用GSVAT模型研究地表/大气界面传输

用麦男水分和能量平衡资料验证了包括地下水的土壤－植被－大气水热传输模型（ＧＳＶＡＴ）,结果显示,模型能较好地模拟地表／大气水热传输特征,以及土壤水分的动态变化。通过敏感性分析,探讨叶／气界面,土壤／空气界面的节水调控效应,结果发现这两个界面对水汽传输的交互影响显著,若同时增加它们的水汽传输阻力,节水效果尤其明显。     

Surface behaviour of bile salts and tetrahydrolipstatin at air/water and oil/water interfaces.

The surface behaviour of two bile salts, sodium deoxycholate (NaDC) and sodium taurodeoxycholate (NaTDC), as well as that of tetrahydrolipstatin (THL), a potent gastrointestinal lipase inhibitor, was studied at air/water and oil/water interfaces, using interfacial tensiometry methods. The surface behaviour of NaDC and NaTDC was comparable at both oil/water and air/water interfaces. A fairly compact interfacial monolayer of bile salts is formed well below the critical micellar concentration (CMC) and can help to explain the well-known effects of bile salts on the kinetic behaviour of pancreatic lipases. Using the Wilhelmy plate technique, the surface pressure-molecular area curves recorded with THL at the air/water interface showed a collapse point at a surface pressure of 24.5 mN.m(-1), corresponding to a molecular area of 70 A(2). Surprisingly, using the oil drop method, a limiting molecular area of 160 A(2) was found to exist at the oil/water interface. On the basis of the above data, space-filling models were proposed for bile salts and THL at air/water and oil/water interfaces.     

Physicochemical characterisation of enzymatically hydrolysed derivatives of acetylated starch

Potato starch modified to different degrees by substitution with acetyl groups was the subject of this study undertaken to determine the influence of conditions of enzymatic hydrolysis on the surface-active properties of hydrolysates of acetylated starch. The effect of acetylation of starch preparation on its susceptibility to enzymatic hydrolysis in the membrane reactor was also considered. All hydrolysates of acetylated starch samples investigated were found to bring a decrease in the surface/interfacial tension, both at the air/water and the toluene/water interfaces. For binary hydrolysate-surfactant systems, the surface mole fractions in the mixed adsorbed monolayer at the air/water interface were estimated. For mixed systems, the synergism in reducing the surface tension at the air/water interface was observed. The experimentally obtained dynamic surface tension data for the aqueous solution of acetylated starch hydrolysates were used to estimate the diffusion coefficients. Particle size distributions of the hydrolysates formed in the aqueous solutions were compared to those of commercial maltodextrin.     

Molecular dynamics simulations of the hydrophobin SC3 at a hydrophobic/hydrophilic interface

Hydrophobins are small ( approximately 100 aa) proteins that have an important role in the growth and development of mycelial fungi. They are surface active and, after secretion by the fungi, self-assemble into amphipathic membranes at hydrophobic/hydrophilic interfaces, reversing the hydrophobicity of the surface. In this study, molecular dynamics simulation techniques have been used to model the process by which a specific class I hydrophobin, SC3, binds to a range of hydrophobic/hydrophilic interfaces. The structure of SC3 used in this investigation was modeled based on the crystal structure of the class II hydrophobin HFBII using the assumption that the disulfide pairings of the eight conserved cysteine residues are maintained. The proposed model for SC3 in aqueous solution is compact and globular containing primarily beta-strand and coil structures. The behavior of this model of SC3 was investigated at an air/water, an oil/water, and a hydrophobic solid/water interface. It was found that SC3 preferentially binds to the interfaces via the loop region between the third and fourth cysteine residues and that binding is associated with an increase in alpha-helix formation in qualitative agreement with experiment. Based on a combination of the available experiment data and the current simulation studies, we propose a possible model for SC3 self-assembly on a hydrophobic solid/water interface.     

Interfacial rheology of surface-active biopolymers: Acacia senegal gum versus hydrophobically modified starch

Acacia gum is a hybrid polyelectrolyte containing both protein and polysaccharide subunits. We study the interfacial rheology of its adsorption layers at the oil/water interface and compare it with adsorbed layers of hydrophobically modified starch, which for economic and political reasons is often used as a substitute for Acacia gum in technological applications. Both the shear and the dilatational rheological responses of the interfaces are considered. In dilatational experiments, the viscoelastic response of the starch derivative is just slightly weaker than that for Acacia gum, whereas we found pronounced differences in shear flow: The interfaces covered with the plant gum flow like a rigid, solidlike material with large storage moduli and a linear viscoelastic regime limited to small shear deformations, above which we observe apparent yielding behavior. In contrast, the films formed by hydrophobically modified starch are predominantly viscous, and the shear moduli are only weakly dependent on the deformation. Concerning their most important technological use as emulsion stabilizers, the dynamic interfacial responses imply not only distinct interfacial dynamics but also different stabilizing mechanisms for these two biopolymers.     

Interactions of lipid monolayers with the natural biopolymer hyaluronic acid

The interaction of the natural mucopolysaccharide hyaluronic acid with different lipids, present in the natural membranes, was studied at the lipid/water interface using thermodynamic methods and X-ray diffraction. The results show that this biopolymer modifies the properties and the structure of the lipid monolayer. The two-dimensional crystalline lattice and domain structure of the charged octadecylamine monolayer are strongly disturbed by the hyaluronic acid, the monolayer compressibility increases and the monolayer collapse pressure drops down. In addition, the presence of charged lipid interfaces influences the structural organisation of the hyaluronic acid at the membrane/water interfaces. The impacts of these results on the structural organisation at the membrane interface are discussed.     

Interactions of lipid monolayers with the natural biopolymer hyaluronic acid

The interaction of the natural mucopolysaccharide hyaluronic acid with different lipids, present in the natural membranes, was studied at the lipid/water interface using thermodynamic methods and X-ray diffraction. The results show that this biopolymer modifies the properties and the structure of the lipid monolayer. The two-dimensional crystalline lattice and domain structure of the charged octadecylamine monolayer are strongly disturbed by the hyaluronic acid, the monolayer compressibility increases and the monolayer collapse pressure drops down. In addition, the presence of charged lipid interfaces influences the structural organisation of the hyaluronic acid at the membrane/water interfaces. The impacts of these results on the structural organisation at the membrane interface are discussed.     

Changes in helical content or net charge of apolipoprotein C-I alter its affinity for lipid/water interfaces

Amphipathic α-helices mediate binding of exchangeable apolipoproteins to lipoproteins. To probe the role of α-helical structure in protein-lipid interactions, we used oil-drop tensiometry to characterize the interfacial behavior of apolipoprotein C-I (apoC-I) variants at triolein/water (TO/W) and 1-palmitoyl-2-oleoylphosphatidylcholine/triolein/water (POPC/TO/W) interfaces. ApoC-I, the smallest apolipoprotein, has two amphipathic α-helices. Mutants had single Pro or Ala substitutions that resulted in large differences in helical content in solution and on phospholipids. The ability of apoC-I to bind TO/W and POPC/TO/W interfaces correlated strongly with α-helical propensity. On binding these interfaces, peptides with higher helical propensity increased surface pressure to a greater extent. Likewise, peptide exclusion pressure at POPC/TO/W interfaces increased with greater helical propensity. ApoC-I retention on TO/W and POPC/TO/W interfaces correlated strongly with phospholipid-bound helical content. On compression of these interfaces, peptides with higher helical content were ejected at higher pressures. Substitution of Arg for Pro in the N-terminal α-helix altered net charge and reduced apoC-I affinity for POPC/TO/W interfaces. Our results suggest that peptide-lipid interactions drive α-helix binding to and retention on lipoproteins. Point mutations in small apolipoproteins could significantly change α-helical propensity or charge, thereby disrupting protein-lipid interactions and preventing the proteins from regulating lipoprotein catabolism at high surface pressures.     

How spermatozoa come to be confined to surfaces

In most detailed studies, sea urchin sperm movement has been analyzed mainly from observations of spermatozoa swimming at the interface between two media: water/air or water/glass. When spermatozoa are placed on a microscope slide, they rapidly appear to swim near those interfaces. The aim of this article is to determine how they become confined to the vicinity of surfaces. High-speed observations of moving spermatozoa reveal blurred portions in the flagellum images that propagate from base to tip, suggesting that flagellar waves contain an out-of-plane component. The model we have developed depicts how this tri-dimensional component tends to keep spermatozoa close to interfaces and, as a consequence, increases the time of contact between the egg surface and spermatozoa.     

Microbial biosurfactants production, applications and future potential

Microorganisms synthesise a wide range of surface-active compounds (SAC), generally called biosurfactants. These compounds are mainly classified according to their molecular weight, physico-chemical properties and mode of action. The low-molecular-weight SACs or biosurfactants reduce the surface tension at the air/water interfaces and the interfacial tension at oil/water interfaces, whereas the high-molecular-weight SACs, also called bioemulsifiers, are more effective in stabilising oil-in-water emulsions. Biosurfactants are attracting much interest due to their potential advantages over their synthetic counterparts in many fields spanning environmental, food, biomedical, and other industrial applications. Their large-scale application and production, however, are currently limited by the high cost of production and by limited understanding of their interactions with cells and with the abiotic environment. In this paper, we review the current knowledge and the latest advances in biosurfactant applications and the biotechnological strategies being developed for improving production processes and future potential.     

A Study on the Dose Distributions in Various Materials from an Ir-192 HDR Brachytherapy Source

Dose distributions of (192)Ir HDR brachytherapy in phantoms simulating water, bone, lung tissue, water-lung and bone-lung interfaces using the Monte Carlo codes EGS4, FLUKA and MCNP4C are reported. Experiments were designed to gather point dose measurements to verify the Monte Carlo results using Gafchromic film, radiophotoluminescent glass dosimeter, solid water, bone, and lung phantom. The results for radial dose functions and anisotropy functions in solid water phantom were consistent with previously reported data (Williamson and Li). The radial dose functions in bone were affected more by depth than those in water. Dose differences between homogeneous solid water phantoms and solid water-lung interfaces ranged from 0.6% to 14.4%. The range between homogeneous bone phantoms and bone-lung interfaces was 4.1% to 15.7%. These results support the understanding in dose distribution differences in water, bone, lung, and their interfaces. Our conclusion is that clinical parameters did not provide dose calculation accuracy for different materials, thus suggesting that dose calculation of HDR treatment planning systems should take into account material density to improve overall treatment quality.     

Water Replacement Hypothesis in Atomic Detail—Factors Determining the Structure of Dehydrated Bilayer Stacks

According to the water replacement hypothesis, trehalose stabilizes dry membranes by preventing the decrease of spacing between membrane lipids under dehydration. In this study, we use molecular-dynamics simulations to investigate the influence of trehalose on the area per lipid (APL) and related structural properties of dehydrated bilayers in atomic detail. The starting conformation of a palmitoyloleolylphosphatidylcholine lipid bilayer in excess water was been obtained by self-assembly. A series of molecular-dynamics simulations of palmitoyloleolylphosphatidylcholine with different degrees of dehydration (28.5, 11.7, and 5.4 waters per lipid) and different molar trehalose/lipid ratios (<1:1, 1:1, and >1:1) were carried out in the NPT ensemble. Water removal causes the formation of multilamellar “stacks” through periodic boundary conditions. The headgroups reorient from pointing outward to inward with dehydration. This causes changes in the electrostatic interactions between interfaces, resulting in interface interpenetration. Interpenetration creates self-spacing of the bilayers and prevents gel-phase formation. At lower concentrations, trehalose does not separate the interfaces, and acting together with self-spacing, it causes a considerable increase of APL. APL decreases at higher trehalose concentrations when the layer of sugar physically separates the interfaces. When interfaces are separated, the model confirms the water replacement hypothesis.     

Ultrasound and matter--physical interactions

The basic physical characteristics of ultrasound waves are reviewed in terms of the typical displacements, velocities, accelerations and pressures generated in various fluid media as a function of frequency. The effects on wave propagation of interfaces are considered, and the way in which waves are reflected, transmitted and mode converted at interfaces introduced. Then the nonlinear propagation of high amplitude ultrasound is explained, and its consequences, including the generation of harmonic frequencies and enhanced attenuation, considered. The absorption of ultrasonic waves and the resulting heat deposition in absorbing media are described together with factors determining the resulting temperature rises obtained. In the case of tissue these include conduction and perfusion. The characteristics of cavitation in fluid media are also briefly covered. Finally, secondary nonlinear physical effects are described. These include radiation forces on interfaces and streaming in fluids.     

Cavities in protein–DNA and protein–RNA interfaces

An analysis of cavities present in protein–DNA and protein–RNA complexes is presented. In terms of the number of cavities and their total volume, the interfaces formed in these complexes are akin to those in transient protein–protein heterocomplexes. With homodimeric proteins protein–DNA interfaces may contain cavities involving both the protein subunits and DNA, and these are more than twice as large as cavities involving a single protein subunit and DNA. A parameter, cavity index, measuring the degree of surface complementarity, indicates that the packing of atoms in protein–protein/DNA/RNA is very similar, but it is about two times less efficient in the permanent interfaces formed between subunits in homodimers. As within the tertiary structure and protein–protein interfaces, protein–DNA interfaces have a higher inclination to be lined by β-sheet residues; from the DNA side, base atoms, in particular those in minor grooves, have a higher tendency to be located in cavities. The larger cavities tend to be less spherical and solvated. A small fraction of water molecules are found to mediate hydrogen-bond interactions with both the components, suggesting their primary role is to fill in the void left due to the local non-complementary nature of the surface patches.     

Aromatic residues in the C terminus of apolipoprotein C-III mediate lipid binding and LPL inhibition

Plasma apoC-III levels correlate with triglyceride (TG) levels and are a strong predictor of CVD outcomes. ApoC-III elevates TG in part by inhibiting LPL. ApoC-III likely inhibits LPL by competing for lipid binding. To probe this, we used oil-drop tensiometry to characterize binding of six apoC-III variants to lipid/water interfaces. This technique monitors the dependence of lipid binding on surface pressure, which increases during TG hydrolysis by LPL. ApoC-III adsorption increased surface pressure by upward of 18 mN/m at phospholipid/TG/water interfaces. ApoC-III was retained to high pressures at these interfaces, desorbing at 21–25 mN/m. Point mutants, which substituted alanine for aromatic residues, impaired the lipid binding of apoC-III. Adsorption and retention pressures decreased by 1–6 mN/m in point mutants, with the magnitude determined by the location of alanine substitutions. Trp42 was most critical to mediating lipid binding. These results strongly correlate with our previous results, linking apoC-III point mutants to increased LPL binding and activity at lipid surfaces. We propose that aromatic residues in the C-terminal half of apoC-III mediate binding to TG-rich lipoproteins. Increased apoC-III expression in the hypertriglyceridemic state allows apoC-III to accumulate on lipoproteins and inhibit LPL by preventing binding and/or access to substrate.     

Interfacial properties of amphipathic beta strand consensus peptides of apolipoprotein B at oil/water interfaces

The region between residues 968 and 1882 of apolipoprotein B (apoB-21 to apoB-41) is rich in amphipathic beta strands (AbetaSs) and promotes the assembly of primordial triacylglyceride (TAG)-rich lipoproteins. To understand the importance of AbetaS in recruiting TAG, the interfacial properties of two AbetaS consensus peptides, P12 and P27, were studied at dodecane/water (DD/W) and triolein/water (TO/W) interfaces. P12 (acetyl-LSLSLNADLRLK-amide) and P27 (acetyl-LSLSLNADLRLKNGNLSLSLNADLRLK-amide), when added into the aqueous phase surrounding a suspended oil drop (dodecane or triolein), decreased the interfacial tension (gamma) in a concentration-dependent manner. At the DD/W interface, 1 x 10(-5) M P12 decreased gamma to approximately 20 mN/m and 6.6 x 10(-6) M P27 decreased gamma to approximately 13 mN/m. At the TO/W interface, 1.5 x 10(-5) M P12 decreased gamma to approximately 14 mN/m and 9.0 x 10(-6) M P27 decreased gamma to approximately 12 mN/m. The surface area of both peptides was between 11.2 and 15.1 angstroms2 per residue, consistent with beta sheets lying flat on DD/W and TO/W interfaces. P12 and P27 are almost purely elastic on DD/W, TO/W, and air/water interfaces. When P12 and P27 were compressed beyond the equilibrium gamma to as low as 4 mN/m, they could not be readily desorbed from either interface. These properties probably help in assembling nascent TAG-rich lipoproteins, and AbetaS may anchor apoB to beta lipoproteins.     

Hydration of protein-protein interfaces

We present an analysis of the water molecules immobilized at the protein-protein interfaces of 115 homodimeric proteins and 46 protein-protein complexes, and compare them with 173 large crystal packing interfaces representing nonspecific interactions. With an average of 15 waters per 1000 A2 of interface area, the crystal packing interfaces are more hydrated than the specific interfaces of homodimers and complexes, which have 10-11 waters per 1000 A2, reflecting the more hydrophilic composition of crystal packing interfaces. Very different patterns of hydration are observed: Water molecules may form a ring around interfaces that remain "dry," or they may permeate "wet" interfaces. A majority of the specific interfaces are dry and most of the crystal packing interfaces are wet, but counterexamples exist in both categories. Water molecules at interfaces form hydrogen bonds with protein groups, with a preference for the main-chain carbonyl and the charged side-chains of Glu, Asp, and Arg. These interactions are essentially the same in specific and nonspecific interfaces, and very similar to those observed elsewhere on the protein surface. Water-mediated polar interactions are as abundant at the interfaces as direct protein-protein hydrogen bonds, and they may contribute to the stability of the assembly.     

Effects of interfaces on aggregates of peptides derived from pancreatic islet amyloid polypeptide

Recent experimental studies indicate that oligomeric complexes of misfolded proteins and peptides are the primary agents of cytotoxicity in amyloid-related diseases. Given the prevalence of mixed-polarity interfaces in physiological environments, an understanding of the mechanisms of interactions between amorphous (pre-fibrillar) aggregates and surfaces is important for completing our knowledge of the behaviour of peptide aggregation phenomena. We have employed fully solvated molecular dynamics simulations to study the morphology, interactions and peptide conformations of disordered aggregates of the amyloidogenic NFGAIL (derived from human islet amyloid polypeptide) and non-amyloidogenic AGAIL peptides upon adsorption to vapour–water, decane–water, bilayer and solid–water interfaces. All of the interfaces studied promote elongation and surface-spreading of both peptide aggregates, with the liquid–liquid interface being particularly efficient at altering the gross morphology of disordered aggregates. NFGAIL aggregates are more effective at disrupting lipid bilayers compared to AGAIL. Additionally, the interfaces studied cause greater changes in peptide conformations within the NFGAIL aggregates compared to AGAIL. We propose that simulations may elucidate the capability of interfaces to effect changes in the behaviour of disordered peptide aggregates, which may also serve to provide measures of the intrinsic fibrillogenicity of a given peptide sequence.     

Investigating the effects of wettability and gaseous nanobubbles on roughened wall–fluid interface using molecular dynamics simulation

This paper reports on the use of molecular dynamics (MD) simulation to investigate the coupling effects of wettability, surface roughness and interfacial nanobubbles (INBs) on wall–fluid interfaces. The fluid properties close to the wall–fluid interface, such as potential energy, density, diffusion coefficients of fluid molecules and effective slip length are simulated. In the cases without surface nanobubbles, regions with lower potential energy have a higher probability of hosting water molecules. The local translational and rotational diffusion coefficients of water within the cavities are strongly influenced by wettability but largely unaffected by hydrodynamic effects. In cases where INBs exist, variations in wettability result in distinctly different argon morphologies. Argon nanobubbles form a convex shape on Wenzel-like interfaces but a shallow concave shape on Cassie-like interfaces. The phenomenon of water molecules invading grooves tends to occur on Wenzel-like interfaces; however, this depends largely on the morphology of the grooves. The high mobility and high density of argon molecules indicate that the state of the argon molecules within the grooves may require further investigation. Our results also show that the effective slip length is significantly influenced by wall–fluid wettability as well as the morphology of INBs.