Kinetics and mechanism of the interconversion of inverse bicontinuous cubic mesophases

This paper describes time-resolved x-ray diffraction data monitoring the transformation of one inverse bicontinuous cubic mesophase into another, in a hydrated lipid system. The first section of the paper describes a mechanism for the transformation that conserves the topology of the bilayer, based on the work of Charvolin and Sadoc, Fogden and Hyde, and Benedicto and O'Brien in this area. We show a pictorial representation of this mechanism, in terms of both the water channels and the lipid bilayer. The second section describes the experimental results obtained. The system under investigation was 2:1 lauric acid: dilauroylphosphatidylcholine at a hydration of 50% water by weight. A pressure-jump was used to induce a phase transition from the gyroid (Q(II)(G)) to the diamond (Q(II)(D)) bicontinuous cubic mesophase, which was monitored by time-resolved x-ray diffraction. The lattice parameter of both mesophases was found to decrease slightly throughout the transformation, but at the stage where the Q(II)(D) phase first appeared, the ratio of lattice parameters of the two phases was found to be approximately constant for all pressure-jump experiments. The value is consistent with a topology-preserving mechanism. However, the polydomain nature of our sample prevents us from confirming that the specific pathway is that described in the first section of the paper. Our data also reveal signals from two different intermediate structures, one of which we have identified as the inverse hexagonal (H-II) mesophase. We suggest that it plays a role in the transfer of water during the transformation. The rate of the phase transition was found to increase with both temperature and pressure-jump amplitude, and its time scale varied from the order of seconds to minutes, depending on the conditions employed

The 7Li(p,n)7 Be Reaction as a Source of Fast Neutrons for Smaller Compact Cyclotrons

The usefulness of the 7Li(p,n)7Be reaction as a fast neutron source for applications, such as neutron therapy etc., using smaller compact cyclotrons (proton energies of up to 15-18 MeV) has been investigated by measuring thin and thick target neutron spectra, absolute cross sections and angular distributions of various neutron groups produced in this reaction at 10.45 MeV. Our results indicate that the forward direction is still the preferred one for obtaining the most suitable fast neutron beam for biomedical application, and that moderately thick, rather than infinitely thick, target would provide higher mean energy. Moreover, it has also been shown that the 7Li(p,n)7 Be reaction is more suited for producing neutron beams for therapy than proton and deuteron induced reactions on Be at corresponding energies, and that a therapeutically useful neutron beam can be produced even with smaller compact cyclotrons

Membrane-protein crystallization in cubo: Temperature-dependent phase behaviour of monoolein-detergent mixtures

The lipidic cubic phase of monoolein has proved to be a matrix well suited to the production of three-dimensional crystals of membrane proteins. It consists of a single continuous bilayer, which is contorted in three-dimensional space and separates two distinct water channels. It has previously been proposed that on the addition of precipitants, membrane proteins embedded in the cubic phase migrate through the matrix to nucleation sites and that this process is dependent upon the stability of the lipidic cubic phase. Here, the effect of detergent type (C-8-C-12 glucosides, C-8-C-12 maltosides and C-7 thioglucoside) and concentration (1-3 x the critical micelle concentration; CMC) on cubic phase stability are reported in the form of the temperature-dependent phase behaviour (268-313 K) in 40% aqueous solution. The results are tabulated to show the best monoolein (MO)-detergent mixtures, mixing temperatures and crystallization temperatures identified. Monoolein-detergent mixtures suited for low-temperature in cubo crystallization of temperature-sensitive proteins are also reported for the first time. These mixtures can be prepared at low temperatures (mixed at less than or equal to 288 K) and remain stable at 277 K for a period of at least one month. They include MO- heptyl thioglucoside (1x and 3x CMC), MO-nonyl glucoside (3 x CMC), MO-octyl maltoside (3 x CMC), MO-nonyl maltoside (1 x CMC) and MO-decyl maltoside (1 x CMC)

Long-Term Integrated Studies Show Complex and Surprising Effects of Climate Change in the Northern Hardwood Forest

Evaluations of the local effects of global change are often confounded by the interactions of natural and anthropogenic factors that overshadow the effects of climate changes on ecosystems. Long-term watershed and natural elevation gradient studies at the Hubbard Brook Experimental Forest and in the surrounding region show surprising results demonstrating the effects of climate change on hydrologic variables (e.g., evapotranspiration, streamflow, soil moisture); the importance of changes in phenology on water, carbon, and nitrogen fluxes during critical seasonal transition periods; winter climate change effects on plant and animal community composition and ecosystem services; and the effects of anthropogenic disturbances and land-use history on plant community composition. These studies highlight the value of long-term integrated research for assessments of the subtle effects of changing climate on complex ecosystems

The minimum energy expenditure shortest path method

This article discusses the addition of an energy parameter to the shortest path execution process; namely, the energy expenditure by a character during execution of the path. Given a simple environment in which a character has the ability to perform actions related to locomotion, such as walking and stair stepping, current techniques execute the shortest path based on the length of the extracted root trajectory. However, actual humans acting in constrained environments do not plan only according to shortest path criterion, they conceptually measure the path that minimizes the amount of energy expenditure. On this basis, it seems that virtual characters should also execute their paths according to the minimization of actual energy expenditure as well. In this article, a simple method that uses a formula for computing vanadium dioxide ($VO_2$) levels, which is a proxy for the energy expenditure by humans during various activities, is presented. The presented solution could be beneficial in any situation requiring a sophisticated perspective of the path-execution process. Moreover, it can be implemented in almost every path-planning method that has the ability to measure stepping actions or other actions of a virtual character

Generalized Centrifugal Force Model for Pedestrian Dynamics

A spatially continuous force-based model for simulating pedestrian dynamics is introduced which includes an elliptical volume exclusion of pedestrians. We discuss the phenomena of oscillations and overlapping which occur for certain choices of the forces. The main intention of this work is the quantitative description of pedestrian movement in several geometries. Measurements of the fundamental diagram in narrow and wide corridors are performed. The results of the proposed model show good agreement with empirical data obtained in controlled experiments.Comment: 10 pages, 14 figures, accepted for publication as a Regular Article in Physical Review E. This version contains minor change

Authigenic carbonates from cold-water carbonate mounds in the Gulf of Cadiz: Microbial diversity and imprint on carbonate minerals

The Pen Duick Escarpment off Morocco consists of recent carbonate mounds in water depths of 500-600 m, flanked by giant mud volcanoes. These mounds are covered by mainly lifeless cold-water corals and have been associated with extensive fields of seep-related carbonates in off-reef regions. Three piston cores (from 350 to 640 cm long), coming from different sites on these juvenile mounds, were sampled and analyzed for mineralogy, stable isotopic composition of carbonates, geochemistry, and microbial communities. Most of the sediment comprises pelagic calcite (coccoliths), detrital quartz and authigenic dolomite, often observed encasing coccoliths. The decalcification of the sediment resulted in a dolomite dominated matrix that showed stable carbon istotope values of as low as -30 permil in contrast to the bulk sample values of -7 to -15 permil, which implies the involvement of microbes in the production of bicarbonate ions. Initial results from 16S rRNA gene clone libraries support the theory, that anaerobic oxidation of methane is one of the most important biogeochemical process leading to carbonate precipitation. Preliminary results of stable carbon isotopes of bulk samples from different carbonate mounds from the same area, indicates that the sulphate-methane transition zone moves in depth through time. We will show and discuss multidisciplinary data obtained after several cruises aimed to elucidate the impact of microorganisms on the construction of these carbonate mounds. The special emphasis in this research will be on the correlation between microbial ecosystems and their metabolic influence on mineral formation and diagenesis