פרסומים

We present a theoretical study of the energetics, equilibrium size, and size distribution of membrane pores composed of electrically charged amphipathic peptides. The peptides are modeled as cylinders (mimicking alpha-helices) carrying different amounts of charge, with the charge being uniformly distributed over a hydrophilic face, defined by the angle subtended by polar amino acid residues. The free energy of a pore of a given radius, R, and a given number of peptides, s, is expressed as a sum of the peptides’ electrostatic charging energy (calculated using Poisson-Boltzmann theory), and the lipid-perturbation energy associated with the formation of a membrane rim (which we model as being semitoroidal) in the gap between neighboring peptides. A simple phenomenological model is used to calculate the membrane perturbation energy. The balance between the opposing forces (namely, the radial free energy derivatives) associated with the electrostatic free energy that favors large R, and the membrane perturbation term that favors small R, dictates the equilibrium properties of the pore. Systematic calculations are reported for circular pores composed of various numbers of peptides, carrying different amounts of charge (1-6 elementary, positive charges) and characterized by different polar angles. We find that the optimal R’s, for all (except, possibly, very weakly) charged peptides conform to the ‘‘toroidal’’ pore model, whereby a membrane rim larger than similar to1 nm intervenes between neighboring peptides. Only weakly charged peptides are likely to form ‘‘barrel-stave’’ pores where the peptides essentially touch one another. Treating pore formation as a two-dimensional self-assembly phenomenon, a simple statistical thermodynamic model is formulated and used to calculate pore size distributions. We find that the average pore size and size polydispersity increase with peptide charge and with the amphipathic polar angle. We also argue that the transition of peptides from the adsorbed to the inserted (membrane pore) state is cooperative and thus occurs rather abruptly upon a change in ambient conditions.

Meteorological radar is a remote sensing system that provides rainfall estimations at high spatial and temporal resolutions. The radar-based rainfall intensities (R) are calculated from the observed radar reflectivities (Z). Often, rain gauge rainfall observations are used in combination with the radar data to find the optimal parameters in the Z–R transformation equation. The scale dependency of the power-law Z–R parameters when estimated from radar reflectivity and rain gauge intensity data is explored herein. The multiplicative (a) and exponent (b) parameters are said to be “scale dependent” if applying the observed and calculated rainfall intensities to objective function at different scale results in different “optimal” parameters. Radar and gauge data were analyzed from convective storms over a midsize, semiarid, and well-equipped watershed. Using the root-mean-square difference (rmsd) objective function, a significant scale dependency was observed. Increased time- and space scales resulted in a considerable increase of the a parameter and decrease of the b parameter. Two sources of uncertainties related to scale dependency were examined: 1) observational uncertainties, which were studied both experimentally and with simplified models that allow representation of observation errors; and 2) model uncertainties. It was found that observational errors are mainly (but not only) associated with positive bias of the b parameter that is reduced with integration, at least for small scales. Model errors also result in scale dependency, but the trend is less systematic, as in the case of observational errors. It is concluded that identification of optimal scale for Z–R relationship determination requires further knowledge of reflectivity and rain-intensity error structure.

An interfacial Maillard reaction between furfural and cysteine in two different food-grade nano-sized self-assembled solns. with two oppositely curved interfaces (W/O and O/W microemulsions) have been studied and compared. These microemulsions are selective microreactors strongly enhancing the generation of sulfur-contg. flavors. The Maillard reactions occur at lower temps. than in water and are much faster. The interfaces of both W/O and O/W microemulsions are capable of enhancing the Maillard reactions in which the selectivity and reactivity are controlled by the compn. of the interface and its curvature. In the W/O microemulsions the Maillard reaction was controlled and enhanced by the interfacial concn. of a co-emulsifier such as butanol and are restricted by the concn. of the core water reservoir. On the other hand, in the O/W microemulsions, where water is the continuous phase, the reaction rates are enhanced by increase in the water content and the microemulsion curvature. The Maillard product internal compn. (regioselectivity and type of products) is dictated by temp., time, pH and mainly by the nature of the interface, and by the surfactant nature and its interfacial compn. [on SciFinder(R)]

An interfacial Maillard reaction between furfural and cysteine in two different food-grade nano-sized self-assembled solutions with two oppositely curved interfaces (W/O and O/W microemulsions) have been studied and compared. These microemulsions are selective microreactors strongly enhancing the generation of sulfur-containing flavors. The Maillard reactions occur at lower temperatures than in water and are much faster. The interfaces of both W/O and O/W microemulsions are capable of enhancing the Maillard reactions in which the selectivity and reactivity are controlled by the composition of the interface and its curvature. In the W/O microemulsions the Maillard reaction was controlled and enhanced by the interfacial concentration of a co-emulsifier such as butanol and are restricted by the concentration of the core water reservoir. On the other hand,, in the O/W microemulsions, where water is the continuous phase, the reaction rates are enhanced by increase in the water content and the microemulsion curvature. The Maillard product internal composition (regioselectivity and type of products) is dictated by temperature, time, pH and mainly by the nature of the interface, and by the surfactant nature and its interfacial composition.

In a previous communication (Kindt et al., 2001) we reported preliminary results of Brownian dynamics simulation and analytical theory which address the packaging and ejection forces involving DNA in bacteriophage capsids. In the present work we provide a systematic formulation of the underlying theory, featuring the energetic and structural aspects of the strongly confined DNA. The free energy of the DNA chain is expressed as a sum of contributions from its encapsidated and released portions, each expressed as a sum of bending and interstrand energies but subjected to different boundary conditions. The equilibrium structure and energy of the capsid-confined and free chain portions are determined, for each ejected length, by variational minimization of the free energy with respect to their shape profiles and interaxial spacings. Numerical results are derived for a model system mimicking the lambda-phage. We find that the fully encapsidated genome is highly compressed and strongly bent, forming a spool-like condensate, storing enormous elastic energy. The elastic stress is rapidly released during the first stage of DNA injection, indicating the large force (tens of pico Newtons) needed to complete the (inverse) loading process. The second injection stage sets in when similar to1/3 of the genome has been released, and the interaxial distance has nearly reached its equilibrium value (corresponding to that of a relaxed torus in solution); concomitantly the encapsidated genome begins a gradual morphological transformation from a spool to a torus. We also calculate the loading force, the average pressure on the capsid’s walls, and the anisotropic pressure profile within the capsid. The results are interpreted in terms of the (competing) bending and interaction components of the packing energy, and are shown to be in good agreement with available experimental data.

Gender gaps in physics in favour of boys are more prominent in Israel than in other countries. The main research question is to find out what gender issues are at play in Israeli advanced placement physics classes. Matriculation exam scores from approximately 400 high schools were analysed across 12 years. In addition, semi-constructed interviews were conducted with 50 advanced placement physics students (25 girls and 25 boys). In terms of participation, it was found that the ratio of girls to boys has been unchanged from 1988 to 2000 and is roughly 1:3. In terms of performance, it was found that the final matriculation scores of boys and girls are similar. However, breaking up the final scores into its two components - teachers’ given grades and matriculation test scores - showed that boy’s test scores are usually higher than girls’ test scores, while girls’ teachers’ given grades are usually higher than boys’. Results from semi-constructed interviews pointed to two factors that are especially unfavourable to many girls: excessive competitiveness and lack of teaching for understanding. Girls’ yearning for deep understanding is seen as a form of questing for connected knowledge. It is suggested that instructional methods that foster students’ understanding while decreasing competitiveness in physics classes might contribute to girls’ participation and performance in advanced physics classes while also supporting the learning of many boys.

Gender gaps in physics in favour of boys are more prominent in Israel than in other countries. The main research question is to find out what gender issues are at play in Israeli advanced placement physics classes. Matriculation exam scores from approximately 400 high schools were analysed across 12 years. In addition, semi-constructed interviews were conducted with 50 advanced placement physics students (25 girls and 25 boys). In terms of participation, it was found that the ratio of girls to boys has been unchanged from 1988 to 2000 and is roughly 1:3. In terms of performance, it was found that the final matriculation scores of boys and girls are similar. However, breaking up the final scores into its two components - teachers' given grades and matriculation test scores - showed that boy's test scores are usually higher than girls' test scores, while girls' teachers' given grades are usually higher than boys'. Results from semi-constructed interviews pointed to two factors that are especially unfavourable to many girls: excessive competitiveness and lack of teaching for understanding. Girls' yearning for deep understanding is seen as a form of questing for connected knowledge. It is suggested that instructional methods that foster students' understanding while decreasing competitiveness in physics classes might contribute to girls' participation and performance in advanced physics classes while also supporting the learning of many boys.

Fostering students’ higher order thinking skills is considered an important educational goal. Although learning theories see the development of students’ thinking as an important goal for all students, teachers often believe that stimulating higher order thinking is appropriate only for high-achieving students. According to this view, low-achieving students are, by and large, unable to deal with tasks that require higher order thinking skills and should thus be spared the frustration generated by such tasks. Because this view may cause teachers to treat students in a nonegalitarian way, it is important to find out whether or not it is supported by empirical evidence. The goal of this study is to examine this issue in light of four different studies, by asking the following question: Do low-achieving students gain from teaching and learning processes that are designed to foster higher order thinking skills? Each of the4 studies addressed a different project whose goal was to teach higher order thinking in science classrooms. Following a brief general description of each project, we provide an analysis of its effects on students with low and high achievements. The findings show that by the end of each of the 4 programs, students with high academic achievements gained higher thinking scores than their peers with low academic achievements. However, students of both subgroups made considerable progress with respect to their initial score. In one of the 4 studies the net gain of low achievers was significantly higher than for high achievers. Our findings strongly suggest that teachers should encourage students of all academic levels to engage in tasks that involve higher order thinking skills.

The formation of amorphous calcium phosphate (ACP) and its transformation into octacalcium phosphate (OCP) in the presence of poly-l-glutamic acid (PGA, Mw 3 000 D and 50 000 D), poly-l-lysine (PLL, Mw 50 000 D) and polystyrene sulfonate (PSS, MW 70 000 D) has been investigated. All polyelectrolytes at low concentrations induced and at high concentrations retarded nucleation of the crystalline precipitate. In addition, the polyelectrolytes inhibited aggregation of ACP particles and growth of the crystalline phase. The intensity of the effects depended on the charge, molecular mass, and concentration of the specific polymer.

Alcohols and polyols are essential components (in addition to the surfactant, water, and oil) in the formation of U-type self-assembled nano-structures, (sometimes called L-phases or U-type microemulsions). These microemulsions are characterized by large isotropic regions ranging from the oil side of the phase diagram up to the aqueous corner. The isotropic oily solutions of reverse micelles (''the concentrates'') can be diluted along some dilution lines with aqueous phase to the ``direct micelles'' corner via a bicontinuous mesophases (i.e., two structural transitions). This dilution takes place with no phase separations or occurrence of liquid crystalline phases. The structural transitions were determined by viscosity, conductivity, and pulsed gradient spin echo NMR (PGSE NMR), and are not visible to the eye. Two guest nutraceutical molecules (lutem and phytosterols) were solubilized, at their maximum solubilization capacity, in the reversed micellar solutions (L-2 phase) and were further diluted with the aqueous phase to the aqueous micellar comer (L-1 phase). Structural transitions (for the two types of molecule) from water-in-oil to bicontinuous microstructures were induced by the guest molecules. The transitions occurred at an earlier stage of dilution, at a lower water content (20 wt.% aqueous phase), than in the empty (blank) microemulsions (transitions at 30 wt.% aqueous phase). The transitions from the bicontinuous microstructure to the oilin-water microemulsions were retarded by the solubilizates and occurred at later dilution stage at higher aqueous phase contents (50 wt.% aqueous region for empty microemulsion and >60 wt.% for solubilized microemulsion). As a result, the bicontinuous isotropic region, in the presence of the guest molecules, becomes much broader. It seems that the main reason for such ``guest-induced structural transitions'' is related to a significant flattening and enhanced rigidity of the interface. The guest molecules of the high molecular volume are occupying high volume fraction of the interface (when the solubilization is maximal).

In this chapter some problems connected with the utilization of subzero temperature differential scanning calorimetry (SZT-DSC) are discussed. Among them are the determination of hydration numbers of surfactants and organic compounds, the determination of the hydration shell thickness, the effect of alcohol on the distribution of water between free and bound states in nonionic surfactant-based systems, and some considerations regarding the problem of phase separation of such systems in subzero temperatures. The significance of SZT-DSC for some novel applications is also discussed.

Winner of The Polonsky Prize for Creativity and Originality in the Humanistic Disciplines, The Hebrew University, 2004Avihu Zakai analyzes Jonathan Edwards’s redemptive mode of historical thought in the context of the Enlightenment. As theologian and philosopher, Edwards has long been a towering figure in American intellectual history. Nevertheless, and despite Edwards’s intense engagement with the nature of time and the meaning of history, there has been no serious attempt to explore his philosophy of history. Offering the first such exploration, Zakai considers Edwards’s historical thought as a reaction, in part, to the varieties of Enlightenment historical narratives and their growing disregard for theistic considerations.Zakai analyzes the ideological origins of Edwards’s insistence that the process of history depends solely on God’s redemptive activity in time as manifested in a series of revivals throughout history, reading this doctrine as an answer to the threat posed to the Christian theological teleology of history by the early modern emergence of a secular conception of history and the modern legitimation of historical time. In response to the Enlightenment refashioning of secular, historical time and its growing emphasis on human agency, Edwards strove to re-establish God’s preeminence within the order of time. Against the de-Christianization of history and removal of divine power from the historical process, he sought to re-enthrone God as the author and lord of history–and thus to re-enchant the historical world.Placing Edwards’s historical thought in its broadest context, this book will be welcomed by those who study early modern history, American history, or religious culture and experience in America. – Publisher’s Description