Temperature- and pressure-dependent phase behavior of monoacylglycerides monoolein and monoelaidin.

We used x-ray and neutron diffraction to study the temperature- and pressure-dependent structure and phase behavior of the monoacylglycerides 1-monoelaidin (ME) and 1-monoolein (MO) in excess water. The monoacylglycerides were chosen for investigation of their phase behavior because they exhibit mesomorphic phases with one-, two-, and three-dimensional periodicity, such as lamellar, an inverted hexagonal and bicontinuous cubic phases, in a rather easily accessible temperature and pressure range. We studied the structure, stability, and transformations of the different phases over a wide temperature and pressure range, explored the epitaxial relations that exist between different phases, and established a relationship between the chemical structure of the lipid molecules and their phase behavior. For both systems, a temperature-pressure phase diagram has been determined in the temperature range from 0 to 100 degrees C at pressures from ambient up to 1400 bar, and drastic differences in phase behavior are found for the two systems. In MO-water dispersions, the cubic phase Pn3m extends over a large phase field in the T,p-plane. At temperatures above 95 degrees C, the inverted hexagonal phase is found. In the lower temperature region, a crystalline lamellar phase is induced at higher pressures. The phases found in ME-water include the lamellar crystalline Lc phase, the L beta gel phase, the L alpha liquid-crystalline phase, and two cubic phases belonging to the crystallographic space groups Im3m and Pn3m. In addition, the existence of metastable phases has been exploited. Between coexisting metastable cubic structures, a metric relationship has been found that is predicted theoretically on the basis of the curvature elastic energy approximation only.     

Calorimetric and x-ray diffraction studies of rye glucocerebroside mesomorphism.

Glucocerebrosides (GlcCer) isolated from the leaves of winter rye (Secale cereale L. cv Puma) differ from the more commonly investigated natural and synthetic cerebrosides, in that greater than 95% of the fatty acids are saturated and monounsaturated hydroxy fatty acids. Isomers of the trihydroxy long chain base hydroxysphingenine (t1(8:18 cis or trans)) and isomers of sphingadienine (d18:2(4trans, 8 cis or trans)) comprise 77% and 17%, respectively, of the total long chain bases. The phase behavior of fully hydrated and dry rye leaf GlcCer was investigated using differential scanning calorimetry (DSC) and x-ray diffraction. On initial heating, aqueous dispersions of GlcCer exhibit a single endothermic transition at 56 degrees C and have an enthalpy (delta H) of 46 J/g. Cooling to 0 degrees C is accompanied by a small exothermic transition (delta H = -8 J/g) at 8 degrees C. On immediate reheating, a broad exothermic transition (delta H = -39 J/g) is observed between 10 and 20 degrees C in addition to a transition at 56 degrees C. These transitions are not reversible, and the exothermic transition rapidly diminishes when the sample is held at low temperature. Using x-ray diffraction, it was determined that the endotherm at 56 degrees C represents a transition from a highly ordered lamellar crystalline phase (Lc) with a d-spacing of 57 A and a series of wide-angle reflections in the 3-10 A range, to a lamellar liquid crystalline (L alpha) phase having a d-spacing of 55 A and a diffuse wide-angle scattering peak centered at 4.7 A. Cooling leads to the formation of a metastable gel phase (L beta) with a d-spacing of 64.0 A and a single broad reflection at 4.28 A. Subsequent warming to above 15 degrees C restores the original Lc phase. Thus, rye GlcCer in excess water exhibit a series of irreversible transitions and gel phase metastability. Dry GlcCer undergo an initial heating endothermic transition at 130 degrees C, which is ascribed to a transformation into the HII phase from a two phase state characterized by the coexistence of phases with disordered (alpha) and helical (delta) type chain conformations but of unknown lattice identity: An exotherm at 67.5 degrees C observed upon subsequent cooling is of unknown origin. Since an undercooled HII phase persists down to 19 degrees C, the exotherm may derive in part from an alpha-to-delta type chain packing conformational change especially under slow cooling conditions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Simulation study of a gramicidin/lipid bilayer system in excess water and lipid. II. Rates and mechanisms of water transport

A gramicidin channel in a fluid phase DMPC bilayer with excess lipid and water has been simulated. By use of the formal correspondence between diffusion and random walk, a permeability for water through the channel was calculated, and was found to agree closely with the experimental results of Rosenberg and Finkelstein (Rosenberg, P.A., and A. Finkelstein. 1978. J. Gen. Physiol. 72:327-340; 341-350) for permeation of water through gramicidin in a phospholipid membrane. By using fluctuation analysis, components of resistance to permeation were computed for movement through the channel interior, for the transition step at the channel mouth where the water molecule solvation environment changes, and for the process of diffusion up to the channel mouth. The majority of the resistance to permeation appears to occur in the transition step at the channel mouth. A significant amount is also due to structurally based free energy barriers within the channel. Only small amounts are due to local friction within the channel or to diffusive resistance for approaching the channel mouth.     

Thermotropic mesomorphism of cholesteryl myristate. An electron diffraction study

Electron diffraction measurements on heated or cooled microcrystals of cholesteryl myristate, which are grown from solution or epitaxially, on benzoic acid, provide further structural information about its mesomorphic behavior. At subambient temperatures (less than -65 degrees C), a new crystal form is observed which doubles the unit cell axes in the (001) plane. At the major crystalline in equilibrium with smectic endotherm at 70 degrees C, evidence is found for the existence of a pretransition crystal packing. The smectic phase, which coexists with this pretransition crystal form, is composed of relatively well-ordered layers, probably with a monolayer-type packing. Cooling the cholesteric phase to the crystalline form causes a rotational disorder which is not yet understood.     

Phase diagram of stigmasterol-dipalmitoylphosphatidylcholine mixtures dispersed in excess water

As a simple model of rafts in plant cells, the effect of stigmasterol, one of the predominant sterols in plant plasma membranes, on the phase behavior of dipalmitoylphosphatidylcholine (DPPC) multilayers has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and freeze-fracture electron microscopy (FFEM) techniques. A partial phase diagram of the binary system has been constructed. Particularly, the stigmasterol concentrations of the “left endpoint” and “right endpoint” of the three-phase line have been determined using the newly developed linear and nonlinear fitting method. They are 6.2 and 23.7 mol%, respectively. Furthermore, the resemblance and difference of phase diagrams of DPPC/stigmasterol, DPPC/cholesterol, and DPPC/ergosterol have been compared and the efficiency of these sterols in promoting the formation of the liquid-ordered domains (rafts) have also been discussed.     

Phase diagram of stigmasterol-dipalmitoylphosphatidylcholine mixtures dispersed in excess water

As a simple model of rafts in plant cells, the effect of stigmasterol, one of the predominant sterols in plant plasma membranes, on the phase behavior of dipalmitoylphosphatidylcholine (DPPC) multilayers has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and freeze-fracture electron microscopy (FFEM) techniques. A partial phase diagram of the binary system has been constructed. Particularly, the stigmasterol concentrations of the "left endpoint" and "right endpoint" of the three-phase line have been determined using the newly developed linear and nonlinear fitting method. They are 6.2 and 23.7 mol%, respectively. Furthermore, the resemblance and difference of phase diagrams of DPPC/stigmasterol, DPPC/cholesterol, and DPPC/ergosterol have been compared and the efficiency of these sterols in promoting the formation of the liquid-ordered domains (rafts) have also been discussed.     

Lysolecithin and cholesterol interact stoichiometrically forming bimolecular lamellar structures in the presence of excess water, of lysolecithin or cholesterol.

The structural interaction of egg lysolecithin, derived from egg lecithin, and cholesterol in aqueous solution has been investigated using X-ray diffraction. When mixed in any proportions, either suspended in excess buffer or up to 85% lipid by dry weight, a separate lamellar phase containing equimolar proportions of lysolecithin and cholesterol forms, separate from excess water, or lysolecithin or cholesterol. The cholesterol disorders the crystalline chains of the lysolecithin. The equimolar phase is stable up to 50 degrees C unlike lysolecithin alone, which forms micelles, Thes results show that lysolecithin and cholesterol combine stoichiometrically in a stable complex. We propose as a structural model, that cholesterol fills the space of the missing fatty acyl chain making the lysolecithin more cylindrical rather than wedge shaped. This interaction could reduce both the lytic action of lysolecithin on membranes and its induction of cell fusion. It suggest another role of cholesterol in cell membranes: namely, to act as a stabilizer of bilayer structure by being a mobile component that can fill free volume in the hydrocarbon interior. Lysolecithin-cholesterol interaction may also be important in the early events of atherosclerosis where lysolecithin levels in vessel walls increase fivefold.     

Self-organizing maps: stationary states,metastability and convergence rate

We investigate the effect of various types of neighborhood function on the convergence rates and the presence or absence of metastable stationary states of Kohonen's self-organizing feature map algorithm in one dimension. We demonstrate that the time necessary to form a topographic representation of the unit interval [0, 1] may vary over several orders of magnitude depending on the range and also the shape of the neighborhood function, by which the weight changes of the neurons in the neighborhood of the winning neuron are scaled. We will prove that for neighborhood functions which are convex on an interval given by the length of the Kohonen chain there exist no metastable states. For all other neighborhood functions, metastable states are present and may trap the algorithm during the learning process. For the widely-used Gaussian function there exists a threshold for the width above which metastable states cannot exist. Due to the presence or absence of metastable states, convergence time is very sensitive to slight changes in the shape of the neighborhood function. Fastest convergence is achieved using neighborhood functions which are "convex" over a large range around the winner neuron and yet have large differences in value at neighboring neurons.     

Effect of excess water on the desilylation of oligoribonucleotides using tetrabutylammonium fluoride.

The most commonly available 2' hydroxyl protecting group used in the synthesis of oligoribonucleotides is the tert-butyldimethylsilyl moiety. This protecting group is generally cleaved with 1 M tetrabutylammonium fluoride (TBAF) in tetrahydrofuran (THF). The efficiency of this reaction was tested on ribonucleotidyldeoxythymidine dinucleotides (AT, CT, GT, and UT). We have found that the efficiency of desilylation of uridine and cytidine is greatly dependent on the water content of the TBAF reagent. Conversely, the water content of the TBAF reagent [up to 17% (w/w)] had no detectable effect on the rate of desilylation of adenosine and guanosine. It was concluded that for effective desilylation of pyrimidine nucleosides the water content of the TBAF reagent must be 5% or less, which is readily achieved using molecular sieves. TBAF dried in such a manner was shown to be effective in deprotecting an oligoribonucleotide containing both purine and pyrimidine residues.     

Polymorphism of 14C vitamin D3 binding protein in cattle and water buffalo serum.

Cattle and water buffalo sera labelled with vitamin D3[14C] (300 and 480 individual samples respectively) were subjected to starch gel electrophoresis followed by autoradiography in an attempt to identify a possible polymorphism of the proteins capable of binding this vitamin. Three phenotypes controlled by two codominant autosomal alleles were identified in cattle while in water buffalo six phenotypes controlled by three codominant autosomal alleles were observed.     

Role of metastability and acidic pH in membrane fusion by tick-borne encephalitis virus.

The envelope protein E of the flavivirus tick-borne encephalitis (TBE) virus is, like the alphavirus E1 protein, a class II viral fusion protein that differs structurally and probably mechanistically from class I viral fusion proteins. The surface of the native TBE virion is covered by an icosahedrally symmetrical network of E homodimers, which mediate low-pH-induced fusion in endosomes. At the pH of fusion, the E homodimers are irreversibly converted to a homotrimeric form, which we have found by intrinsic fluorescence measurements to be more stable than the native dimers. Thus, the TBE virus E protein is analogous to the prototypical class I fusion protein, the influenza virus hemagglutinin (HA), in that it is initially synthesized in a metastable state that is energetically poised to be converted to the fusogenic state by exposure to low pH. However, in contrast to what has been observed with influenza virus HA, this transition could not be triggered by input of heat energy alone and membrane fusion could be induced only when the virus was exposed to an acidic pH. In a previous study we showed that the dimer-to-trimer transition appears to be a two-step process involving a reversible dissociation of the dimer followed by an irreversible trimerization of the dissociated monomeric subunits. Because the dimer-monomer equilibrium in the first step apparently depends on the protonation state of E, the lack of availability of monomers for the trimerization step at neutral pH could explain why low pH is essential for fusion in spite of the metastability of the native E dimer.     

Effects of excess centromeres and excess telomeres on chromosome loss rates.

The linear chromosomes of eukaryotes contain specialized structures to ensure their faithful replication and segregation to daughter cells. Two of these structures, centromeres and telomeres, are limited, respectively, to one and two copies per chromosome. It is possible that the proteins that interact with centromere and telomere DNA sequences are present in limiting amounts and could be competed away from the chromosomal copies of these elements by additional copies introduced on plasmids. We have introduced excess centromeres and telomeres into Saccharomyces cerevisiae and quantitated their effects on the rates of loss of chromosome III and chromosome VII by fluctuation analysis. We show that (i) 600 new telomeres have no effect on chromosome loss; (ii) an average of 25 extra centromere DNA sequences increase the rate of chromosome III loss from 0.4 x 10(-4) events per cell division to 1.3 x 10(-3) events per cell division; (iii) centromere DNA (CEN) sequences on circular vectors destabilize chromosomes more effectively than do CEN sequences on 15-kb linear vectors, and transcribed CEN sequences have no effect on chromosome stability. We discuss the different effects of extra centromere and telomere DNA sequences on chromosome stability in terms of how the cell recognizes these two chromosomal structures.     

Osmoregulation in a nectar-feeding insect, the carpenter bee Xylocopa capitata: water excess and ion conservation

Abstract The liquid diet and high metabolic water production during flight in the carpenter bee Xylocopa capitata Smith 1854 (Hymenoptera, Anthophoridae) causes a water excess, and this is exacerbated by a low dietary intake of ions. The nectar and pollen of the preferred food-plants, Virgilia divaricata Adamson and Podalyria calyptrata Willd., and other Fabaceae had low levels of sodium. Analyses of the bees and their body fluids showed that the bees have an exceptionally low Na content, and Na homeostasis seems to depend on recycling almost all Na which enters the rectum. The copious dilute urine (137 mOsm) had Na and K concentrations of only 3.4 and 7.0 ITIM, respectively. Isolated preparations of Xylocopa Malpighian tubules secreted a fluid with a K concentration 10 times that of the haemolymph. This means that recycling of K is as important as that of Na, and the bulk of K resorption probably occurs passively in the ileum. This study is the first to examine hymenopteran Malpighian tubules. Their stimulation by cAMP is indicative of the presence of a diuretic hormone in Xylocopa.     

Water use and yield of tomatoes under limited water and excess boron

The role of tripartite associations among Frankia, Alpova diplophloeus (an ectomycorrhizal fungus) and Alnus tenuifolia in growth, nitrogen fixation, ectomycorrhizal formation, and mineral acquisition of A. tenuifolia was investigated. Seedlings of A. tenuifolia were planted in pots containing a mixture of ground basalt–perlite, or perlite alone, which served as the control. The seedlings were inoculated with Frankia isolated from root nodules of alder, followed by spores of A. diplophloeus and grown for 5 months in a greenhouse. The seedlings grown in the pots with a mixture of ground basalt–perlite after dual inoculation with Frankia and A. diplophloeus had the heaviest shoots and root nodules in dry weight, and showed the greatest nitrogen-fixing ability measured by acetylene reduction. Ectomycorrhizae formed with A. diplophloeus increased when this fungus was inoculated together with Frankia. The mineral composition (P, K, Ca, Fe, Mg, Mn, Na, Si and Al) in the seedlings was also determined. The results of these experiments showed that the tripartite associations could improve the growth, nitrogen fixation and mineral acquisition (rock solubilization) of A. tenuifolia.