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

Learning for Sustainability: Partnership for the Goals

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Pressure-dependent inverse bicontinuous cubic phase formation in a phosphatidylinositol 4-phosphate/phosphatidylcholine system

In this paper, we report the inositide-driven formation of an inverse bicontinuous cubic phase with space group Ia3d (QIIG, gyroid phase). The system under study consisted of distearoylphosphatidylinositol 4-phosphate (DSPIP) and dioleoylphosphatidylcholine at a molar ratio of 1:49, with a physiological concentration of magnesium ions at pH 7·4. The behaviour of the system was monitored as a function of temperature and pressure. The formation of the phase with Ia3d geometry was recorded repeatably at high pressure, and occurred more readily at higher temperatures. We conclude that the Ia3d phase formed is a thermodynamically stable structure, and that DSPIP is a potent source of membrane curvature that can drive the formation of mesophases with both 2- and 3D geometry

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

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

The promise and peril of intensive-site-based ecological research: insights from the Hubbard Brook ecosystem study

Abstract. Ecological research is increasingly concentrated at particular locations or sites. This trend reflects a variety of advantages of intensive, site-based research, but also raises important questions about the nature of such spatially delimited research: how well does site based research represent broader areas, and does it constrain scientific discovery?We provide an overview of these issues with a particular focus on one prominent intensive research site: the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. Among the key features of intensive sites are: long-term, archived data sets that provide a context for new discoveries and the elucidation of ecological mechanisms; the capacity to constrain inputs and parameters, and to validate models of complex ecological processes; and the intellectual cross-fertilization among disciplines in ecological and environmental sciences. The feasibility of scaling up ecological observations from intensive sites depends upon both the phenomenon of interest and the characteristics of the site. An evaluation of deviation metrics for the HBEF illustrates that, in some respects, including sensitivity and recovery of streams and trees from acid deposition, this site is representative of the Northern Forest region, of which HBEF is a part. However, the mountainous terrain and lack of significant agricultural legacy make the HBEF among the least disturbed sites in the Northern Forest region. Its relatively cool, wet climate contributes to high stream flow compared to other sites. These similarities and differences between the HBEF and the region can profoundly influence ecological patterns and processes and potentially limit the generality of observations at this and other intensive sites. Indeed, the difficulty of scaling up may be greatest for ecological phenomena that are sensitive to historical disturbance and that exhibit the greatest spatiotemporal variation, such as denitrification in soils and the dynamics of bird communities. Our research shows that end member sites for some processes often provide important insights into the behavior of inherently heterogeneous ecological processes. In the current era of rapid environmental and biological change, key ecological responses at intensive sites will reflect both specific local drivers and regional trends

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