Crowding effects in binary mixtures of rod-like and spherical particles

Crowding effects relevant to the phase stability of binary mixtures of rod-like and spherical particles are investigated by means of Monte Carlo simulations in the isobaric NPT ensemble. The two types of particles are represented, respectively, by freely rotating hard spherocylinders of a moderate aspect ratio (L/sigma = 5) and hard spheres of the same diameter sigma. Molar fractions of spheres ranging xHS = 0.00-0.37 are considered with the aim of characterizing the crowding effects on the liquid crystal phases of the hard spherocylinder fluid induced by the spherical component as depleting agent. We find that the addition of the spherical crowder is beneficial for the stabilization of the layers of the rod-like particles characteristic of the smectic phase. On the contrary, the addition of spheres has a negative impact upon the stability of the nematic phase, where the rod-like particles tend to align collectively parallel to each other. Interestingly, the spheres tend to arrange forming rod-like clusters in the nematic phase and lamellar structures in the smectic phase, which is compensated by the entropy gained by the spherocylinder particles in each phase. The main results are in qualitative agreement with recent experimental and theoretical studies and serve to test the prediction of current equations of state for these types of binary mixtures.     

Transport numbers in transdermal iontophoresis

Parameters determining ionic transport numbers in transdermal iontophoresis have been characterized. The transport number of an ion (its ability to carry charge) is key to its iontophoretic delivery or extraction across the skin. Using small inorganic ions, the roles of molar fraction and mobility of the co- and counterions present have been demonstrated. A direct, constant current was applied across mammalian skin in vitro. Cations were anodally delivered from either simple M(+)Cl(-) solutions (single-ion case, M(+) = sodium, lithium, ammonium, potassium), or binary and quaternary mixtures thereof. Transport numbers were deduced from ion fluxes. In the single-ion case, maximum cationic fluxes directly related to the corresponding ionic aqueous mobilities were found. Addition of co-ions decreased the transport numbers of all cations relative to the single-ion case, the degree of effect depending upon the molar fraction and mobility of the species involved. With chloride as the principal counterion competing to carry current across the skin (the in vivo situation), a maximum limit on the single or collective cation transport number was 0.6-0.8. Overall, these results demonstrate how current flowing across the skin during transdermal iontophoresis is distributed between competing ions, and establish simple rules with which to optimize transdermal iontophoretic transport.     

Limited evidence that mixing leaf litter accelerates decomposition or increases diversity of decomposers in streams of eastern Canada

Most studies of terrestrial litter decomposition in streams and rivers have used leaves from a single tree species, but leaf packs in streams in eastern North America are usually mixtures of two or more species. Litter mixtures may decay more quickly than either of the component species. If so, estimates of stream energy and nutrient budgets may be inaccurate. In northern Nova Scotia, Canada, we measured mass loss from binary mixtures (1:1 mass ratio) of leaf litter in mesh bags, using freshly fallen or air-dried litter from five species of canopy trees. We repeated the experiment eight times, in summer and fall, in two streams and a small river, over 3 years. In some trials we enumerated benthic invertebrate and fungal colonization of decaying litter. Although there were marked differences in mass loss rates among litter types, decomposition was accelerated in mixtures relative to the mean of the component species in only three of eight trials, and only in mixtures containing N-rich speckled alder leaves. Mixing yellow birch and red maple leaves inhibited decomposition. Diversity (Shannon–Weaver Index), species richness, and abundance of aquatic hyphomycete fungi, as indexed by conidial production, were never greater (and sometimes less) on litter mixtures than on the component species. Total numbers, taxonomic richness and diversity of benthic invertebrates generally, and litter-feeding species in particular, were not augmented by mixing litter types. Litter mixtures appear to dilute a preferred substrate with patches of a less preferred substrate. Our results provide only weak support for the contention that combining two litter types leads to acceleration of decomposition rates. Handling editor: K. Martens     

The influence of sodium salts on binary mixtures of bitter-tasting compounds

In order to study potential mixture interactions among bitter compounds, selected sodium salts were added to five compounds presented either alone or as binary bitter-compound mixtures. Each compound was tested at a concentration that elicited 'weak' perceived bitterness. The bitter compounds were mixed at these concentrations to form a subset of possible binary mixtures. For comparison, the concentration of each solitary compound was doubled to measure bitterness inhibition at the higher intensity level elicited by the mixtures. The following sodium salts were tested for bitterness inhibition: 100 mM sodium chloride (salty), 100 mM sodium gluconate (salty), 100 and 20 mM monosodium glutamate (umami), and 50 mM adenosine monophosphate disodium salt (umami). Sucrose (sweet) was also employed as a bitterness suppressor. The sodium salts differentially suppressed the bitterness of compounds and their binary combinations. Although most bitter compounds were suppressed, the bitterness of tetralone was not suppressed, nor was the bitterness of the binary mixtures that contained it. In general, the percent suppression of binary mixtures of compounds was predicted by the average percent suppression of its two components. Within the constraints of the present study, the bitterness of mixtures was suppressed by sodium salts and sucrose independently, with few bitter interactions. This is consistent with observations that the bitter taste system integrates the bitterness of multi-compound solutions linearly.     

Studies on rickettsia-like organism disease of the tropical marine pearl oyster I: the fine structure and morphogenesis of pinctada maxima pathogen rickettsia-like organism

The present paper reports for the first time the discovery of a rickettsia-like organism (RLO) in the cultured tropical marine pearl oyster Pinctada maxima with mass mortality in the Hainan Province of China. This organism parasitizes the cytoplasm of host cells and forms intracytoplasmic eosinophilic inclusions. These organisms are extremely pleiomorphic in shape and average 967 x 551 nm in size, as measured in cross sections of transmission electron micrographs. The organisms exhibit clearly recognizable ultrastructural characteristics of prokaryotic bacteria-like cells, including two trilaminar membranes, an increasing electron-dense periplasmic ribosome zone, and a thread-like DNA nucleoidal structure. In addition to the above prokaryotic characteristics, the following unique biological characteristics were confirmed by TEM: (i) These organisms are usually located in host cells in two ways, namely, free in the cell cytoplasm and involved within membrane-limited phagolysosomes; (ii) The organisms exist in two morphological cell types, namely a small cell variant (SCV) and a large cell variant (LCV). The most important morphological difference between two cell types is that the SCV is obviously ribosome-rich in the periphery of the body, which makes SCV more electron-dense in the cytoplasm and narrower in the central nucleoid area than the LCV; (iii) Two propagative modes of the organisms, transverse binary fission and budding, are observed in cytoplasm and phagolysosomes of host cells under TEM, in which the budding is more often seen in phagolysosomes. These characteristics indicate that the organism is a separate species in the family Rickettsiaceae and should be classified into the genus Rickettsia. On the basis of the existence of the two propagative modes and two cell types, and intracellular location, we propose a developmental cycle for this organism which includes a vegetative differentiation stage to develop LCV by transverse binary fisson and a budding differentiation stage to develop resistant SCV. Copyright 1999 Academic Press.     

Miscibility critical pressures in monolayers of ternary lipid mixtures

When phospholipids are mixed with cholesterol in a monolayer at an air-water interface, coexisting 2-dimensional liquid phases can be observed if the surface pressure, pi, is lower than the miscibility critical pressure, pi(c). Ternary mixtures of two phospholipid species with dihydrocholesterol have been reported to have critical pressures that are linearly proportional to the relative composition of the phospholipids. However, we report here that, if the acyl chains of the two phospholipids differ significantly in length or unsaturation, the behavior is markedly different. In this case, the critical pressure of the ternary mixture can be remarkably high, exceeding the critical pressures of the corresponding binary mixtures. High critical pressures are also seen in binary mixtures of phospholipid and dihydrocholesterol when the two acyl chains of the phospholipid differ sufficiently in length. Using regular solution theory, we interpret the elevated critical pressures of these mixtures as an attractive interaction between the phospholipid components.     

Coupling flash liquid chromatography with mass spectrometry for enrichment and isolation of milk oligosaccharides for functional studies

Mass spectrometry has been coupled with flash liquid chromatography to yield new capabilities for isolating nonchromophoric material from complicated biological mixtures. A flash liquid chromatography/tandem mass spectrometry (LC/MS/MS) method enabled fraction collection of milk oligosaccharides from biological mixtures based on composition and structure. The method is compatible with traditional gas pressure-driven flow flash chromatography widely employed in organic chemistry laboratories. The online mass detector enabled real-time optimization of chromatographic parameters to favor separation of oligosaccharides that would otherwise be indistinguishable from coeluting components with a nonspecific detector. Unlike previously described preparative LC/MS techniques, we have employed a dynamic flow connection that permits any flow rate from the flash system to be delivered from 1 to 200 ml/min without affecting the ionization conditions of the mass spectrometer. A new way of packing large amounts of graphitized carbon allowed the enrichment and separation of milligram quantities of structurally heterogeneous mixtures of human milk oligosaccharides (HMOs) and bovine milk oligosaccharides (BMOs). Abundant saccharide components in milk, such as lactose and lacto-N-tetraose, were separated from the rarer and less abundant oligosaccharides that have greater structural diversity and biological functionality. Neutral and acidic HMOs and BMOs were largely separated and enriched with a dual binary solvent system.     

Algal toxins alter copepod feeding behavior

Using digital holographic cinematography, we quantify and compare the feeding behavior of free-swimming copepods, Acartia tonsa, on nutritional prey (Storeatula major) to that occurring during exposure to toxic and non-toxic strains of Karenia brevis and Karlodinium veneficum. These two harmful algal species produce polyketide toxins with different modes of action and potency. We distinguish between two different beating modes of the copepod's feeding appendages-a "sampling beating" that has short durations (<100 ms) and involves little fluid entrainment and a longer duration "grazing beating" that persists up to 1200 ms and generates feeding currents. The durations of both beating modes have log-normal distributions. Without prey, A. tonsa only samples the environment at low frequency. Upon introduction of non-toxic food, it increases its sampling time moderately and the grazing period substantially. On mono algal diets for either of the toxic dinoflagellates, sampling time fraction is high but the grazing is very limited. A. tonsa demonstrates aversion to both toxic algal species. In mixtures of S. major and the neurotoxin producing K. brevis, sampling and grazing diminish rapidly, presumably due to neurological effects of consuming brevetoxins while trying to feed on S. major. In contrast, on mixtures of cytotoxin producing K. veneficum, both behavioral modes persist, indicating that intake of karlotoxins does not immediately inhibit the copepod's grazing behavior. These findings add critical insight into how these algal toxins may influence the copepod's feeding behavior, and suggest how some harmful algal species may alter top-down control exerted by grazers like copepods.     

Patterns of ciliary currents in Atlantic reef corals and their functional significance

Patterns of ciliary currents of 35 species of Atlantic reef corals are described and compared with currents of Pacific corals. Observations were made during the day and at night, during feeding and without food. There is a basic pattern of ciliary currents common to both Atlantic and Pacific species. In all but the family Agaricidae currents flow off the oral disk and up or out between the tentacles. In the centre of the disk region currents flow towards the mouth or the peristome. On the polyp stalk or column there was considerable variation between species in both Atlantic and Pacific forms. In some species currents flow downwards toward the coenosarc while in others, current pass up the stalk towards the tentacles.
In the Atlantic Agaricidae there may be an inward flow towards the mouth, an outward flow or a unidirectional flow across the corallum. The patterns of flow depend upon the state of contraction of the polyps or the shape and proximity of adjacent polyps.
No ciliary current reversal was observed in Atlantic species. Ciliary currents are functional as a cleansing mechanism and facilitated the ability of mucus nets and strands to gather particles.     

Calculation of the surface tension of planar interfaces by molecular simulations: from Lennard-Jones fluids to binary mixtures

The relative longevity of the research in the field of the molecular simulations of the liquid–vapour interfaces of Lennard-Jones (LJ) particles can be explained by the dependence of the surface tension on many methodological factors. After a few illustrations on the parameters that can impact the results of surface tension on the LJ interfaces, we establish the ability of the current methodologies to quantitatively predict the surface tension of various liquid–vapour interfaces of pure components at different temperatures. We also show that the methods perform very well for the reproduction of the interfacial tension of binary mixtures in a wide range of pressures.     

Differences in the modulation of collective membrane motions by ergosterol, lanosterol, and cholesterol: a dynamic light scattering study

A dynamic light scattering setup was used to study the undulations of freely suspended planar lipid bilayers, the so-called black lipid membranes, over a previously inaccessible range of frequency and wave number. A pure synthetic lecithin bilayer, 1,2-dielaidoyl-sn-3-glycero-phoshatidylcholine (DEPC), and binary mixtures of DEPC with 40 mol % of cholesterol, ergosterol, or lanosterol were studied. By analyzing the dynamic light scattering data (oscillation and damping curves) in terms of transverse shear motion, we extracted the lateral tension and surface viscosity of the composite bilayers for each sterol. Cholesterol gave the strongest increase in lateral tension (approximately sixfold) with respect to the DEPC control, followed by lanosterol (approximately twofold), and ergosterol (1.7-fold). Most interestingly, only cholesterol simultaneously altered the surface viscosity of the bilayer by almost two orders of magnitude, whereas the other two sterols did not affect this parameter. We interpret this unique behavior of cholesterol as a result of its previously established out-of-plane motion which allows the molecule to cross the bilayer midplane, thereby effectively coupling the bilayer leaflets to form a highly flexible but more stable composite membrane.     

Dynamics in a protein-lipid complex: nuclear magnetic resonance measurements on the headgroup of cardiolipin when bound to cytochrome c.

Deuterium and phosphorus nuclear magnetic resonance (NMR) has been used to investigate the dynamics of slow motional processes induced in bilayer cardiolipin upon binding with cytochrome c. 31P NMR line shapes suggest that protein binding induces less restricted, isotropic-like motions in the lipid phosphates within the ms time scale of this measurement. However, these motions impart rapid transverse relaxation to methylene deuterons adjacent to the phosphate in the lipid headgroup and so did not feature strongly in the NMR line shapes recorded from these nuclei by using the quadrupolar echo. Nonetheless, motional characteristics of the headgroup deuterons were accessible to a dynamic NMR approach using the Carr-Purcell-Meiboom-Gill multiple-pulse experiment. Compared to the well-studied case of deuterons in fatty acyl chains of bilayer phosphatidylcholine, the motions determining the 2H spin transverse relaxation in the headgroup of bilayer cardiolipin were much faster, having a lower limit in the 5-10 kHz range. On binding with cytochrome c, the T2 effecting motions in the cardiolipin headgroup became faster still, with rates comparable to the residual quadrupolar coupling frequency of the headgroup deuterons (approximately 25 kHz) and so coincided with the time scale for recording the quadrupolar echo (approximately 40 microseconds). It is concluded that the headgroup of cardiolipin does not exclusively report localized dynamic information but is particularly sensitive to collective motions occurring throughout the bilayer molecules. Although the rates of collective modes of motion may be dependent on the lipid type in pure lipid bilayers, these low-frequency fluctuations appear to occupy a similar dynamic range in a variety of lipid-protein systems, including the natural membranes.     

Polarographic Studies on Sarkomycin

By polarography, isoniazid (INH) gives three reduction waves and sarkomycin-isoniazid derivative (SK-INH) gives three or four waves. Polarography of the mixtures of two out of sarkomycin (SK), INH and SK-INH suggests that the first, the second and the fourth wave of SK-INH are derived from the first, the second and the third wave of INH, respectively, and the third wave from SK. Alternating current polarography of SK-INH shows three or four maxima, whose numbers are the same as wave numbers in direct current polarography.     

Fully automated multifunctional ultrahigh pressure liquid chromatography system for advanced proteome analyses

A multifunctional liquid chromatography system that performs 1-dimensional, 2-dimensional (strong cation exchange/reverse phase liquid chromatography or SCX/RPLC) separations and online phosphopeptide enrichment using a single binary nanoflow pump has been developed. With a simple operation of a function selection valve equipped with a SCX column and a TiO(2) (titanium dioxide) column, a fully automated selection of three different experiment modes was achieved. Because the current system uses essentially the same solvent flow paths, the same trap column, and the same separation column for reverse-phase separation of 1D, 2D, and online phosphopeptides enrichment experiments, the elution time information obtained from these experiments is in excellent agreement, which facilitates correlating peptide information from different experiments. The final reverse-phase separation of the three experiments is completely decoupled from all of the function selection processes; thereby salts or acids from SCX or TiO(2) column do not affect the efficiency of the reverse-phase separation.     

Non Equilibrium Molecular Dynamics Simulation of Coupled Heat- and Mass Transport Across a Liquid/Vapor Interface

Abstract

The transport properties of bulk liquid, gas and at the gas/liquid interface were studied for two binary Lennard-Jones/spline mixtures by use of nonequilibrium molecular dynamics. One of the mixtures was an ideal isotope mixture, the other a non-ideal mixture. The simulations gave the thermal conductivity, mutual diffusion coefficient, heat flux, mass flux, and the changes in these quantities across the interface. The local entropy production was expressed in terms of fluxes and thermodynamic forces, and numercial estimates are given. It was shown that the largest contribution to the total entropy production occurs in the vapor phase under the chosen conditions. We expect, however that if the mass flux were larger, the major contribution to the entropy production would come from the liquid phase.     

Effect of salts on the properties of aqueous sugar systems, in relation to biomaterial stabilization. 1. Water sorption behavior and ice crystallization/melting

Trehalose and sucrose, two sugars that are involved in the protection of living organisms under extreme conditions, and their mixtures with salts were employed to prepare supercooled or freeze-dried glassy systems. The objective of the present work was to explore the effects of different salts on water sorption, glass transition temperature (T(g)), and formation and melting of ice in aqueous sugar systems. In the sugar-salt mixtures, water adsorption was higher than expected on the basis of the water uptake by each pure component. In systems with a reduced mass fraction of water (w less-than-or-equal 0.4), salts delayed water crystallization, probably due to ion-water interactions. In systems where > 0.6, water crystallization could be explained by the known colligative properties of the solutes. The glass transition temperature of the maximally concentrated matrix (T(g)') was decreased by the presence of salts. However, the actual T(g) values of the systems were not modified. Thus, the effect of salts on sorption behavior and formation of ice may reflect dynamic water-salt-sugar interactions which take place at a molecular level and are related to the charge/mass ratio of the cation present without affecting supramolecular or macroscopic properties.     

Solvent structure and enzyme activity

Enzymes are fluctuating particles in thermal equilibrium with their solvent environment. A variety of models of enzyme action have postulated selective excitation of enzyme vibrational modes or triggering of correlated motion of catalytic groups through collisions with solvent particles as the basis of catalytic activity. Solvent composition and structure are expected to influence such interactions. Solutes such as p-dioxane, t-butanol, and tetraalkylammonium chlorides are known to be strong perturbants of the structure of water. However, when the kinetic parameters of two enzymes, carboxypeptidase A and alpha-chymotrypsin, were examined carefully in aqueous mixtures containing these solutes, no significant influence of solvent structure or mass composition on the catalytic rate constant was found. The results indicate, furthermore, that, within the low viscosity limit, fluctuations in enzyme structure that are responsible for activated processes in the catalytically rate limiting step appear not to be significantly influenced by dynamic processes in the bulk solvent.     

Solid-liquid phase behavior of binary fatty acid mixtures. 1. Oleic acid/stearic acid and oleic acid/behenic acid mixtures

Solid-liquid phase behavior of binary fatty acid mixtures was investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR) for the mixture composed of oleic acid (OA) and stearic acid (SA) and that composed of OA and behenic acid (BA). The DSC results provided a monotectic type T-X phase diagram for these mixtures, from which it was suggested that the two fatty acid species are completely immiscible in a solid phase regardless of the two polymorphs of OA, i.e., alpha-form or gamma-form. The solid phase immiscibility was confirmed by the FT-IR observation that the spectra obtained for the mixtures correspond to the superposition of the two spectra for respective components. Thermodynamic analysis of liquidus line demonstrated that OA and SA form an ideal mixture in a liquid phase, whereas the mixing of OA and BA in a liquid phase is slightly non-ideal.     

Physical state of cholesteryl esters deposited in cultured macrophages

J774 macrophages load with cholesteryl ester (CE) when incubated with acetylated low-density lipoprotein and cholesterol-rich liposomes; the CE accumulates as cytoplasmic inclusions 1-2 micron in diameter. The CE core of the droplet comprises about 90% of its mass; the predominant CE species present are cholesteryl palmitate (CP, 41%) and cholesteryl oleate (CO, 37%). The thermotropic properties of the inclusions, both in intact cells and after isolation, have been characterized by differential scanning calorimetry. On heating, the inclusions exhibit two endothermic transitions at about 41 and 53 degrees C with a total enthalpy of 7.7 +/- 1.2 cal/g of CE. Very similar thermal behavior is exhibited by a binary mixture containing equal weights of CO and CP; this indicates that these two species dominate the phase behavior of CE in J774 inclusions. A phase diagram for the CO/CP system has been generated, and this reflects simple eutectic behavior. The eutectic is 83% w/w CO, and it melts at 49-50 degrees C. Below this temperature, CO and CP form two immiscible crystalline phases due to the very limited ability of the unsaturated oleate and saturated palmitate acyl chains to mix in the crystal phase. On heating a 1/1 w/w CO/CP mixture, an isotropic liquid of eutectic composition forms at 49 degrees C, and the remaining crystalline cholesteryl palmitate melts over the temperature range 50-69 degrees C. The phase diagram indicates that bulk mixtures of CE molecules in J774 inclusions should be crystalline at 37 degrees C, the growth temperature of the cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of two novel human small heat shock proteins: protein kinase-related HspB8 and testis-specific HspB9

Some binary mixtures of cholesterol and phospholipids in monolayers have thermodynamic phase diagrams with two upper miscibility critical points. This feature has been interpreted in terms of 'condensed complexes' between the phospholipid and cholesterol. The present work gives evidence for the formation of complexes with a common simple integral stoichiometry in binary mixtures of cholesterol and a series of five sphingomyelins where the amide-linked acyl chain length is varied. This indicates that these complexes have a distinct geometry even though they form a liquid phase.