The effect of hydration stress solutes on the phase behavior of hydrated dipalmitoylphosphatidylcholine

Photosynthesis in newly developed leaves of heat-tolerant wheat acclimates to long-term nocturnal warming

We examined photosynthetic traits of pre-existing and newly developed flag leaves of four wheat genotypes grown in controlled-environment experiments. In newly developed leaves, acclimation of the maximum rate of net CO2 assimilation (An) to warm nights (i.e. increased An) was associated with increased capacity of Rubisco carboxylation and photosynthetic electron transport, with Rubisco activation state probably contributing to increased Rubisco activity. Metabolite profiling linked acclimation of An to greater accumulation of monosaccharides and saturated fatty acids in leaves; these changes suggest roles for osmotic adjustment of leaf turgor pressure and maintenance of cell membrane integrity. By contrast, where An decreased under warm nights, the decline was related to lower stomatal conductance and rates of photosynthetic electron transport. Decreases in An occurred despite higher basal PSII thermal stability in all genotypes exposed to warm nights: Tcrit of 45-46.5 °C in non-acclimated versus 43.8-45 °C in acclimated leaves. Pre-existing leaves showed no change in An-temperature response curves, except for an elite heat-tolerant genotype. These findings illustrate the impact of night-time warming on the ability of wheat plants to photosynthesize during the day, thereby contributing to explain the impact of global warming on crop productivity.

Influence of Humectants on the Thermotropic Behaviour and Nanostructure of Fully Hydrated Lecithin Bilayers

Humectants are used widely in topical formulations as they provide cosmetic and health benefits to skin. Of particular interest to our laboratories is the interaction of humectants in phospholipid based topical skin care formulations. This study probed the effects of three exemplary humectants on a fully hydrated lecithin system (DPPC) by use of X-ray scattering and differential scanning calorimetry. While the three humectants affected the nanostructure of 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC, bilayers in a similar manner; leading to an increased membrane order, differences in the effect on the thermal behaviour of DPPC suggest that betaine and sarcosine interacted via a different mechanism compared to acetic monoethanolamide, AMEA. At concentrations above 0.4M, betaine and sarcosine stabilised the gel phase by depletion of the interfacial water via the preferential exclusion mechanism. At the same time, a slight increase in the rigidity of the membrane was observed with an increase in the membrane thickness. Overall, the addition of betaine or sarcosine resulted in an increase in the pre- and main transition temperatures of DPPC. AMEA, on the other hand, decreases both transition temperatures and although the interlamellar water layer was also decreased, there was evidence from the altered lipid chain packing, that AMEA molecules are present also at the bilayer interface, at least at high concentrations. Above the melting point in the fluid lamellar phase, none of the humectants induced significant structural changes, neither concerning the bilayer stacking order nor its overall membrane fluidity.

To Acclimate or Not to Acclimate? Simultaneous Positive and Negative Effects of Acclimation on Susceptibility of Tribolium confusum (Coleoptera: Tenebrionidae) and Oryzaephilus surinamensis (Coleoptera: Silvanidae) to Low Temperatures

Laboratory tests on acclimated and nonacclimated life stages of Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae) (adults, pupae, larvae, and eggs) and Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae) (adults, larvae, and eggs) were conducted at 0, -5, -10, and -15°C to evaluate effects of acclimation on susceptibility to cold treatment. Acclimation of all tested life stages for 7 d at 15°C affected susceptibility of both species to the cold temperatures. After 1 d exposures for ≥2 h, acclimated adults had a noticeable increase in cold tolerance compared with nonacclimated adults for both tested species. Nonacclimated pupae of T. confusum were equally susceptible to cold compared with acclimated pupae at short exposures to low temperatures. Exposure of nonacclimated life stages of T. confusum, at -10°C for 1 d gave 0% survival. Similarly, almost all (99.6%) nonacclimated individuals of O. surinamensis died at -10°C. At 0°C, nonacclimated larvae were more cold tolerant than acclimated larvae, but this trend was reversed when larvae were exposed to -5°C. Mixed results were obtained for larvae of O. surinamensis because in some of the combinations tested, nonacclimated larvae were more tolerant, even at temperatures that were lower than 0°C. In contrast to O. surinamensis, eggs of T. confusum that had not been exposed to cold were not affected by acclimation, while exposure to cold showed increased cold hardiness in acclimated eggs. Results show that individual stored-product insect species may have mixed susceptibility to cold temperatures, which must be taken into account when using cold treatment as a management strategy.

Glucose Can Protect Membranes against Dehydration Damage by Inducing a Glassy Membrane State at Low Hydrations

The physical effects of small sugars on membranes have been studied for decades, primarily because of their membrane stabilization in cold or dehydrated environments. We studied the effects of up to 20 mol% glucose in bilayers made of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at low hydration by combining X-ray diffraction and Molecular Dynamics (MD) simulations. In agreement with previous studies, we observe membrane thinning at low and membrane thickening at high sugar concentrations. Glucose was found to preferentially localize to the outer head region of phospholipid bilayers at all concentrations, and partitioning of sugar in the membranes was found to monotonically increase with increasing sugar concentration. While the number of gauche defects in the lipid acyl tails and the lipid packing in the presence of sugar resembled values of a fluid lipid bilayer, tail dynamics, as assessed by autocorrelation of the carbon atoms in the phospholipid tails, were slowed down significantly with increasing glucose content. Thus, our findings suggest that sugar leads to a a disordered, glassy state of the hydrophobic membrane core. The non-monotonic effect of glucose on membrane thickness was found to be an effect of fluidification at low concentrations and decreased interdigitation in the higher sugar concentration regime.

A Comparative Study of the Influence of Sugars Sucrose, Trehalose, and Maltose on the Hydration and Diffusion of DMPC Lipid Bilayer at Complete Hydration: Investigation of Structural and Spectroscopic Aspect of Lipid-Sugar Interaction

It is well-known that sugars protect membrane structures against fusion and leakage. Here, we have investigated the interaction between different sugars (sucrose, trehalose, and maltose) and phospholipid membrane of 1,2-dimyristoyl-sn-glycero-3-phoshpocholine (DMPC) using dynamic light scattering (DLS), transmission electron microscopy (TEM), and other various spectroscopic techniques. DLS measurement reveals that the addition of sugar molecule results a significant increase of the average diameter of DMPC membrane. We have also noticed that in the presence of different sugars the rotational relaxation and solvation time of coumarin 480 (C480) and coumarin 153 (C153) surrounding DMPC membrane increases, suggesting a marked reduction of the hydration behavior at the surface of phospholipid membrane. In addition, we have also investigated the effect of sugar molecules on the lateral mobility of phospholipids. Interestingly, the relative increase in rotational, solvation and lateral diffusion is more prominent for C480 than that of C153 because of their different location in lipid bilayer. It is because of preferential location of comparatively hydrophilic probe C480 in the interfacial region of the lipid bilayer. Sugars intercalate with the phospholipid headgroup through hydrogen bonding and replace smaller sized water molecules from the membrane surface. Therefore, overall, we have monitored a comparative analysis regarding the interaction of different sugar molecules (sucrose, trehalose, and maltose) with the DMPC membrane through DLS, TEM, solvation dynamics, time-resolved anisotropy, and fluorescence correlation spectroscopy (FCS) measurements to explore the structural and spectroscopic aspect of lipid-sugar interaction.

Rapid cold hardening of Thrips palmi (Thysanoptera: Thripidae)

Cold tolerance of the palm thrips, Thrips palmi Karny, was investigated to predict its survival in field during winter. Supercooling points of T. palmi were varied among the developmental stages and ranged from -26.4 to -18.4°C. However, the cold injuries occurred above supercooling points in terms of higher mortality. The exposure to subzero temperatures (-5° to -15°C) resulted in significant mortalities to all developmental stages of T. palmi. A preexposure to a low temperature (4°C) for 7 h significantly increased the cold tolerance of all stages of T. palmi with respect to survival at -10°C and supercooling capacity. The rapid cold hardening (RCH) was dependent on the duration of the preexposure period at 4°C in adult stage. Polyol and sugar analysis using an high-performance liquid chromatography analysis showed that 4°C preexposure caused accumulation of glycerol, trehalose, mannitol, and mannose in the adults. The increase in trehalose levels was more significant than the others. This study suggests that all stages of T. palmi are able to become cold-hardy by RCH, in which several polyols and sugars may play crucial roles as cryoprotectants.

Taste of sugar at the membrane: thermodynamics and kinetics of the interaction of a disaccharide with lipid bilayers

Sugar recognition at the membrane is critical in various physiological processes. Many aspects of sugar-membrane interaction are still unknown. We take an integrated approach by combining conventional molecular-dynamics simulations with enhanced sampling methods and analytical models to understand the thermodynamics and kinetics of a di-mannose molecule in a phospholipid bilayer system. We observe that di-mannose has a slight preference to localize at the water-phospholipid interface. Using umbrella sampling, we show the free energy bias for this preferred location to be just -0.42 kcal/mol, which explains the coexistence of attraction and exclusion mechanisms of sugar-membrane interaction. Accurate estimation of absolute entropy change of water molecules with a two-phase model indicates that the small energy bias is the result of a favorable entropy change of water molecules. Then, we incorporate results from molecular-dynamics simulation in two different ways to an analytical diffusion-reaction model to obtain association and dissociation constants for di-mannose interaction with membrane. Finally, we verify our approach by predicting concentration dependence of di-mannose recognition at the membrane that is consistent with experiment. In conclusion, we provide a combined approach for the thermodynamics and kinetics of a weak ligand-binding system, which has broad implications across many different fields.

Reconciliation of opposing views on membrane-sugar interactions

It is well established that small sugars exert different types of stabilization of biomembranes both in vivo and in vitro. However, the essential question of whether sugars are bound to or expelled from membrane surfaces, i.e., the sign and size of the free energy of the interaction, remains unresolved, and this prevents a molecular understanding of the stabilizing mechanism. We have used small-angle neutron scattering and thermodynamic measurements to show that sugars may be either bound or expelled depending on the concentration of sugar. At low concentration, small sugars bind quite strongly to a lipid bilayer, and the accumulation of sugar at the interface makes the membrane thinner and laterally expanded. Above ∼0.2 M the sugars gradually become expelled from the membrane surface, and this repulsive mode of interaction counteracts membrane thinning. The dual nature of sugar-membrane interactions offers a reconciliation of conflicting views in earlier reports on sugar-induced modulations of membrane properties.

The effect of compatible solute ectoines on the structural organization of lipid monolayer and bilayer membranes

Compatible solutes are small organic osmolytes responsible for osmotic balance and at the same time compatible with the cellular metabolism. Here, we have investigated the effect of the compatible solutes, ectoine and hydroxyectoine, on the fluid-rigid domain structure of lipid monolayer and bilayer membranes. Mainly saturated dipalmitoyl-phosphatidylcholine membranes exhibiting a clear le/lc phase transition were used. Fluorescence microscopy showed that ectoines added to the aqueous subphase expand and fluidize the lipid monolayers especially at surface pressures below 30mN/m. The domain structure at the le/lc phase transition is sensitively modified leading to smaller but more numerous domains in the presence of ectoines. Hydroxyectoine was more efficient than ectoine. These results are explained by the replacement theory assuming that the ectoines are likely to be expelled from the membrane surface thus favoring the hydration of the lipid membrane. This effect reduces the line tension, which is the interfacial energy at the domain edges leading to reduced domain sizes and increased number of rigid domains. Isotherms of negatively charged phosphatidylglycerol membranes show a similar expansion, while unsaturated lipids are less affected. Mixed phosphatidylcholine/phosphatidylglycerol membranes exhibit the same effect on the line tension increasing the tendency for a phase separation. This could be shown also in bilayer vesicles, where the compatible solutes have only a minor effect on the lipid main phase transition in pure DPPC membranes but reduce the extent of the pretransition. In mixed DPPC/DPPG bilayer membranes ectoines cause a phase separation leading to the enrichment of expanded DPPC domains. In conclusion, our study gives for the first time evidence that ectoines have an effect on lipid membranes increasing the hydration of the surface and thus increasing the mobility of the lipid head groups and fluidizing the lipid layer accordingly. This increased fluidity may be of advantage for cell membranes to withstand extreme conditions like temperature or osmotic pressure and might also accelerate cellular repair mechanisms.

Adsorption behaviors of DPPC/MO aggregates on SiO2 surfaces

The adsorption kinetics of extruded 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC)/1-(cis-9-octadecenoyl)- rac-glycerol (monoolein, MO) aggregates on SiO 2 surface at 25 degrees C is investigated in real time, using the dissipative quartz crystal microbalance (QCM) technique. Four adsorption pathways have been identified depending on the molar fraction of MO in the DPPC/MO system: (I) intact vesicle adsorption, (II) vesicle reorganization on a SiO 2 surface, (III) supported lipid bilayer (SLB) formation, and (IV) cubosome adsorption. The results can be understood by the fact that DPPC is a lamellar phase-forming lipid, whereas MO prefers the cubic phase. Therefore, the incorporation of MO in DPPC increases the packing parameter. Equally important, MO also increases the mobility of lipid molecules and lateral pressure in the bilayers as a result of the presence of a unique cis- double bond. Before extrusion, the vesicles size increases with the MO content when X MO <or= 0.7 and cubosomes are formed for X MO >or= 0.8. The extruded DPPC/MO suspensions consist of reformed vesicles for X MO <or= 0.7 and filtered cubosomes for X MO >or= 0.8, all with a uniform diameter of approximately 100 nm. Differential scanning calorimetry (DSC) further indicates that the addition of MO lowers the main phase transition temperature of DPPC and thus makes the hydrophobic interior more fluid.

High concentrations of the compatible solute glycinebetaine destabilize model membranes under stress conditions

Compatible solutes are accumulated by diverse organisms in response to environmental stresses such as drought, salt, or cold. Glycinebetaine (Bet) is such a solute that is accumulated by many plants and microorganisms to high concentrations under stress conditions. It is an osmoprotectant in bacteria and stabilizes both soluble and peripherally membrane-bound proteins in vitro. Here, the effects of Bet on the stability of model lipid membranes are compared to the effects of two other compatible solutes, sucrose and trehalose. Both in the presence of 1M NaCl and during freezing to -20 degrees C, Bet is highly destabilizing to liposomes containing nonbilayer lipids, while the disaccharides are either protective or, in some cases, much less destabilizing. The destabilizing effect of Bet is more pronounced in membranes containing the nonbilayer galactolipid monogalactosyldiacylglycerol from plant chloroplasts than in membranes containing the nonbilayer phospholipid phosphatidylethanolamine. The most dramatic differences between the sugars and Bet were observed in liposomes made from a combination of lipids resembling plant chloroplast thylakoid membranes. Measurements with the dye merocyanine 540 indicate that the water-membrane interface was affected in opposite directions by the presence of high concentrations of sucrose or Bet. The dynamics of the lipids, however, were not differentially affected by the solutes, making direct solute-lipid interactions an unlikely explanation for the different effects on stability. The data offer an explanation, why Bet at high concentrations achieved during exogenous feeding of leaf tissues can be detrimental to cellular stability and survival under stress, while bacterial membranes that contain phosphatidylethanolamine instead of monogalactosyldiacylglycerol, or cyanobacteria that contain highly saturated monogalactosyldiacylglycerol are less susceptible.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

The effect of cold acclimation and deacclimation on cold tolerance, trehalose and free amino acid levels in Sitophilus granarius and Cryptolestes ferrugineus (Coleoptera)

Canadian and French laboratory strains of Sitophilus granarius (L.) and Cryptolestes ferrugineus (Stephens) were cold acclimated by placing adults at 15, 10 and 5 degrees C successively for 2wk at each temperature before deacclimating them for 1wk at 30 degrees C. Unacclimated S. granarius had an LT(50) (lethal time for 50% of the population) of 12days at 0 degrees C compared with 40days after the full cold acclimation. At -10 degrees C, unacclimated C. ferrugineus had an LT(50) of 1.4days compared with 24days after the full acclimation. Cold acclimation was lost within a week after returning insects to 30 degrees C. Trehalose, as well as the amino acids proline, asparagine, glutamic acid and lysine were higher in cold acclimated insects for both species. For S. granarius, glutamine was higher in cold acclimated insects and isoleucine, ethanolamine and phosphoethanolamine, a precursor of phospholipids, were lower in cold acclimated insects. For C. ferrugineus, alanine, aspartic acid, threonine, valine, isoleucine, leucine, phenylalanine and phosphoethanolamine were higher in cold acclimated insects. For both species tyrosine was lower in cold acclimated insects. There were small but significant differences between Canadian and French strains of S. granarius, with the Canadian strain being more cold hardy and having higher levels of trehalose. There were small but significant differences between male and female S. granarius, with males being more cold hardy and having higher levels of proline, asparagine and glutamic acid. In conclusion, high levels of trehalose and proline were correlated with cold tolerance, as seen in several other insects. However, correlation does not prove that these compounds are responsible for cold tolerance, and we outline further tests that could demonstrate a causal relationship between trehalose and proline and cold tolerance.

Effect of water on lamellar structure of DPPC/sugar systems

The ability of two monosaccharides, four disaccharides and one trisaccharide, to lower the transition temperature of L-alpha-dipalmitoyl phosphatidylcholine (DPPC) was investigated using differential scanning calorimetry (DSC) and the ability of these sugars to change the lateral packing of the acyl chains of DPPC was investigated using wide-angle X-ray diffraction. The sugars affected the gel-liquid crystal transition temperature (Tc) of DPPC when the water content of the DPPC/sugar systems was less than 20 wt.%. Specifically, Tc of the DPPC without sugar increased to approximately 106 degrees C, the Tc of the DPPC/monosaccharide system remained almost constant at 43 degrees C and of the DPPC/disaccharide or trisaccharide systems decreased to approximately 24 degrees C. In the dehydrated state, di- and trisaccharides caused looser packing of the DPPC hydrocarbon chains than the monosaccharides did, and the sugars affected the packing mode in different ways. The addition of water caused this difference in the sugars' effects on the packing mode to disappear and further addition of water caused the effect of sugar to almost disappear. Thus, the addition of water to a DPPC/sugar system weakens the interaction between the sugar and lipid and strengthens the DPPC chain packing.

The action in vivo of glycine betaine in enhancement of tolerance of Synechococcus sp. strain PCC 7942 to low temperature

The cyanobacterium Synechococcus sp. strain PCC 7942 was transformed with the codA gene for choline oxidase from Arthrobacter globiformis under the control of a constitutive promoter. This transformation allowed the cyanobacterial cells to accumulate glycine betaine at 60 to 80 mM in the cytoplasm. The transformed cells could grow at 20 degrees C, the temperature at which the growth of control cells was markedly suppressed. Photosynthesis of the transformed cells at 20 degrees C was more tolerant to light than that of the control cells. This was caused by the enhanced ability of the photosynthetic machinery in the transformed cells to recover from low-temperature photoinhibition. In darkness, photosynthesis of the transformed cells was more tolerant to low temperature such as 0 to 10 degrees C than that of the control cells. In parallel with the improvement in the ability of the transformed cells to tolerate low temperature, the lipid phase transition of plasma membranes from the liquid-crystalline state to the gel state shifted toward lower temperatures, although the level of unsaturation of the membrane lipids was unaffected by the transformation. These findings suggest that glycine betaine enhances the tolerance of photosynthesis to low temperature.

The interaction of polyphenols with bilayers: conditions for increasing bilayer adhesion

Because proteins and other molecules with a high polyphenol content are commonly involved in adhesion processes, we are investigating the interactions between polyphenols and biological materials. A naturally occurring polyphenol that binds a variety of proteins and lipids is tannic acid (TA), which contains five digallic acid residues covalently linked to a central D-glucose. A previous study has shown that TA increases the adhesion between apposing phosphatidylcholine (PC) bilayers and over a very narrow concentration range collapses the interbilayer fluid space from about 15 A to 5 A. To determine the chemical requirements a polyphenolic molecule must possess to increase bilayer adhesion, we have synthesized several simpler TA analogs that vary in their size, shape, and number of gallic acid and hydroxyl groups. X-ray diffraction, absorbance, binding, and differential scanning calorimetry measurements were used to investigate the interaction of these polyphenolic molecules with egg PC (EPC) and dipalmitoyl PC (DPPC) bilayers. Of these synthetic polyphenols, only penta-O-galloyl-alpha-D-glucose (PGG) was able to completely mimic the effects of TA by collapsing the interbilayer fluid space from 15 A to 5 A, decreasing the dipole potential by about 300 mV, increasing the transition enthalpy of DPPC liposomes, and inducing an interdigitated phase in DPPC. Binding studies indicated that the fluid space was reduced to 5 A at an EPC:PGG mole ratio of 5:1. We conclude that these polyphenols collapse the fluid space of PC bilayers because they 1) are amphipathic and partition into the bilayers interfacial region, 2) are long enough to span the interbilayer space, 3) contain several gallic acids distributed so that they can partition simultaneously into apposing bilayers, and 4) have sufficient gallic acid residues to interact with all lipid headgroups and cover the bilayer surface. Under these conditions we conclude that the polyphenols from interbilayer bridges. We argue that these bridges are stabilized by increased adhesion arising from an increased van der Waals interaction between apposing bilayers, electrostatic interactions between the pi electrons in the phenol ring and the -(N+CH3)3 groups on the PC headgroups, decreased hydration repulsion between bilayers, and hydrogen bonds between the H-bond-donating moieties on the polyphenols and H-bond-accepting groups in the bilayer.