstructure studies revealed that denser structure with smaller voids in gel network was observed in the mixed gel in the presence of CaCl₂ at higher levels. However, mixed gels incorporated with GL above 5%, regardless of CaCl₂ levels, yielded the lower likeness score than FG gel (control) (P < 0.05). The addition of GL at low level (2.5%) with CaCl₂ (up to 6 mM) had no adverse effect on sensory property of mixed gels but could improve gelling property of FG via increasing gel strength and gelling point.     

Neutrophilic, nitrate-dependent, Fe(II) oxidation by a Dechloromonas species

A species of Dechloromonas, strain UWNR4, was isolated from a nitrate-reducing, enrichment culture obtained from Wisconsin River (USA) sediments. This strain was characterized for anaerobic oxidation of both aqueous and chelated Fe(II) coupled to nitrate reduction at circumneutral pH. Dechloromonas sp. UWNR4 was incubated in anoxic batch reactors in a defined medium containing 4.5–5 mM NO₃ ^?, 6 mM Fe²⁺ and 1–1.8 mM acetate. Strain UWNR4 efficiently oxidized Fe²⁺ with 90 % oxidation of Fe²⁺ after 3 days of incubation. However, oxidation of Fe²⁺ resulted in Fe(III)-hydroxide-encrusted cells and loss of metabolic activity, suggested by inability of the cells to utilize further additions of acetate. In similar experiments with chelated iron (Fe(II)-EDTA), encrusted cells were not produced and further additions of acetate and Fe(II)-EDTA could be oxidized. Although members of the genus Dechloromonas are primarily known as perchlorate and nitrate reducers, our findings suggest that some species could be members of microbial communities influencing iron redox cycling in anoxic, freshwater sediments. Our work using Fe(II)-EDTA also demonstrates that Fe(II) oxidation was microbially catalyzed rather than a result of abiotic oxidation by biogenic NO₂ ^?.     

Combining Effect of Microbial Transglutaminase and Bambara Groundnut Protein Isolate on Gel Properties of Surimi from Sardine (Sardinella albella)

Effects of protein isolate from bambara groundnut (BGPI) at different levels (0–6 %, w/w) in combination with microbial transglutaminase (MTGase) at different concentrations (0, 0.3 and 0.6 U g^?1surimi) on gels properties of sardine (Sardinella albella) surimi were investigated. In the absence of MTGase, the increases in breaking force and deformation of gels were obtained when BGPI at levels of 1.5–3 % was incorporated (P?<?0.05). The further increases in BGPI levels (4.5–6 %) resulted in the decrease in breaking force and deformation (P?<?0.05). When MTGase (0.3 and 0.6 U g^?1surimi) was added, the increase in breaking force and deformation were noticed, regardless of BGPI levels, and the strengthening effect was in dose-dependent manner. The increases in hardness, gumminess and chewiness were also observed when surimi gel was added with BGPI and MTGase (P?<?0.05). Water-holding capacity of gels was improved with increasing level of BGPI, and MTGase incorporated (P?<?0.05). Whiteness of gels slightly decreased with increasing BGPI levels, however the addition of MTGase had no impact on whiteness (P?>?0.05). Based on electrophoretic study, myosin heavy chain decreased with addition of MTGase, indicating the formation of cross-links. More compact structure was observed in gel added with MTGase (0.6 U g^?1surimi) and 6 % BGPI, and was accompanied by an increased gel strength.     

The oxidation of ferrocytochrome c by Br−2, (SCN)−2, N3 and OH radicals studied by pulsed-electron and γ-ray radiolysis

The reactions of ferrocytochrome c with Br^?₂, (SCN)^?₂, N₃ and OH radicals were followed by measuring the change in the optical spectra of cytochrome c on γ-irradiation as well as the rate of change of absorbance upon pulse irradiation.Ferrocytochrome c is oxidized to ferricytochrome c by Br^?₂, (SCN)^?₂ or N₃ radical with an efficiency of about 100% through a second-order process in which no intermediates were observed. The rate constants in neutral solutions at I = 0.073 are 9.7 · 10⁸ M^?1 · s^?1, 7.9 · 10⁸ M^?1 · s^?1, 1.3 · 10⁹ M^?1 · s^?1 for the oxidation by Br^?₂, (SCN)^?₂ and N₃ radicals, respectively. The rate constants do not vary appreciably in alkaline solutions (pH 8.9). The ionic strength dependence was observed for the rate constants of the oxidation by Br^?₂ and (SCN)^?₂. Those rate constants estimated on the assumption that the radicals react only with the amino acid residues with the characteristic steric correction factors were less than one-tenth of the observed ones. These results suggest that the partially exposed region of the heme is the probable site of electron transfer from ferrocytochrome c to the radical.Hydroxyl radicals also oxidize ferrocytochrome c with a high rate constant (k > 1 · 10¹⁰ M^?1 · s^?1), but with a very small efficiency (5%).     

Textural and Rheological Properties of Soy Protein Isolate Tofu-Type Emulsion Gels: Influence of Soybean Variety and Coagulant Type

The contribution of soybean variety and coagulant type to the textural and rheological properties of soy protein isolate (SPI) tofu-type emulsion gels was studied. SPIs from eight soybean varieties were subjected to amino acid and sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) analysis, and results showed that the 11S fraction proteins (r?=?0.833, p?<?0.05) and the ratio of 11S to 7S (r =?0.920, p <?0.01) were positively correlated with the hardness of CaSO₄-induced emulsion gels and glucono-δ-lactone (GDL)-induced gels, with the correlation coefficients of 0.827 (p <?0.05) and 0.893 (p <?0.01), respectively. In the case of microbial transglutaminase (MTGase), strong relations between the content of glutamate (r =?0.886, p?<?0.01) and lysine (r =?0.810, p <?0.05) and gel hardness were found. Rheological data demonstrated that CaSO₄-induced emulsion gel was stiffer with high rigidity but gel induced by MTGase performed better elasticity. The findings of this study are of great importance to further understand the gelation mechanisms of different coagulants and provide useful information for the development of SPI-based filled tofu.     

Critical moisture content for successful cryopreservation of embryonic axes of <Emphasis Type="Italic">Fortunella polyandra</Emphasis> determined by differential scanning calorimetry (DSC)

The relationships between water content of desiccated embryonic axes (using different methods of desiccation), the availability of water determined by differential scanning calorimetry (DSC) analysis and recovery percentage after liquid nitrogen (LN) exposure of Fortunella polyandra embryonic axes were investigated. The objectives were to understand thermal properties of desiccated embryonic axes during cryopreservation and to determine the critical moisture contents for successful cryopreservation of the embryonic axes. Excised embryonic axes were desiccated under laminar air flow (0, 10, 15, 30 and 45 min), over silica gel (0, 5, 15, 30 and 60 min), and ultra-rapidly (0, 5, 10, 20 and 25 min). Desiccation under laminar air flow resulted in an optimal water content of 0.150 gH₂O g^?1 dw and a survival of 50 % after cryopreservation, while the unfrozen water content (WC_u) was 0.126 gH₂O g^?1 dw. After drying over silica gel, the optimal water content was 0.190 gH₂O g^?1 dw, where the survival was 40 % after cryopreservation and the WC_u was determined as 0.177 gH₂O g^?1 dw. Using the flash-drying method, the optimal water content was found to be 0.145 gH₂O g^?1 dw, the survival was 50 % after cryopreservation and the WC_u was 0.133 gH₂O g^?1 dw. Embryonic axes of F. polyandra showed low-to-moderate tolerance to desiccation. The results of the freezing transitions for all the desiccation times and methods showed that the onset temperature and the peak of the mean enthalpy decreased in size with decreasing water content. DSC elucidated the critical moisture contents and the cooling and melt enthalpies for successful cryopreservation of F. polyandra embryonic axes.     

Tea Polyphenols Affect Oxidative Modification and Solution Stability of Myofibrillar Protein from Grass Carp (Ctenopharyngodon idellus)

This study investigated the effects of different concentrations (0, 5, 10, 20, 50, and 100 μmol/g protein) of tea polyphenols (TP) on the oxidative modification and the physicochemical, structural, and gelling properties of myofibrillar protein (MP) from grass carp (Ctenopharyngodon idellus) oxidized by a hydroxyl radical-generating system. The results showed that low concentrations (5 and 10 μmol/g protein) of TP could effectively inhibit the formation of carbonyl groups and dityrosine, the loss of sulfhydryl groups and α-helix conformation, and the change of the tertiary structure of MP caused by hydroxyl radicals. Moreover, the presence of TP in all concentrations decreased the surface hydrophobicity of MP. TP at 10 μmol/g protein had better effects on preventing the oxidation-induced cross-linking and aggregation of myosin heavy chain (MHC) and actin of MP, keeping the stability of MP solutions with lower turbiscan stability index (TSI) values, and improving gelling properties characterized by higher hardness and gel strength of MP gels. By contrast, excessive presence of TP (20, 50, and 100 μmol/g protein) showed pro-oxidative effects on oxidatively stressed MP, which was detrimental to the MP and contributed to the denaturation and irregular aggregation of MP, the loss of MP solution stability, and lower gelling capacity with poor texture and gel strength. The concentration-dependent effects of TP on MP depended on the extent of MP oxidative modification and MP-TP interactions, indicating that a proper amount of TP has the potential to protect MP from oxidation and to enhance the gelling capacity of surimi during processing.

     

Antioxidant activity of brown alga Saccharina bongardiana from Kamchatka (Pacific coast of Russia). A methodological approach

The present study aims to investigate the levels of polyphenols and antioxidant activity in one of the most important commercial species of seaweeds in Kamchatka, an edible brown seaweed Saccharina bongardiana. Six extracts of S. bongardiana, acetone, methanol, ethanol, and the respective 70 % aqueous solutions, were assessed for total phenol content in order to determine the most efficient extracting solvent. The total phenol content was measured by the Folin–Ciocalteu method and expressed as phloroglucinol equivalents (PGE). The antioxidant tests used were 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay, linoleic acid-β carotene oxidation inhibiting assay, and Fe²⁺ ion chelating method. Higher phenolic contents were obtained using aqueous organic solvents, as compared to the respective absolute solvents; 70 % acetone was found to be the most efficient solvent (1.039 mg PGE 100 mg^?1 dry algal powder). High significant correlations were noted between total phenol content and the tested antioxidant activities; so the aqueous organic extracts exhibited the highest antioxidant activities versus DPPH radicals (EC₅₀ values of 0.6–1.1 mg dry weight (DW) mL^?1), linoleic acid-β carotene oxidation (74–78 % at 0.8 mg DW mL^?1), as well as ferrous ions (EC₅₀ values of 5.0–7.9 mg DW mL^?1). Some methodological recommendations regarding the assays used and the expression of results are proposed.     

Antioxidant, cytotoxic and hemolytic effects of sulfated galactans from edible red alga Hypnea musciformis

Polysaccharides, galactans, obtained from edible red seaweed Hypnea musciformis were characterized by molecular weight and infrared spectroscopy analysis and were evaluated for antioxidant activity in vitro and for their effects on cell viability. The main components were galactose and sulfate presenting low protein contamination. These sulfated galactans (F1.0) showed a polydisperse profile, and signs in infrared analysis were attributed to a sulfate ester S?=?O bond, the presence of a 3,6-anhydrogalactose C–O bond, nonsulfated β-d-galactose, and a C–O–SO₄ bond in galactose C4. The NMR analysis showed signals at about 95 and 92 attributed to anomeric carbon of 4-linked 3,6-anhydro-α-d-galactopyranose residue of κ-carrageenans and 4-linked 3,6-anhydro-α-d-galactopyranose2-sulfate of ι-carrageenans. Sulfated galactan F1.0 showed strong antioxidant activity under lipid peroxidation assay where F1.0 at 8 mg mL^?1 promoted 57.92% peroxidation inhibition and displayed the scavenging activity on hydroxyl radicals in a dose-dependent manner leading to 32.5% scavenging of these radicals when 5 mg mL^?1 of sulfated galactan F1.0 was used. The sulfated galactan fraction also exhibited strong inhibition on the H₂O₂-induced hemolysis model. Sulfated galactan F1.0 displayed low cytotoxic action in 3 T3 cells and moderate antitumoral action in HeLa cells. These results suggest that sulfated galactan F1.0 from H. musciformis has antioxidant potential, which is a great effect for a compound used as food and in the food industry.     

Matrix-dependent modulation of anisotropic effects on NMR spectra from 7Li+ and 23Na+ encapsulated in cryptands

⁷Li and ²³Na NMR spectra of the respective cations in gelatin and ι-carrageenan gels containing cryptand-[2.1.1] (for Li⁺) or cryptand-[2.2.2] (for Na⁺) displayed two transitions: the one at higher frequency corresponded to the cation surrounded by gel, the other to cation inside its appropriately sized cryptand. While binding to cryptands yielded much broader lines and shorter T ₁ relaxation times, anisotropic splitting in first order ⁷Li or ²³Na NMR spectra was not detected. Stretching the gels resulted in increasing the anisotropic electric field gradient tensor; thus, the NMR transitions of the cation in the gel were split (removal of degeneracy) to display its characteristic 3:4:3 triplet for spin = 3/2 nuclei. The transitions of the cryptand-bound cations (Li⁺-cryptand-[2.1.1] and Na⁺-cryptand-[2.2.2]) showed different extents of interaction with the electric field gradient tensor depending on the composition of the gel matrix. The NMR signal for ⁷Li⁺-cryptand-[2.1.1] in stretched gelatin gel showed a five-fold increased splitting as compared to the ⁷Li⁺ signal in the reference gel. In stretched ι-carrageenan gels, no anisotropic splitting from the cryptand-bound Li⁺ was recorded. Steady-state irradiation envelopes or z-spectra showed evidence of Li⁺ exchange between isotropic (cryptand) and anisotropic (gel) sites only at higher temperatures (55 °C). For Na⁺ bound to the cryptand-[2.2.2], anisotropic splitting (three-fold smaller compared with the ²³Na signal in the reference gel) was only recorded in stretched ι-carrageenan gels, whereas gelatin gels showed only anisotropic splitting for the ²³Na signal in the reference gel.     

Pulse-radiolytic investigations of catechols and catecholamines II. Reactions of Tiron with oxygen radical species

Transient spectra and kinetic data of Tiron (1,2-dihydroxybenzene-3,5-disulphonic acid) are reported, obtained after pulse-radiolytic oxidation by hydroxyl radicals (°OH), superoxide anions (O₂^?) or a combination of both oxygen radicals. The rate constant with °OH radicals was determined at 1.0·10⁹ M^?1·s^?1. Contrary to a previous report (Greenstock, C.L. and Miller, R.W. (1975) Biochim. Biophys. Acta 396, 11–16), the rate constant with O₂^? of 1.0·10⁷ M^?1·s^?1 is lower by one order of magnitude; also the semiquinone absorbs at 300 nm rather than at 400 nm. The ratio of the rate constants with °OH and O₂^? of 100 again demonstrates that any oxidation reaction by the latter radical is unspecific due to the more efficient reaction of °OH radicals, leading to the same products with catechol compounds.     

Comparative studies of water permeability of red blood cells from humans and over 30 animal species: an overview of 20 years of collaboration with Philip Kuchel

NMR measurements of the diffusional permeability of the human adult red blood cell (RBC) membrane to water (P _d) and of the activation energy (E _a,d) of the process furnished values of P _d ~ 4 × 10^?3 cm/s at 25 °C and ~6.1 × 10^?3 cm/s at 37 °C, and E _a,d ~ 26 kJ/mol. Comparative NMR measurements for other species showed: (1) monotremes (echidna and platypus), chicken, little penguin, and saltwater crocodile have the lowest P _d values; (2) sheep, cow, and elephant have P _d values lower than human P _d values; (3) cat, horse, alpaca, and camel have P _d values close to those of humans; (4) guinea pig, dog, dingo, agile wallaby, red-necked wallaby, Eastern grey kangaroo, and red kangaroo have P _d values higher than those of humans; (5) mouse, rat, rabbit, and “small and medium size” marsupials have the highest values of P _d (>8.0 × 10^?3 cm/s at 25 °C and >10.0 × 10^?3 cm/s at 37 °C). There are peculiarities of E _a,d values for the RBCs from different species. The maximum inhibition of diffusional permeability of RBCs induced by incubation with p-chloromercuribenzene sulfonate varied between 0 % (for the chicken and little penguin) to ~50 % (for human, mouse, cat, sheep, horse, camel, and Indian elephant), and ~60–75 % (for rat, guinea pig, rabbit, dog, alpaca, and all marsupials). These results indicate that no water channel proteins (WCPs) or aquaporins are present in the membrane of RBCs from monotremes (echidna, platypus), chicken, little penguin and saltwater crocodile whereas WCPs from the membranes of RBCs from marsupials have peculiarities.     

Hydroxyl radical production from hydrogen peroxide and enzymatically generated paraquat radicals: Catalytic requirements and oxygen dependence

The ability of paraquat radicals (PQ^+.) generated by xanthine oxidase and glutathione reductase to give H₂O₂-dependent hydroxyl radical production was investigated. Under anaerobic conditions, paraquat radicals from each source caused chain oxidation of formate to CO₂, and oxidation of deoxyribose to thiobarbituric acid-reactive products that was inhibited by hydroxyl radical scavengers. This is in accordance with the following mechanism derived for radicals generated by γ-irradiation [H. C. Sutton and C. C. Winterbourn (1984) Arch. Biochem. Biophys.235, 106–115] PQ^+. + Fe³⁺ (chelate) → Fe²⁺ (chelate) + PQ⁺⁺ H₂O₂ + Fe²⁺ (chelate) → Fe³⁺ (chelate) + OH^? + OH.. Iron-(EDTA) and iron-(diethylenetriaminepentaacetic acid) (DTPA) were good catalysts of the reaction; iron complexed with desferrioxamine or transferrin was not. Extremely low concentrations of iron (0.03 μm) gave near-maximum yields of hydroxyl radicals. In the absence of added chelator, no formate oxidation occurred. Paraquat radicals generated from xanthine oxidase (but not by the other methods) caused H₂O₂-dependent deoxyribose oxidation. However, inhibition by scavengers was much less than expected for a reaction of hydroxyl radicals, and this deoxyribose oxidation with xanthine oxidase does not appear to be mediated by free hydroxyl radicals. With O₂ present, no hydroxyl radical production from H₂O₂ and paraquat radicals generated by radiation was detected. However, with paraquat radicals continuously generated by either enzyme, oxidation of both formate and deoxyribose was measured. Product yields decreased with increasing O₂ concentration and increased with increasing iron(DTPA). These results imply a major difference in reactivity between free and enzymatically generated paraquat radicals, and suggest that the latter could react as an enzyme-paraquat radical complex, for which the relative rate of reaction with Fe³⁺ (chelate) compared with O₂ is greater than is the case with free paraquat radicals.     

The Effects of Lipid Oxidation Product Acrolein on the Structure and Gel Properties of Rabbit Meat Myofibrillar Proteins

We investigated the effect of acrolein, a byproduct of lipid oxidation, on the structure and gel properties of myofibrillar proteins (MPs) isolated from rabbit meat. As the acrolein concentration increased, the protein carbonyl compounds significantly accumulated (p?<?0.05), and the total sulfhydryl content was significantly lost (p?<?0.05). The results of circular dichroism spectra, surface hydrophobicity, UV absorption spectra and intrinsic fluorescence spectra evidenced that acrolein caused the disruption of α-helix structure, the exposure of hydrophobic sites and the unfolding of MPs. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis suggested that medium (0–1 mM) and high (5–10 mM) concentrations of acrolein could induce protein cross-linkage and protein aggregation, respectively. These structural changes could affect gelling properties of MPs involving gel strength and water holding capacity (WHC). The results of Raman spectroscopy indicated that moderate oxidative modification caused protein unfolding as well as the decline of α-helix structure and the increase of β-sheets structure in gels, thereby influencing the gel properties. Moderate oxidative modification (0–1 mM) improved gel strength and WHC, while excessive oxidative modification (5–10 mM) resulted in decreased gel properties.     

Nitrofuran enhancement of microsomal electron transport, superoxide anion production and lipid peroxidation

Addition of nifurtimox (a nitrofuran derivative used for the treatment of Chagas' disease) to rat liver microsomes produced an increase of (a) electron flow from NADPH to molecular oxygen, (b) generation of both superoxide anion radical (O₂^?) and hydrogen peroxide, and (c) lipid peroxidation. The nifurtimox-stimulated NADPH oxidation was greatly inhibited by NADP⁺ and p-chloromercuribenzoate, and to a lesser extent by SKF-525-A and metyrapone. These inhibitions reveal the function of both the NADPH-cytochrome P-450 (c) reductase and cytochrome P-450 in nifurtimox reduction. Superoxide dismutase, catalase (in the presence of superoxide dismutase), and hydroxyl radical scavengers (mannitol, 5,5-dimethyl-1-pyrroline-1-oxide) inhibited the nifurtimox-stimulated NADPH oxidation, in accordance with the additional operation of a reaction chain including the hydroxyl radical. Further evidence supporting the role of superoxide anion and hydroxyl radicals in the nifurtimox-induced NADPH oxidation resulted from the effect of specific inhibitors on NADPH oxidation by O₂^? (generated by the xanthine oxidase reaction) and by OH. (generated by an iron chelate or the Fenton reaction). Production of O₂^? by rat kidney, testes and brain microsomes was significantly stimulated by nifurtimox in the presence of NADPH. It is postulated that enhanced formation of free radicals is the basis for nifurtimox toxicity in mammals, in good agreement with the postulated mechanism of the trypanocide effect of nifurtimox on Trypanosoma cruzi.     

Methane oxidation and methane driven redox process during sequential reduction of a flooded soil ecosystem

A laboratory incubation study conducted to assess the temporal variation of CH₄ oxidation during soil reduction processes in a flooded soil ecosystem. A classical sequence of microbial terminal electron accepting process observed following NO₃ ^? reduction, Fe³⁺ reduction, SO₄ ^2? reduction and CH₄ production in flooded soil incubated under initial aerobic and helium-flushed anaerobic conditions. CH₄ oxidation in the slurries was influenced by microbial redox process during slurry reduction. Under aerobic headspace condition, CH₄ oxidation rate (k) was stimulated by 29 % during 5 days (NO₃ ^? reduction) and 32 % during both 10 days (Fe³⁺) and 20 days (early SO₄ ^2? reduction) over unreduced slurry. CH₄ oxidation was inhibited at the later methanogenic period. Contrastingly, CH₄ oxidation activity in anaerobic incubated slurries was characterized with prolonged lag phase and lower CH₄ oxidation. Higher CH₄ oxidation rate in aerobically incubated flooded soil was related to high abundance of methanotrophs (r?=?0.994, p?<?0.01) and ammonium oxidizers population (r?=?0.184, p?<?0.05). Effect of electron donors NH₄ ⁺, Fe^2+, S^2? on CH₄ oxidation assayed to define the interaction between reduced inorganic species and methane oxidation. The electron donors stimulated CH₄ oxidation as well as increased the abundance of methanotrophic microbial population except S^2? which inhibited the methanotrophic activity by affecting methane oxidizing bacterial population. Our result confirmed the complex interaction between methane-oxidizing microbial groups and redox species during sequential reduction processes of a flooded soil ecosystem.     

Kinetics investigation of the hydrogen abstraction reaction between CH<Subscript>3</Subscript>SS and CN radicals

The reaction mechanisms and rates for the H abstraction reactions between CH₃SS and CN radicals in the gas phase were investigated with density functional theory (DFT) methods. The geometries, harmonic vibrational frequencies, and energies of all stationary points were obtained at B3PW91/6-311G(d,p) level of theory. Relationships between the reactants, intermediates, transition states and products were confirmed, with the frequency and the intrinsic reaction coordinate (IRC) analysis at the same theoretical level. High accurate energy information was provided by the G3(MP2) method combined with the standard statistical thermodynamics. Gibbs free energies at 298.15 K for all of the reaction steps were reported, and were used to describe the profile diagrams of the potential energy surface. The rate constants were evaluated with both the classical transition state theory and the canonical variational transition state theory, in which the small-curvature tunneling correction was included. A total number of 9 intermediates (IMs) and 17 transition states (TSs) were obtained. It is shown that IM1 is the most stable intermediate by the largest energy release, and the channel of CH₃SS?+?CN?→?IM3?→?TS10?→?P1(CH₂SS?+?HCN) is the dominant reaction with the lowest energy barrier of 144.7 kJ mol^?1. The fitted Arrhenius expressions of the calculated CVT/SCT rate constants for the rate-determining step of the favorable channel is k =7.73?×?10⁶? T ^1.40exp(?14,423.8/T) s^?1 in the temperature range of 200–2000 K. The apparent activation energy E _a(app.) for the main channel is ?102.5 kJ mol^?1, which is comparable with the G3(MP2) energy barrier of ?91.8 kJ mol^?1 of TS10 (relative to the reactants).     

Long-lived spin state of a tripeptide in stretched hydrogel

The longitudinal (T ₁), transverse (T ₂), and singlet state (T _s) relaxation times of the geminal backbone protons (CH₂) of l-Leu-Gly-Gly were studied by NMR spectroscopy at 9.4 T in a bovine hide gelatin gel composed in D₂O at 25 °C. Gelatin granules were dissolved in a hot solution of the tripeptide and then the solution was allowed to gel inside a flexible silicone tubing. With increases in gelatin content, the T ₂ and T _s of the CH₂ protons correspondingly decreased (T _s/T ₂ ~ constant), while the change in T ₁ was relatively small. The largest observed T _s/T ₁ value was 3.3 at 46 % w/v gelatin that was the lowest gelatin content examined. Stretching the tubing, and hence the gel, brought about anisotropic alignment of the constituents resulting in residual quadrupolar splitting of the resonance from D₂O in ²H NMR spectra, and residual dipolar splitting of the CH₂ resonance in ¹H NMR spectra. WALTZ-16 decoupling during the relaxation intervals extended the singlet state relaxation time, but the efficacy diminished as the gels were stretched. Theoretically predicted T ₁, T ₂, and T _s values, assuming intramolecular dipolar coupling as the only source of relaxation, were within the same order of magnitude as the experimentally observed values. Overall we showed that it is possible to observe a long-lived spin state in an anisotropic medium when T ₂ is shorter than T ₁ in the presence of non-zero residual dipolar couplings.