Bridges of Calcium Bicarbonate Tightly Couple Dipolar Lipid Membranes.

The interaction between lipid membranes and ions is associated with a range of key physiological processes. Most earlier studies have focused on the interaction of lipids with cations, while the specific effects of the anions have been largely overlooked. Owing to dissolved atmospheric carbon dioxide, bicarbonate is an important ubiquitous anion in aqueous media. In this paper, we report on the effect of bicarbonate anions on the interactions between dipolar lipid membranes in the presence of previously adsorbed calcium cations. Using a combination of solution X-ray scattering, osmotic stress, and molecular dynamics simulations, we followed the interactions between 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) lipid membranes that were dialyzed against CaCl2 solutions in the presence and absence of bicarbonate anions. Calcium cations adsorbed onto DLPC membranes, charge them, and lead to their swelling. In the presence of bicarbonate anions, however, the calcium cations can tightly couple one dipolar DLPC membrane to the other and form a highly condensed and dehydrated lamellar phase with a repeat distance of 3.45 ± 0.02 nm. Similar tight condensation and dehydration has only been observed between charged membranes in the presence of multivalent counterions. Bridging between bilayers by calcium bicarbonate complexes induced this arrangement. Furthermore, in this condensed phase, lipid molecules and adsorbed ions were arranged in a two-dimensional oblique lattice.

Production of Aliphatic and Aromatic Compounds in the High Temperature Decomposition of Propargyl Chloride. Single Pulse Shock Tube Experiments, Quantum Chemical Calculations, and Computer Modeling.

The thermal reactions of propargyl chloride were studied behind reflected shock waves in a pressurized driver 2 in. i.d. single-pulse shock tube over the temperature range 1000-1350 K and pressure range behind the reflected shocks of 2-4 atm. Cooling rates were ∼5 × 105 K/s. The reflected shock temperatures were calculated from the extent of elimination of hydrofluoric acid (HF) from 1,1,1-trifluoroethane, where 1,1,1-trifluoroethane (TFE) → HF + 1,1-difluoroethylene (DFE), that was added in small concentration (0.1%) to the reaction mixture to serve as a chemical thermometer. For comparison, the shock temperatures were obtained also from the measured incident shock velocities, using the three conservation equations and the ideal gas equation of states. Fifteen stable reaction products, containing different numbers of carbon atoms (from two to nine), both aliphatic and aromatic, chain and cyclic, with and without chlorine resulting from the initial rupture of the C-Cl bond in propargyl chloride were identified. On the basis of the results of quantum chemical calculations that were carried out, a chemical kinetic scheme containing 63 elementary steps was constructed. Comparison of the curves that were calculated by using the kinetic scheme with the experimental results shows good agreement.

Changes in aggregation states of light-harvesting complexes as a mechanism for modulating energy transfer in desert crust cyanobacteria.

In this paper we propose an energy dissipation mechanism that is completely reliant on changes in the aggregation state of the phycobilisome light-harvesting antenna components. All photosynthetic organisms regulate the efficiency of excitation energy transfer (EET) to fit light energy supply to biochemical demands. Not many do this to the extent required of desert crust cyanobacteria. Following predawn dew deposition, they harvest light energy with maximum efficiency until desiccating in the early morning hours. In the desiccated state, absorbed energy is completely quenched. Time and spectrally resolved fluorescence emission measurements of the desiccated desert crust Leptolyngbya ohadii strain identified (i) reduced EET between phycobilisome components, (ii) shorter fluorescence lifetimes, and (iii) red shift in the emission spectra, compared with the hydrated state. These changes coincide with a loss of the ordered phycobilisome structure, evident from small-angle neutron and X-ray scattering and cryo-transmission electron microscopy data. Based on these observations we propose a model where in the hydrated state the organized rod structure of the phycobilisome supports directional EET to reaction centers with minimal losses due to thermal dissipation. In the desiccated state this structure is lost, giving way to more random aggregates. The resulting EET path will exhibit increased coupling to the environment and enhanced quenching.

Osmotic Stress Induced Desorption of Calcium Ions from Dipolar Lipid Membranes.

The interaction between multivalent ions and lipid membranes with saturated tails and dipolar (net neutral) headgroups can lead to adsorption of the ions onto the membrane. The ions charge the membranes and contribute to electrostatic repulsion between them, in a similar manner to membranes containing charged lipids. Using solution X-ray scattering and the osmotic stress method, we measured and modeled the pressure-distance curves between partially charged membranes containing mixtures of charged (1,2-dilauroyl-sn-glycero-3-phospho-l-serine, DLPS) and dipolar (1,2-dilauroyl-sn-glycero-3-phosphocholine, DLPC) lipids over a wide range of membrane charge densities. We then compared these pressure-distance curves with those of DLPC membranes in the presence of 10 mM CaCl2. Our data and modeling show that when low osmotic stress is applied to the DLPC bilayers, the membrane charge density is equivalent to that of a charged membrane containing ca. 4 mol % DLPS and 96 mol % DLPC. As the osmotic stress increased, the charge density of the DLPC membrane decreased and resembled that of a membrane containing ca. 1 mol % DLPS. These data are consistent with desorption of the calcium ions from the DLPC membrane with increasing osmotic stress.

A Deep Insight into the Details of the Interisomerization and Decomposition Mechanism of o-Quinolyl and o-Isoquinolyl Radicals. Quantum Chemical Calculations and Computer Modeling.

The isomerization of o-quinolyl ↔ o-isoquinolyl radicals and their thermal decomposition were studied by quantum chemical methods, where potential energy surfaces of the reaction channels and their kinetics rate parameters were determined. A detailed kinetics scheme containing 40 elementary steps was constructed. Computer simulations were carried out to determine the isomerization mechanism and the distribution of reaction products in the decomposition. The calculated mole percent of the stable products was compared to the experimental values that were obtained in this laboratory in the past, using the single pulse shock tube. The agreement between the experimental and the calculated mole percents was very good. A map of the figures containing the mole percent's of eight stable products of the decomposition plotted vs T are presented. The fast isomerization of o-quinolyl → o-isoquinolyl radicals via the intermediate indene imine radical and the attainment of fast equilibrium between these two radicals is the reason for the identical product distribution regardless whether the reactant radical is o-quinolyl or o-isoquinolyl. Three of the main decomposition products of o-quinolyl radical, are those containing the benzene ring, namely, phenyl, benzonitrile, and phenylacetylene radicals. They undergo further decomposition mainly at high temperatures via two types of reactions: (1) Opening of the benzene ring in the radicals, followed by splitting into fragments. (2) Dissociative attachment of benzonitrile and phenyl acetylene by hydrogen atoms to form hydrogen cyanide and acetylene.

Spider Silk-CBD-Cellulose Nanocrystal Composites: Mechanism of Assembly.

The fabrication of cellulose-spider silk bio-nanocomposites comprised of cellulose nanocrystals (CNCs) and recombinant spider silk protein fused to a cellulose binding domain (CBD) is described. Silk-CBD successfully binds cellulose, and unlike recombinant silk alone, silk-CBD self-assembles into microfibrils even in the absence of CNCs. Silk-CBD-CNC composite sponges and films show changes in internal structure and CNC alignment related to the addition of silk-CBD. The silk-CBD sponges exhibit improved thermal and structural characteristics in comparison to control recombinant spider silk sponges. The glass transition temperature (Tg) of the silk-CBD sponge was higher than the control silk sponge and similar to native dragline spider silk fibers. Gel filtration analysis, dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (TEM) indicated that silk-CBD, but not the recombinant silk control, formed a nematic liquid crystalline phase similar to that observed in native spider silk during the silk spinning process. Silk-CBD microfibrils spontaneously formed in solution upon ultrasonication. We suggest a model for silk-CBD assembly that implicates CBD in the central role of driving the dimerization of spider silk monomers, a process essential to the molecular assembly of spider-silk nanofibers and silk-CNC composites.

Critical Conditions for Adsorption of Calcium Ions onto Dipolar Lipid Membranes.

Dipolar lipid membranes may adsorb multivalent ions. The binding constant depends on the type of lipid and ions. In this paper, we focus on the adsorption of calcium ions onto 1,2-dilauroylphosphatidylcholine (DLPC) membrane. Using small-angle-X-ray scattering we found that at ambient room temperature ca. 0.6 mM CaCl2 is a critical concentration at which calcium ions adsorbed to 30 mg/mL (ca. 48 mM) DLPC membrane. We then determined the structure of the lamellar phases formed at CaCl2 concentrations below and above the critical concentration and characterized the effect of temperature and incubation time on the adsorption process. Our findings suggest that calcium adsorption to DLPC membranes requires an initial nucleation phase.

Wet Spinning and Drawing of Human Recombinant Collagen.

The advancement of tissue engineering and regenerative medicine has generated a growing demand for collagen fibers that both resemble native collagen fibers as closely as possible in terms of structure and function, and can be produced in large quantities and processed by current textile technologies. However, the collagen spinning methodologies reported thus far have not matured sufficiently to provide a spinning rate suitable for large-scale production and also generate fibers with insufficient mechanical properties. In the current study, we introduce three new elements into existing collagen fiber spinning technologies: the use of recombinant human collagen, high concentration dope, and spin drawing. At the optimal draw ratio, mechanically strong, aligned, thin fibers, with diameters similar to those of cotton or polyester fibers, are obtained at rates exceeding 1,000 m/h. The resulting fibers display an ultimate tensile strength (UTS) of 150 MPa and a strain of 0.21 after being hydrated in PBS, values which are comparable to and even surpass those reported for human patellar and Achilles tendons. The production technology is simple, based entirely on existing fiber production machinery, and suitable for scale-up and rapid production of large fiber quantities.

Tilted cellulose arrangement as a novel mechanism for hygroscopic coiling in the stork's bill awn.

The sessile nature of plants demands the development of seed-dispersal mechanisms to establish new growing loci. Dispersal strategies of many species involve drying of the dispersal unit, which induces directed contraction and movement based on changing environmental humidity. The majority of researched hygroscopic dispersal mechanisms are based on a bilayered structure. Here, we investigate the motility of the stork's bill (Erodium) seeds that relies on the tightening and loosening of a helical awn to propel itself across the surface into a safe germination place. We show that this movement is based on a specialized single layer consisting of a mechanically uniform tissue. A cell wall structure with cellulose microfibrils arranged in an unusually tilted helix causes each cell to spiral. These cells generate a macroscopic coil by spiralling collectively. A simple model made from a thread embedded in an isotropic foam matrix shows that this cellulose arrangement is indeed sufficient to induce the spiralling of the cells.

Entropic attraction condenses like-charged interfaces composed of self-assembled molecules.

Like-charged solid interfaces repel and separate from one another as much as possible. Charged interfaces composed of self-assembled charged-molecules such as lipids or proteins are ubiquitous. The present study shows that although charged lipid-membranes are sufficiently rigid, in order to swell as much as possible, they deviate markedly from the behavior of typical like-charged solids when diluted below a critical concentration (ca. 15 wt %). Unexpectedly, they swell into lamellar structures with spacing that is up to four times shorter than the layers should assume (if filling the entire available space). This process is reversible with respect to changing the lipid concentration. Additionally, the research shows that, although the repulsion between charged interfaces increases with temperature, like-charged membranes, remarkably, condense with increasing temperature. This effect is also shown to be reversible. Our findings hold for a wide range of conditions including varying membrane charge density, bending rigidity, salt concentration, and conditions of typical living systems. We attribute the limited swelling and condensation of the net repulsive interfaces to their self-assembled character. Unlike solids, membranes can rearrange to gain an effective entropic attraction, which increases with temperature and compensates for the work required for condensing the bilayers. Our findings provide new insight into the thermodynamics and self-organization of like-charged interfaces composed of self-assembled molecules such as charged biomaterials and supramolecular assemblies that are widely found in synthetic and natural constructs.

Effect of temperature on the structure of charged membranes.

Interactions between charged and neutral self-assembled phospholipid membranes are well understood and take into account temperature dependence. Yet, the manner in which the structure of the membrane is affected by temperature was hardly studied. Here we study the effect of temperature on the thickness, area per lipid, and volume per lipid of charged membranes. Two types of membranes were studied: membranes composed of charged lipids and dipolar (neutral) membranes that adsorbed divalent cations and became charged. Small-angle X-ray scattering data demonstrate that the thickness of charged membranes decreases with temperature. Wide-angle X-ray scattering data show that the area per headgroup increases with temperature. Intrinsically charged membranes linearly thin with temperature, whereas neutral membranes that adsorb divalent ions and become charged show an exponential decrease of their thickness. The data indicate that, on average, the tails shorten as the temperature rises. We attribute this behavior to higher lipid tail entropy and to the weaker electrostatic screening of the charged headgroups, by their counterions, at elevated temperatures. The latter effect leads to stronger electrostatic repulsion between the charged headgroups that increases the area per headgroup and decreases the bilayer thickness.

The structure of ions and zwitterionic lipids regulates the charge of dipolar membranes.

In pure water, zwitterionic lipids form lamellar phases with an equilibrium water gap on the order of 2 to 3 nm as a result of the dominating van der Waals attraction between dipolar bilayers. Monovalent ions can swell those neutral lamellae by a small amount. Divalent ions can adsorb onto dipolar membranes and charge them. Using solution X-ray scattering, we studied how the structure of ions and zwitterionic lipids regulates the charge of dipolar membranes. We found that unlike monovalent ions that weakly interact with all of the examined dipolar membranes, divalent and trivalent ions adsorb onto membranes containing lipids with saturated tails, with an association constant on the order of ∼10 M(-1). One double bond in the lipid tail is sufficient to prevent divalent ion adsorption. We suggest that this behavior is due to the relatively loose packing of lipids with unsaturated tails that increases the area per lipid headgroup, enabling their free rotation. Divalent ion adsorption links two lipids and limits their free rotation. The ion-dipole interaction gained by the adsorption of the ions onto unsaturated membranes is insufficient to compensate for the loss of headgroup free-rotational entropy. The ion-dipole interaction is stronger for cations with a higher valence. Nevertheless, polyamines behave as monovalent ions near dipolar interfaces in the sense that they interact weakly with the membrane surface, whereas in the bulk their behavior is similar to that of multivalent cations. Advanced data analysis and comparison with theory provide insight into the structure and interactions between ion-induced regulated charged interfaces. This study models biologically relevant interactions between cell membranes and various ions and the manner in which the lipid structure governs those interactions. The ability to monitor these interactions creates a tool for probing systems that are more complex and forms the basis for controlling the interactions between dipolar membranes and charged proteins or biopolymers for encapsulation and delivery applications.

Reactions of 1-naphthyl radicals with acetylene. Single-pulse shock tube experiments and quantum chemical calculations. Differences and similarities in the reaction with ethylene.

The reactions of 1-naphthyl radicals with acetylene were studied behind reflected shock waves in a single-pulse shock tube, covering the temperature range 950-1200 K at overall densities behind the reflected shocks of approximately 2.5 x 10(-5) mol/cm3. 1-Iodonaphthalene served as the source for 1-naphthyl radicals. The [acetylene]/[1-iodonaphthalene] ratio in all of the experiments was approximately 100 to channel the free radicals into reactions with acetylene rather than iodonaphthalene. Only two major products resulting from the reactions of 1-naphthyl radicals with acetylene and with hydrogen atoms were found in the post shock samples. They were acenaphthylene and naphthalene. Some low molecular weight aliphatic products at rather low concentrations, resulting from an attack of various free radicals on acetylene, were also found in the shocked samples. In view of the relatively low temperatures employed in the present experiments, the unimolecular decomposition rate of acetylene is negligible. One potential energy surface describes the production of acenaphthylene and 1-naphthyl acetylene, although the latter was not found experimentally due to the high barrier (calculated) required for its production. Using quantum chemical methods, the rate constants for three unimolecular elementary steps on the surface were calculated using transition state theory. A kinetics scheme containing 16 elementary steps was constructed, and computer modeling was performed. An excellent agreement between the experimental yields of the two major products and the calculated yields was obtained. Differences and similarities in the potential energy surfaces of 1-naphthyl radical + acetylene and those of ethylene are presented, and the kinetics mechanisms are discussed.

Reactions of 1-naphthyl radicals with ethylene. Single pulse shock tube experiments, quantum chemical, transition state theory, and multiwell calculations.

The reactions of 1-naphthyl radicals with ethylene were studied behind reflected shock waves in a single pulse shock tube, covering the temperature range 950-1200 K at overall densities behind the reflected shocks of approximately 2.5 x 10(-5) mol/cm3. 1-Iodonaphthalene served as the source for 1-naphthyl radicals as its C-I bond dissociation energy is relatively small. It is only approximately 65 kcal/mol as compared to the C-H bond strength in naphthalene which is approximately 112 kcal/mol and can thus produce naphthyl radicals at rather low reflected shock temperatures. The [ethylene]/[1-iodo-naphthalene] ratio in all of the experiments was approximately 100 in order to channel the free radicals into reactions with ethylene rather than iodonaphthalene. Four products resulting from the reactions of 1-naphthyl radicals with ethylene were found in the post shock samples. They were vinyl naphthalene, acenaphthene, acenaphthylene, and naphthalene. Some low molecular weight aliphatic products at rather low concentrations, resulting from the attack of various free radicals on ethylene were also found in the shocked samples. In view of the relatively low temperatures employed in the present experiments, the unimolecular decomposition rate of ethylene is negligible. Three potential energy surfaces describing the production of vinyl naphthalene, acenaphthene, and acenaphthylene were calculated using quantum chemical methods and rate constants for the elementary steps on the surfaces were calculated using transition state theory. Naphthalene is not part of the reactions on the surfaces. Acenaphthylene is obtained only from acenaphthene. A kinetics scheme containing 27 elementary steps most of which were obtained from the potential energy surfaces was constructed and computer modeling was performed. An excellent agreement between the experimental yields of the four major products and the calculated yields was obtained.

Decomposition and isomerization of 1,2-benzisoxazole: single-pulse shock-tube experiments, quantum chemical and transition-state theory calculations.

Isomerization and decomposition of 1,2-benzisoxazole were studied behind reflected shock waves in a pressurized driver, single-pulse shock tube. It isomerizes to o-hydroxybenzonitrile, and no fragmentation is observed up to a temperature where the isomerization is almost complete (approximately 1040 K at 2 ms reaction time). The isomerization experiments in this investigation covered the temperature range 900-1040 K. The lack of fragmentation is in complete contrast to the thermal behavior of isoxazole, where no isomerization was observed and the main decomposition products over the same temperature range were carbon monoxide and acetonitrile. In a series of experiments covering the temperature range 1190-1350 K, a plethora of fragmentation products appear in the post shock samples of 1,2-benzisoxazole. The product distribution is exactly the same regardless of whether the starting material is 1,2-benzisoxazole or o-hydroxybenzonitrile, indicating that over this temperature range the 1,2-benzisoxazole has completely isomerized to o-hydroxybenzonitrile prior to fragmentation. Two potential energy surfaces that lead to the isomerization were evaluated by quantum chemical calculations. One surface with one intermediate and two transition states has a high barrier and does not contribute to the process. The second surface is more complex. It has three intermediates and four transition states, but it has a lower overall barrier and yields the isomerization product o-hydroxybenzonitrile at a much higher rate. The unimolecular isomerization rate constants kinfinity at a number of temperatures in the range of 900-1040 K were calculated from the potential energy surface using transition-state theory and then expressed in an Arrhenius form. The value obtained is kfirst=4.15x10(14) exp(-51.7x10(3)/RT) s-1, where R is expressed in units of cal/(K mol). The calculated value is somewhat higher than the one obtained from the experimental results. When it is expressed in terms of energy difference it corresponds of ca. 2 kcal/mol.

Decomposition of anthranil. Single pulse shock-tube experiments, potential energy surfaces and multiwell transition-state calculations. The role of intersystem crossing.

The thermal decomposition of anthranil diluted in argon was studied behind reflected shock waves in a 2 in. i.d. pressurized driver single-pulse shock tube over the temperature range 825-1000 K and overall densities of approximately 3 x 10(-5) mol/cm(3). Two major products: aniline and cyclopentadiene carbonitrile (accompanied by carbon monoxide) and four minor products resulting from the decomposition were found in the postshock samples. They were, in order of decreasing abundance, pyridine, CH(2)=CHCN, HCN and CHC-CN, and comprised only a few percents of the overall product distribution. Quantum chemical calculations were carried out to determine the sequence of the unimolecular reactions that lead to the formation of cyclopentadiene carbonitrile and of phenylnitrene/phenylimine that are the precursors of aniline. They form aniline by reactions with traces of water impurities. To produce cyclopentadiene carbonitrile, two main processes must take place: CO elimination and ring contraction from a six- to a five-membered ring. It was shown that this can occur via two parallel pathways where CO elimination takes place prior to or following ring contraction. Singlet potential energy surfaces for all the elementary reactions that lead to the formation of cyclopentadiene carbonitrile and phenylnitrene/phenylimine were obtained. Their rate constants were calculated on the basis of the results of the quantum chemical calculations using transition-state theory. A kinetic scheme containing these reactions was constructed and multiwell calculations were performed to evaluate the mole percent of the products as a function of temperature. A very serious disagreement between the experimental results and the results of calculations showed that the singlet PESs could not account for the observed experimental rates. No other singlet PESs that lead to the formation of these products could be found. In view of this observation, attempts to find pathways that lead to the formation of cyclopentadiene carbonitrile and phenylnitrene/phenylimine on triplet surfaces were made. Such surfaces were found, and singlet <--> triplet intersystem crossing probabilities and crossing rate constants were calculated as well as the rate constants of all the elementary steps on the triplet surfaces. A reaction scheme was constructed and multiwell calculations were performed, including also the pathways on the singlet surfaces, to evaluate the mole percent of the products as a function of temperature. The agreement between the experimental results and these calculations was quite satisfactory.

Thermal reactions of benzoxazole. Single pulse shock tube experiments and quantum chemical calculations.

The thermal decomposition of benzoxazole diluted in argon was studied behind reflected shock waves in a 2 in. i.d. single-pulse shock tube over the temperature range 1000-1350 K and at overall densities of approximately 3 x 10(-5) mol/cm(3). Two major products, o-hydroxybenzonitrile at high concentration and cyclopentadiene carbonitrile (accompanied by carbon monoxide) at much lower concentration, and four minor fragmentation products resulting from the decomposition were found in the postshock samples. They were, in order of decreasing abundance, benzonitrile, acetylene, HCN, and CH=C-CN and comprised of only a few percent of the overall product distribution. Quantum chemical calculations were carried out to determine the sequence of the unimolecular reactions that led to the formation of o-hydroxybenzonitrile and cyclopentadiene carbonitrile, the major products of the thermal reactions of benzoxazole. A potential energy surface leading directly from benzoxazole to cyclopentadiene carbonitrile could not be found, and it was shown that the latter is formed from the product o-hydroxybenzonitrile. In order that cyclopentadiene carbonitrile be produced, CO elimination and ring contraction from a six- to a five-membered ring must take place. A surface where CO elimination occurs prior to ring contraction was found to have very high barriers compared to the ones where ring contraction occurs prior to CO elimination and was not considered in our discussion. Rates for all the steps on the various surfaces were evaluated, kinetic schemes containing these steps were constructed, and multiwell calculations were performed to evaluate the mole percent of the two major products as a function of temperature. The agreement between the experimental results and these calculations, as shown graphically, is very good.