Interfacial free energies of intact and reconstituted erythrocyte surfaces: Implications for biological adhesion

Model cell surfaces consisting of phospholipids or phospholipids and the erythrocyte membrane glycoprotein glycophorin have been formed at an oil/water interface. Interfacial free energies have been estimated from surface wetting by both hydrophobic and hydrophilic test droplets on both the model surfaces and on intact erythrocytes. The use of a dense fluorocarbon oil to form the oil/water interface facilitates analysis by minimising surface deformation by the test drop. Hydrophobic test droplets (polar hydrocarbon oils) show increasing contact angles (decreasing wetting) with increasing hydrophilicity (decreasing interfacial free energy) of the model interface. Hydrophilic test droplets (phase separated aqueous polymer systems) show the opposite behaviour, spreading more as the interfacial free energy is decreased. Both systems give similar estimates of the interfacial free energy. Glycophorin reproduces the wetting properties of intact cell surfaces by reducing the lipid-water interfacial free energy from 5·10^?3 J·m^?2 to 1·10^?6 J·m^?2. From molecular considerations it is concluded that ‘cell surface free energy’ is an ambiguous term; its magnitude depends on the location of the interface in question. Thus, in a thermodynamic analysis of interactions at biosurfaces (such as cellular adhesion, chemotaxis or membrane fusion), the interfacial free energies may vary by more than three orders of magnitude depending on the location of the particular interface.     

Complex Formation Equilibria in Two-Component Bilayer Lipid Membrane: Interfacial Tension Method

We measured the interfacial tension of lipid membranes composed of ceramide-cholesterol, ceramide-sphingomyelin, and sphingomyelin-cholesterol. The membrane components formed 1:1 complexes. Interfacial tension measurements were used to determine the membrane surface concentration A ₃⁻¹, the membrane interfacial tension γ₃, and the stability constant K.     

Elevated CO2 and drought alter tissue water relations of birch (Betula populifolia Marsh.) seedlings

The effect of increasing atmospheric CO₂ concentrations on tissue water relations was examined in Betula populifolia, a common pioneer tree species of the northeastern U.S. deciduous forests. Components of tissue water relations were estimated from pressure volume curves of tree seedlings grown in either ambient (350 l l^–1) or elevated CO₂ (700 l l^–1), and both mesic and xeric water regimes. Both CO₂ and water treatment had significant effects on osmotic potential at full hydration, apoplasmic fractions, and tissue elastic moduli. Under xeric conditions and ambient CO₂ concentrations, plants showed a decrease in osmotic potentials of 0.15 MPa and an increase in tissue elastic moduli at full hydration of 1.5 MPa. The decrease in elasticity may enable plants to improve the soil-plant water potential gradient given a small change in water content, while lower osmotic potentials shift the zero turgor loss point to lower water potentials. Under elevated CO₂, plants in xeric conditions had osmotic potentials 0.2 MPa lower than mesic plants and decreased elastic moduli at full hydration. The increase in tissue elasticity at elevated CO₂ enabled the xeric plants to maintain positive turgor pressures at lower water potentials and tissue water contents. Surprisingly, the elevated CO₂ plants under mesic conditions had the most inelastic tissues. We propose that this inelasticity may enable plants to generate a favorable water potential gradient from the soil to the plant despite the low stomatal conductances observed under elevated CO₂ conditions.     

Composition and Oil-Water Interfacial Tension Studies in Different Vegetable Oils

The interfacial tension is one of the most important fundamental properties and presents crucial impacts throughout vegetable oil production, application and digestion. In this study, composition of vegetable oil including tocopherols, phytosterols, phenolic compounds, phospholipids, fatty acid composition and other constituents were determined. Furthermore, interfacial tension and its relationship with vegetable oil compositions were analyzed. Distribution and profile of composition of vegetable oil were remarkably different. The interfacial tension results showed physical refined vegetable oil exhibited an obviously lower interfacial tension than chemical refined oil attributed to abundant minor compositions. Moreover, the correlation analysis results indicated that phenolic compounds demonstrated the greatest influence on interfacial tension of vegetable oil against water with r = − 0.671, p = 0.009, followed by free fatty acid value, linoleic acid of triglyceride and phospholipids with r = − 0.639, 0.626, − 0.576 and p = 0.014, 0.017 and 0.031, respectively. No significant correlation was found between interfacial tension and other minor compositions. These results contribute to regulating lipid metabolism and evaluating oil quality more scientifically.

     

Oxygen-18 Exchange as a Measure of Accessibility of CO(2) and HCO(3) to Carbonic Anhydrase in Chlorella vulgaris (UTEX 263)

We have measured the exchange of ¹⁸O between CO₂ and H₂O in stirred suspensions of Chlorella vulgaris (UTEX 263) using a membrane inlet to a mass spectrometer. The depletion of ¹⁸O from CO₂ in the fluid outside the cells provides a method to study CO₂ and HCO₃⁻ kinetics in suspensions of algae that contain carbonic anhydrase since ¹⁸O loss to H₂O is catalyzed inside the cells but not in the external fluid. Low-CO₂ cells of Chlorella vulgaris (grown with air) were added to a solution containing ¹⁸O enriched CO₂ and HCO₃⁻ with 2 to 15 millimolar total inorganic carbon. The observed depletion of ¹⁸O from CO₂ was biphasic and the resulting ¹⁸C content of CO₂ was much less than the ¹⁸O content of HCO₃⁻ in the external solution. Analysis of the slopes showed that the Fick's law rate constant for entry of HCO₃⁻ into the cell was experimentally indistinguishable from zero (bicarbonate impermeable) with an upper limit of 3 × 10⁻⁴ s⁻¹ due to our experimental errors. The Fick's law rate constant for entry of CO₂ to the sites of intracellular carbonic anhydrase was large, 0.013 per second, but not as great as calculated for no membrane barrier to CO₂ flux (6 per second). The experimental value may be explained by a nonhomogeneous distribution of carbonic anhydrase in the cell (such as membrane-bound enzyme) or by a membrane barrier to CO₂ entry into the cell or both. The CO₂ hydration activity inside the cells was 160 times the uncatalyzed CO₂ hydration rate.     

Solvatochromic Modeling of Laurdan for Multiple Polarity Analysis of Dihydrosphingomyelin Bilayer

The hydration properties of the interface between lipid bilayers and bulk water are important for determining membrane characteristics. Here, the emission properties of a solvent-sensitive fluorescence probe, 6-lauroyl-2-dimethylamino naphthalene (Laurdan), were evaluated in lipid bilayer systems composed of the sphingolipids D-erythro-N-palmitoyl-sphingosylphosphorylcholine (PSM) and D-erythro-N-palmitoyl-dihydrosphingomyelin (DHPSM). The glycerophospholipids 1-palmitoyl-2-palmitoyl-sn-glycero-3-phosphocholine and 1-oleoyl-2-oleoyl-sn-glycero-3-phosphocholine were used as controls. The fluorescence properties of Laurdan in sphingolipid bilayers indicated multiple excited states according to the results obtained from the emission spectra, fluorescence anisotropy, and the center-of-mass spectra during the decay time. Deconvolution of the Laurdan emission spectra into four components based on the solvent model enabled us to identify the varieties of hydration and the configurational states derived from intermolecular hydrogen bonding in sphingolipids. Sphingolipids showed specific, interfacial hydration properties stemming from their intra- and intermolecular hydrogen bonds. Particularly, the Laurdan in DHPSM revealed more hydrated properties compared to PSM, even though DHPSM has a higher T_m than PSM. Because DHPSM forms hydrogen bonds with water molecules (in 2NH configurational functional groups), the interfacial region of the DHPSM bilayer was expected to be in a highly polar environment. The careful analysis of Laurdan emission spectra through the four-component deconvolution in this study provides important insights for understanding the multiple polarity in the lipid membrane.     

Microbiological and production characteristics of the Dagang high-temperature heavy oil reservoir (block no. 1) during trials of the biotechnology for enhanced oil recovery

Microbiological and biogeochemical data on the Kongdian bed (block no. 1) of the Dagang high-temperature oilfield during trials of the biotechnology for enhanced oil recovery are reported. Oil-bearing horizons of block no. 1 are characterized by high temperature (56.9–58.4°C), complex geological conditions, and heavy oil (density 0.966–0.969 g/cm³). The biotechnology implied injecting oxygen as an air-water mixture or H₂O₂ together with aqueous solution of nitrogen and phosphorus mineral salts through injection wells in order to activate the oilfield microbial community. In the course of trials, an increase in abundance of aerobic and anaerobic microorganisms was revealed, as well as increased methanogenesis rate in formation water. Microbial oxidation of heavy oil resulted in increased concentration of mineral carbonates dissolved in formation water, changes in the stable carbon isotopic composition δ¹³C/Σ(CO₂ + HCO₃ ^- + CO₃ ^2-), formation of biosurfactants, and decreased interfacial tension of formation water. Application of the biotechnology at the Kongdian bed (block no. 1) resulted in additional recovery of 6331 t oil. Oil viscosity in the zone of production wells located at the North block of the Kongdian bed decreased by 11%. A total of 46152 t additional oil was recovered at three experimental sites of the Dagang oilfield (North block and block no. 1 of the Kongdian bed and the Gangxi bed), which is an indication of high efficiency of the technology for activation of the oilfield microflora for heavy oil replacement from high-temperature oilfields.     

Weak acid permeability of a villous membrane: Formic acid transport across rat proximal tubule

Chloride/formate exchange, in parallel with Na⁺/H⁺ exchange and nonionic diffusion of H₂CO₂, has been proposed as a mechanism of electroneutral transcellular Cl⁻ reabsorption by the proximal tubule. However, the measured brush border H₂CO₂ permeability of the rat proximal tubule is at least an order of magnitude too low to support sufficient H₂CO₂ recycling. To investigate the possibility that an unstirred layer within the brush border might depress the measured H₂CO₂ permeability, we constructed a mathematical model of a villous membrane. Axial fluxes along villous and intervillous spaces were specified by Nernst-Planck diffusion equations. Model parameters were set to achieve agreement with ion and water fluxes measured in the rat proximal tubule. The equations were solved numerically to generate steady-state concentration profiles in the villous and intervillous spaces. An apparent brush border H₂CO₂ permeability was determined by perturbing luminal [H₂CO₂] and calculating the change in H₂CO₂ flux. Overall, the ratio of apparent brush border H₂CO₂ permeability to cell membrane H₂CO₂ permeability was greater than 90%. Contributing to the small decrease in apparent permeability are finite diffusion coefficients, folding of the membrane, and acidification of the luminal solution. An approximate analysis of this system shows the critical parameters of brush border formate transport to be the actual membrane H₂CO₂ permeability, and the diffusion coefficients of HCO ₃ ⁻ and HCO ₃ ⁻ . Nevertheless, decreasing the diffusion coefficients by one order of magnitude failed to depress apparent brush border H₂CO₂ permeability by more than an additional 25%. We conclude that although permeability is systematically underestimated across a villous membrane, unstirred layer effects in the brush border are still too small to resolve the discrepancy between the measured value of H₂CO₂ permeability and the value needed to allow recycling.     

Effect of dehydration and high light on photosynthesis of two C3 plants (Phaseolus vulgaris L. and Elatostema repens (Lour.) Hall f.)

The effect of drought on the photosynthetic functioning of two C₃ plants, Phaseolus vulgaris and Elatostema repens, has been examined. Leaf net CO₂ uptake measured in normal air was negligible at a leaf water deficit of about 30% while the calculated leaf intercellular CO₂ concentration (Ci) was unchanged. However, both the maximal photosynthetic capacity (CO₂-dependent O₂ evolution) and apparent quantum yield, measured in the presence of saturating CO₂ levels (5 to 14%), only started to decrease within the range of 25 to 30% leaf water deficit. This shows that the drought-induced inhibition seen in normal air is not caused by an inhibition of the photosynthetic mechanism, and that in this case Ci values can be misleading. Both 77 K and room-temperature fluorescence measurements indicate that the functioning of the photosystem-II reaction centre is hardly modified by water shortage. Furthermore, an analysis of photochemical chlorophyll fluorescence quenching shows, in the absence of CO₂, that O₂ can be an efficient acceptor of photosynthetic energy, even in severly dehydrated plants which do not show net CO₂ uptake in normal air. In these plants, O₂ is probably reduced mainly via Mehler-type reactions. High-light treatment given at low O₂ increases photoinhibition as measured by the decrease of apparent quantum yield in dehydrated P. vulgaris, whereas, interestingly, 1% O₂ protects dehydrated E. repens against high-light damage. The two plants could have different protective mechanisms depending upon the O₂ level or different photoinhibitory sites or mechanisms.Abbreviations and symbols Ca, Ci ambient and calculated intercellular CO₂ concentration - Fm, Fo, Fv maximum, initial and variable fluorescence emission - LWD leaf water deficit - PPFD photosynthetic photon flux density - PSII photosystem II - qQ photochemical quenching of chlorophyll fluorescence     

Modeling and mutagenesis of amino acid residues critical for CO2 hydration by specialized NDH-1 complexes in cyanobacteria

The uptake of inorganic carbon in cyanobacteria is facilitated by an energetically intensive CO₂-concentrating mechanism (CCM). This includes specialized Type-1 NDH complexes that function to couple photosynthetic redox energy to CO₂ hydration forming the bicarbonate that accumulates to high cytoplasmic concentrations during the operation of the CCM, required for effective carbon fixation. Here we used a Synechococcus PCC7942 expression system to investigate the role of conserved histidine and cysteine residues in the CupB (also designated, ChpX) protein, which has been hypothesized to participate in a vectoral CO₂ hydration reaction near the interface between CupB protein and the proton-pumping subunits of the NDH-1 complex. A homology model has been constructed and most of the targeted conserved residues are in the vicinity of a Zn ion modeled to form the catalytic site of deprotonation and CO₂ hydration. Growth and CO₂ uptake assays show that the most severe defects in activity among the targeted residues are due to a substitution of the predicted Zn ligand, CupB-His86. Mutations at other sites produced intermediate effects. Proteomic analysis revealed that some amino acid substitution mutations of CupB caused the induction of bicarbonate uptake proteins to a greater extent than complete deletion of CupB, despite growth under CO₂-enriched conditions. The results are discussed in terms of hypotheses on the catalytic function of this unusual enzyme.     

Physical,chemical and biological characteristics* of CO2 gas flux across the air-water interface

Abstract Carbon dioxide gas flux across the air-water interface is most often treated as a ‘simple’ physical process, primarily responding to wind speed and water temperature. Available experimental data yield an exponential regression equation relating wind speed to the thickness of a stagnant boundary film through which gas diffuses to or from the water. Flux of CO₂ is influenced by CO₂ hydration reactions in the stagnant boundary layer. High pH and a thick stagnant boundary layer favour chemical enhancement of the CO₂ gas flux. The rate of CO₂ flux reflects the sum of net organic metabolism plus CaCO₃ reactions. Some interesting gas-flux constraints on the rate of net organic carbon production and on global geochemical cycling of CaCO₃ emerge. At high pH (circa 10), the maximum net organic carbon production which can be supported by CO₂ flux across the air-water interface is about 0.06 mol C m^&2 d^&1. On a global scale, organic C, not atmospheric C, appears to account for the ‘CO₂’ term in the classical CaCO₃ dissolution-precipitation reaction.     

Carbonic Anhydrase and the Uptake of Inorganic Carbon by Synechococcus sp. (UTEX-2380)

We report the changes in the concentrations and ¹⁸O contents of extracellular CO₂ and HCO₃⁻ in suspensions of Synechococcus sp. (UTEX 2380) using membrane inlet mass spectrometry. This marine cyanobacterium is known to have an active uptake mechanism for inorganic carbon. Measuring ¹⁸O exchange between CO₂ and water, we have found the intracellular carbonic anhydrase activity to be equivalent to 20 times the uncatalyzed CO₂ hydration rate in different samples of cells that were grown on bubbled air (low-CO₂ conditions). This activity was only weakly inhibited by ethoxzolamide with an I₅₀ near 7 to 10 micromolar in lysed cell suspensions. We have shown that even with CO₂-starved cells there is considerable generation of CO₂ from intracellular stores, a factor that can cause errors in measurement of net CO₂ uptake unless accounted for. It was demonstrated that use of ¹³C-labeled inorganic carbon outside the cell can correct for such errors in mass spectrometric measurement. Oxygen-18 depletion experiments show that in the light, CO₂ readily passes across the cell membrane to the sites of intracellular carbonic anhydrase. Although HCO₃⁻ was readily taken up by the cells, these experiments shown that there is no significant efflux of HCO₃⁻ from Synechococcus.     

Pre-and post-branchial blood respiratory status during acute hypercapnia or hypoxia in rainbow trout,Oncorhynchus mykiss

Simultaneous venous (pre-branchial) and arterial (post-branchial) extracorporeal blood circulations were utilized to monitor continuously the rapid and progressive effects of acute environmental hypercapnia (water partial pressure of CO₂ 4.8±0.2 torr) or hypoxia (water partial pressure of O₂ 25±2 torr) on oxygen and carbon dioxide tensions and pH in the blood of rainbow trout (Oncorhynchus mykiss). During hypercapnia, the CO₂ tension in the arterial blood increased from 1.7±0.1 to 6.2±0.2 torr within 20 min and this was associated with a decrease of arterial extracellular pH from 7.95±0.03 to 7.38±0.03; the acid-base status of the mixed venous blood changed in a similar fashion. The decrease in blood pH in vivo was greater than in blood equilibrated in vitro with a similar CO₂ tension indicating a significant metabolic component to the acidosis in vivo. Under normocapnic conditions, venous blood CO₂ tension was slightly higher than arterial blood CO₂ tension difference was abolished or reversed during the initial 25 min of hypercapnia indicating that CO₂ was absorbed from the water during this period. Arterial O₂ tension remained constant during hypercapnia; however, venous blood O₂ tension decreased significantly (from 22.0±2.6 to 9.0±1.0 torr) during the initial 10 min. Hypercapnia elicited the release of catecholamines (adrenaline and noradrenaline) into the blood. The adrenaline concentration increased from 6±3 to 418±141 nmol · l^-1 within 25 min; noradrenaline concentration increased from 3±0.5 to 50±21 nmol · l^-1 within 15 min. During hypoxia arterial blood O₂ tension declined progressively from 108.4±9.9 to 12.8±1.7 torr within 30 min. Venous blood O₂ tension initially was stable but then decreased abruptly as catecholamines were released into the circulation. The release of catecholamines occurred concomitantly with a sudden metabolic acidosis in both blood compartments and a rise in CO₂ tension in the mixed venous blood only.Abbreviations CCO₂ plasmatotal carbondioxide - CtO₂ blood oxygen content - PO₂ partial pressure of oxygen - PCO₂ partial pressure of carbon dioxide - PaO₂ arterial bloodPO₂ - PaCO₂ arterial bloodPCO₂ - PvCO₂ venous bloodPCO₂ - PwO₂ waterPO₂ - PwCO₂ waterPCO₂ - Hb haemoglobin - SHbO₂ haemoglobin oxygen saturation - HPLC high-performance liquid chromatography - rbc red blood cell(s) - Hct haematocrit     

Evaluating the efficiency of enzyme accelerated CO2 capture: chemical kinetics modelling for interpreting measurement results

In this paper, the efficiency of the carbonic anhydrase (CA) enzyme in accelerating the hydration of CO₂ is evaluated using a measurement system which consists of a vessel in which a gaseous flow of mixtures of nitrogen and CO₂ is bubbled into water or water solutions containing a known quantity of CA enzyme. The pH value of the solution and the CO₂ concentration at the measurement system gas exhaust are continuously monitored. The measured CO₂ level allows for assessing the quantity of CO₂, which, subtracted from the gaseous phase, is dissolved into the liquid phase and/or hydrated to bicarbonate. The measurement procedure consists of inducing a transient and observing and modelling the different kinetics involved in the steady-state recovery with and without CA. The main contribution of this work is exploiting dynamical system theory and chemical kinetics modelling for interpreting measurement results for characterising the activity of CA enzymes. The data for model fitting are obtained from a standard bioreactor, in principle equal to standard two-phase bioreactors described in the literature, in which two different techniques can be used to move the process itself away from the steady-state, inducing transients.     

Molecular simulation and experimental studies on the interfacial properties of a mixed surfactant SDS/C4mimBr

Mixed surfactants have potential applications in various fields. The understanding and prediction of their macro- and microscopic properties are of great importance in the designing of these materials. We used molecular dynamics (MD) and experiments to study the interfacial tension and the microscopic structures of the sodium dodecyl sulfate (SDS)/C₄mimBr mixed surfactant at the water/hexane interface. The interfacial tension, density profile, radial distribution function (RDF), orientation distribution of the tails and order parameters have been examined. It seems that the addition of C₄mimBr decreased the interfacial tension; a higher C₄mimBr concentration resulted in a thicker interface, a smaller droplet, and more disordered SDS tails. The competition between free volume and electrostatic shielding seems to be the primary mechanism behind these phenomena.     

Effects of Lecithin Addition in Oil or Water Phase on the Stability of Emulsions Made with Whey Proteins

The effects of lecithin addition in oil or water phase on the stability of oil-in-water emulsions made with 0.1 wt% whey protein and 10 wt% n-tetradecane at neutral and acidic pH were studied by monitoring the gravitational creaming and phase separation. The effects of lecithin addition on the interfacial behavior of β-lactoglobulin were also studied to compare with the results of emulsion stability. At neutral pH, crude phosphatidylcholine (PC) from egg yolk or soybean increased the stability of the emulsion made with protein and lowered the interfacial tension of protein films more effectively than pure egg PC. A more remarkable effect on both the emulsion stability and the interfacial tension was found when crude PC was added in the oil phase rather than in the water phase. The purity of lecithins and the way to add them are suggested to be very important to make a stable emulsion with protein. On acidic pH (4.5 or 3.0), the increased creaming or phase separation in a whey protein-stabilized emulsion, but the lowered interfacial tension of β-lactoglobulin films, were found upon the addition of pure or crude PC in oil or water phase. These results suggest that in acidic pH, densely packed films may be formed on a planar oil–water interface, but not on adsorbed layers around oil droplets in an emulsion.     

In situ bioremediation of hydrocarbon in soil

Laboratory and field experiments were carried out for bioremediation of soils contaminated by fuel oil and motor oil. Bioventing was combined with the application of selected bacteria and dissolved nutrients. In the field experiments, soil gas was evacuated by air pumps from the permeable boreholes. The process was followed by both soil and gas analysis. Biodegradation of oil contamination and the microbial activity was measured by the oil and cell concentration in the soil. In 2 months, the oil content decreased considerably, and the cell number increased by one order of magnitude or more. The evacuated gas was tested for CO₂, O₂ and volatilized hydrocarbon content. The CO₂ level proves the presence of biodegradation: a permanent high value about ten times higher than normal, could be measured for 2 months, followed by a slow decrease in the third month. Volatilized hydrocarbon content was the highest in the first 2 d. After a continuous decrease, it dropped under the threshold of measurability for the third month. Selective biodegradation of hydrocarbon mixtures (oily wastes) was investigated as well: gas Chromatographic oil analysis showed the changes in the oil composition. The appropriate microflora was working in an ideal commensalism, and as a result, all of the hydrocarbon components were degraded nearly to the same extent.     

Deciphering the role of charge,hydration, and hydrophobicity for cytotoxic activities and membrane interactions of bile acid based facial amphiphiles

We synthesized four cationic bile acid based facial amphiphiles featuring trimethyl ammonium head groups. We evaluated the role of these amphiphiles for cytotoxic activities against colon cancer cells and their membrane interactions by varying charge, hydration and hydrophobicity. The singly charged cationic Lithocholic acid based amphiphile (LCA-TMA₁) is most cytotoxic, whereas the triply charged cationic Cholic acid based amphiphile (CA-TMA₃) is least cytotoxic. Light microscopy and Annexin-FITC assay revealed that these facial amphiphiles caused late apoptosis. In addition, we studied the interactions of these amphiphiles with model membrane systems by Prodan-based hydration, DPH-based anisotropy, and differential scanning calorimetry. LCA-TMA₁ is most hydrophobic with a hard charge causing efficient dehydration and maximum perturbations of membranes thereby facilitating translocation and high cytotoxicity against colon cancer cells. In contrast, the highly hydrated and multiple charged CA-TMA₃ caused least membrane perturbations leading to low translocation and less cytotoxicity. As expected, Chenodeoxycholic acid and Deoxycholic acid based amphiphiles (CDCA-TMA₂, DCA-TMA₂) featuring two charged head groups showed intermediate behavior. Thus, we deciphered that charge, hydration, and hydrophobicity of these amphiphiles govern membrane interactions, translocation, and resulting cytoxicity against colon cancer cells.     

Effect of changes in water content on photosynthesis,transpiration and discrimination against 13CO2 and C18O16O in Pleurozium and Sphagnum

Photosynthetic gas exchange characteristics of two common boreal forest mosses, Sphagnum (section acutifolia) and Pleurozium schreberi, were measured continuously during the time required for the moss to dry out from full hydration. Similar patterns of change in CO₂ assimilation with variation in water content occurred for both species. The maximum rates of CO₂ assimilation for Sphagnum (approx. 7 mol m^–2 s^–1) occurred at a water content of approximately 7 (fresh weight/dry weight) while for Pleurozium the maximum rate (approx. 2 mol m^–2 s^–1) occurred at a water content of approximately 6 (fresh weight/dry weight). Above and below these water contents CO₂ assimilation declined. In both species total conductance to water vapour (expressed as a percentage of the maximum rates) remained nearly constant at a water content above 9 (fresh weight/dry weight), but below this level declined in a strong linear manner. Short-term, on-line ¹³CO₂ and C¹⁸O¹⁶O discrimination varied substantially with changes in moss water content and associated changes in the ratio of chloroplast CO₂ to ambient CO₂ partial pressure. At full hydration (maximum water content) both Sphagnum and Pleurozium had similar values of ¹³CO₂ discrimination (approx. 15). Discrimination against ¹³CO₂ increased continuously with reductions in water content to a maximum of 27 in Sphagnum and 22 in Pleurozium. In a similar manner C¹⁸C¹⁶O discrimination increased from approximately 30 at full hydration in both species to a maximum of 150 in Sphagnum and 90 in Pleurozium, at low water content. The observed changes in C¹⁸O¹⁶O were strongly correlated to predictions of a mechanistic model of discrimination processes. Field measurements of moss water content suggested that photosynthetic gas exchange by moss in the understory of a black spruce forest was regularly limited by low water content.     

Study of the molecular array behaviours and interfacial activities of green surfactant alkyl polyglycoside and the mixed systems with other surfactants on oil–water interface

Abstract

The widely performance of surfactants is closely related to their interfacial activity, which is essentially determined by the molecular array behaviours at the interface, of which the studies are significance for clearly understanding their structure-performance relationships. In this paper, the detailed molecular array behaviours of green surfactant alkyl polyglycoside (APG) and the mixed systems with other types of surfactants on oil/water interface have been studied using molecular dynamics simulations, and the key theoretical principle was confirmed by quantum chemistry calculations. It was found that the hydrophilic maltose ring head groups of decyl polyglycoside (C₁₀-APG) are prone to lie flatly at the oil–water interface, the steric hindrance results in the low interfacial density, which critically determines the limit of the interfacial activity. The interfacial adsorption behaviours of the binary mixtures of C₁₀-APG and SDS or DATB and the ternary mixtures of C₁₀-APG, SDS and DATB were studied in detail, how the efficient synergism effect could be achieved for the mixture to get super high interfacial activity was discussed. This study provides a strategy to reveal how the molecular interfacial behaviours determine the key interfacial characteristics of the novel surfactants, which might provide help to promote their applications.