On the Stability of Clathrate Hydrates Encaging Polar Guest Molecules: Contrast in the Hydrogen Bonds of Methylamine and Methanol Hydrates

Abstract

The stability of clathrate hydrates encaging highly polar guests has been investigated in order to explain the experimental observation that some amines form clathrate hydrates but alcohols act as inhibitor to hydrate formation. We choose methylamine and methanol as guest species and examine the stable structure, at which the total potential energy has a minimum value. At the local minima of those two hydrates, the potential energies of water-water and guest-water, and their hydrogen bonded networks are compared. It is found that methanol does not retain the host lattice structure, while the host-network structure is kept in the presence of methylamine. It is shown that the difference in the magnitude of the partial charge on the hydrogen atom between the hydroxyl and amino groups plays a much more significant role on the stability of both clathrate hydrates than the difference in molecular geometry. This is supported from the result of a methylamine-like model that has the same partial charges on the atoms in the hydrophilic site as methanol.     

Identification of hydrogen selenide and other volatile selenols by derivatization with 1-fluoro-2,4-dinitrobenzene

A procedure is described for the trapping and identification of hydrogen selenide and methyl selenol ( CH3SeH ). The volatile selenols were generated by reducing selenious acid or dimethyldiselenide with Zn dust and hydrochloric acid under a stream of nitrogen and passing into a trapping solution composed of 50 mM 1-fluoro-2,4-dinitrobenzene plus 83 mM sodium bicarbonate in 67% dimethylformamide:33% water. The selenols react rapidly to form stable dinitrophenyl (DNP) selenoethers that can be extracted into benzene; these are easily identified by TLC, HPLC, or mass spectrometry. Hydrogen selenide is trapped in 90-99% yield, primarily as the di-DNP- monoselenide with a trace of di-DNP- diselenide .     

Electrochemical identification of microbially mediated hydrogen sulfide oxidation

The process of H₂S oxidation by the phototrophic bacteriaThiocapsa roseopersicina andChlorobium phaeobacteroides, respectively, was monitored using a Pt-glass-Ag⁰, Ag₂S electrode combination without liquid junction. Due to the resulting pe(pH) and pH₂S plottings three steps can be distinguished: oxidation of H₂S to an S(0) state, oxidation of S (0) to SO₄ ^2–, and oxidation of the remaining H₂S directly to SO₄ ^2–. Differences between the investigated bacteria exist with respect to their individual oxidation strategies.Thiocapsa apparently stops oxidizing H₂S at pH₂S 7.5 (e.g. 10^–7.5M H₂S) and shifts to the utilization of the intracellularly stored S (0). In contrastChlorobium utilizes its extracellularly stored sulfur parallel to the extracellular H₂S fraction. The corresponding Pt-sensor responses (pe₇ values) were found to be similar to the corresponding partial redox equilibria (p₇ values) of H₂S oxidation stoichiometries as proposed by Van Niel (1931) and Trüper (1964). It is concluded that the recording of pe enables investigators to understand (and control) in situ redox processes, independent of their thermodynamic equilibration, only bound to changes of electroactivity vs. sensor.     

Cultivar and Rhizobium strain effect on nitrogen fixation and transport inPhaseolus vulgaris L.

The effects of Rhizobium strain and its interaction with plant cultivar were examined in glasshouse-grownPhaseolus vulgaris in two experiments where the physiological attributes defining the symbiotic efficiency were determined. Strains of Rhizobium significantly affected nodulation, rates of N accumulation, partitioning of N within the mature shoot and remobilizaton of the N stored in the vegetative organs to the seeds. The most efficient symbiosis (strain CO5 with Negro Argel), in comparison with the least efficient symbiosis (strain 127 K-17 with Venezuela-350) showed higher rates of C₂H₂ reduction from flowering to mid pod fill stage, evolved less hydrogen from nodules and showed higher rates of N transport as well as higher percentages of ureide-N in the xylem sap. At maturity, the best cultivar/strain association exceeded the total N accumulated in the seed and the harvest index of the poorest symbiosis in 88% and 20%, respectively. The other symbiotic combinations were intermediate in all characteristics. Nitrogen accumulation in plant shoot showed highly significant correlation with acetylene reduction rates, nodule relative efficiency, total N transport in the xylem sap and percentage of N transported as ureides.     

Is functional manganese involved in hydrogen-peroxide-stimulated anomalous oxygen evolution in CACl2-washed photosystem II membranes?

When detergent-derived photosystem II (PSII) membranes are treated with CaCl₂ to remove the three extrinsic proteins associated with the O₂-evolving complex, the resulting membranes (CaPSII) can still catalyze water oxidation if sufficient Ca²⁺ and Cl^- are present. When CaPSII membranes are exposed to single turnover flashes on an O₂ rate electrode, anomalous O₂ is produced by the first two flashes. The addition of catalase to the membrane suspension completely inhibits O₂ produced by the first two flashes, but not by subsequent flashes. Exogenous H₂O₂ stimulates anomalous O₂ production by the first few flashes in CaPSII membranes, but not in control PSII membranes. Diuron (DCMU) does not inhibit H₂O₂-stimulated O₂ production by the first flash. However, it does inhibit the O₂ yield of all subsequent flashes, indicating that all flash-induced O₂ signals in CaPSII membranes are dependent on photosystem II electron transport. H₂O₂ stimulation of O₂ yields is inhibited in Tris-, heat-, and EDTA-(ethylenediaminetetraacetic acid)-treated CaPSII. In the presence of high salt, H₂O₂ (but not EDTA) treatment of CaPSII, extracts Mn functional in normal photosynthetic O₂ evolution. The addition of exogenous Mn²⁺ reconstitutes anomalous O₂ production in Tris-and H₂O₂/EDTA-treated CaPSII preparations but only in the presence of H₂O₂. Anomalous H₂O₂-stimulated O₂ production can be observed both with a Clark electrode (steady state) and an O₂ rate electrode (flash sequence). The mechanism involves electron donation from H₂O₂, mediated by free Mn²⁺, to PSII, and the 33-kDa extrinsic protein under some conditions can block this process. Since H₂O₂ can remove functional Mn from CaPSII membranes, its presence can convert functional Mn to the Mn²⁺ mediator state required for anomalous O₂ production. EDTA binds Mn in CaPSII disrupted by H₂O₂ and prevents anomalous O₂ evolution.Abbreviations CaPSII a PSII preparation washed with approximately 1M CaCl₂ - Chl chlorophyll - DCBQ 2,6-dichloro-p-benzoquinone - DCMU (diuron) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EDTA ethylenediaminetetraacetic acid - MES 2-[N-morpholino]-ethanesulfonic acid - PSII a detergent-derived photosystem II membrane preparation - RC reaction center - Tris tris(hydroxymethyl)-aminomethane - Y_n oxygen rate electrode flash yield resulting from the nth flash of a sequence of single turnover flashes of light Operated by the Midwest Research Institute for the U.S. Department of Energy under contract DE-AC02-83CH10093.     

Synthesis and conformational analysis of aib-containing peptide modelling for N-glycosylation site in N-glycoprotein

A tetrapetide containing an Aib residue, Boc-Asn-Aib-Thr-Aib-OMe, was synthesized as a peptide model for the N-glycosylation site in N-glycoproteins. Backbone conformation of the peptide and possible intramolecular interaction between the Asn and Thr side chains were elucidated by means of n.m.r. spectroscopy. Temperature dependence of NH proton chemical shift and NOE experiments showed that Boc-Asn-Aib-Thr-Aib-OMe has a tendency to form a β-turn structure with a hydrogen bond involving Thr and Aib⁴ NH groups. Incorporation of Aib residues in the peptide model promotes folding of the peptide backbone. With folded backbone conformation, carboxyamide protons of the Asn residue are not involved in hydrogen bond network, while the OH group of the Thr residue is a candidate for a hydrogen bond in DMSO-d₆ solution.     

Alterations in cardiac membrane Ca2+ transport during oxidative stress

Although cardiac dysfunction due to ischemia-reperfusion injury is considered to involve oxygen free radicals, the exact manner by which this oxidative stress affects the myocardium is not clear. As the occurrence of intracellular Ca²⁺ overload has been shown to play a critical role in the genesis of cellular damage due to ischemia-reperfusion, this study was undertaken to examine whether oxygen free radicals are involved in altering the sarcolemmal Ca²⁺-transport activities due to reperfusion injury. When isolated rat hearts were made globally ischemic for 30 min and then reperfused for 5 min, the Ca²⁺ -pump and Na⁺-Ca²⁺ exchange activities were depressed in the purified sarcolemmal fraction; these alterations were prevented when a free radical scavenger enzymes (superoxide dismutase plus catalase) were added to the reperfusion medium. Both the Ca²⁺- pump and Na⁺- Ca²⁺ exchange activities in control heart sarcolemmal preparations were depressed by activated oxygen-generating systems containing xanthine plus xanthine oxidase and H₂O₂; these changes were prevented by the inclusion of superoxide dismutase and catalase in the incubation medium. These results support the view that oxidative stress during ischemia-reperfusion may contribute towards the occurrence of intracellular Ca²⁺ overload and subsequent cell damage by depressing the sarcolemmal mechanisms governing the efflux of Ca²⁺ from the cardiac cell.     

Several new substrates for Desulfovibrio vulgaris strain Marburg and a spontaneous mutant from it

The ability of Desulfovibrio vulgaris strain Marburg (DSM 2119) to oxidize alcohols was surveyed in the presence and absence of hydrogen-scavenging anaerobes, Acetobacterium woodii and Methanospirillum hungatei. In the presence of sulfate, D. vulgaris grew not only on ethanol, 1-propanol, and 1-butanol, but also on isobutanol, 1-pentanol, ethyleneglycol, and 1,3-propanediol. Metabolism of these alcohols was simple oxidation to the corresponding acids, except with the last two substrates: ethyleneglycol was oxidized to glycolate plus acetate, 1,3-propanediol to 3-hydroxypropionate plus acetate. Experimental evidence was obtained, suggesting that 2-methoxyethanol was not utilized by all the cells of strain marburg, but by a spontaneous mutant. 2-Methoxyethanol was oxidized to methoxyacetate by the mutant. Co-culture of strain Marburg plus A. woodii grew on ethanol, 1-propanol, 1-butanol, and 1,3-propanediol in the absence of sulfate. Co-culture of strain Marburg plus M. hungatei grew on ethanol, 1-propanol, and 1-butanol, but not on ethyleneglycol and 1,3-propanediol, Co-culture of the mutant plus A. woodii or M. hungatei did not grow on 2-methoxyethanol.     

Formation of Hydroxyl Radicals in Biological Systems. Does Myoglobin Stimulate Hydroxyl Radical Formation from Hydrogen Peroxide?

Incubation of horse-heart oxymyoglobin or metmyoglobin with excess H₂O₂ causes formation of myoglobin(IV), followed by haem degradation. At the time when haem degradation is observed, hydroxyl radicals (.OH) can be detected in the reaction mixture by their ability to degrade the sugar deoxyribose. Detection of hydroxyl radicals can be decreased by transferrin or by OH scavengers (mannitol, arginine, phenylalanine) but not by urea. Neither transferrin nor any of these scavengers inhibit the haem degradation. It is concluded that intact oxymyoglobin or metmyoglobin molecules do not react with H₂O₂ to form OH detectable by deoxyribose, but that H₂O₂ eventually leads to release of iron ions from the proteins. These released iron ions can react to form OH outside the protein or close to its surface. Salicylate and the iron chelator desferrioxamine stabilize myoglobin and prevent haem degradation. The biological importance of OH generated using iron ions released from myoglobin by H₂O₂ is discussed in relation to myocardial reoxygenation injury.