Carmen Tamburu

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 storks 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.

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.

Following the structural changes during zinc-induced crystallization of charged membranes using time-resolved solution X-ray scattering

Zinc ions are highly abundant in biological systems and interact with various enzymes, proteins and biomembranes. In this paper, an in-house state-of-the-art time-resolved solution X-ray scattering setup was used to study the interactions of divalent ions with charged membranes. We show that unlike calcium ions that strongly couple and crystallize charged membranes very rapidly, zinc ions exhibit a fast time scale (seconds) for the strong coupling of the bilayers and a much slower one (hours) for the 2D lateral crystallization of the bilayers. This is attributed to the smaller zinc ion size (compared to calcium ions), which requires higher energy to shed its hydration shells. The rate of crystallization depends on the structure of the lipid tails and is slower for the unsaturated lipid, DOPS, than the saturated lipid, DLPS. We attribute this to the stronger steric repulsion between unsaturated DOPS tails, which have kinks, and to the weaker cohesive electrostatic energy, induced by the zinc ions, due to the larger area per head-group of DOPS. The Avrami model for a 2D growth mechanism with an instantaneous nucleation describes well the crystallization process. The crystallization involves various structural changes in the bilayer structure and lipid conformations within each bilayer. In this paper, we present those structural changes as a function of time.

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.

THERMAL DECOMPOSITION OF ACETONITRILE. KINETIC MODELING

The thermal decomposition of acetonitrile in the temperature range 1350–1950 K is modeled with a reaction scheme containing 23 species and 43 elementary reactions. Values of {[product]t/[CH3CN]0}/t, which were reported in a previous investigation are computed with this scheme at 50 K intervals and are compared with the values reported in the literature. Except for acrylonitrile and propyl nitrile at the high-temperature end of the study, very good agreement between the calculation and the experiment is obtained. A sensitivity spectrum of the kinetic scheme is shown and a discussion of the overall mechanism is presented.

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.

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.

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.

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.

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.