Purely entropic self-assembly of the bicontinuous Ia3̅d gyroid phase in equilibrium hard-pear systems

We investigate a model of hard pear-shaped particles which forms the bicontinuous Ia3d structure by entropic self-assembly, extending the previous observations of Barmes et al. (2003 Phys. Rev. E 68, 021708. (doi:10.1103/PhysRevE.68.021708)) and Ellison et al. (2006 Phys. Rev. Lett. 97, 237801. (doi:10.1103/PhysRevLett.97.237801)). We specifically provide the complete phase diagram of this system, with global density and particle shape as the two variable parameters, incorporating the gyroid phase as well as disordered isotropic, smectic and nematic phases. The phase diagram is obtained by two methods, one being a compression–decompression study and the other being a continuous change of the particle shape parameter at constant density. Additionally, we probe the mechanism by which interdigitating sheets of pears in these systems create surfaces with negative Gauss curvature, which is needed to form the gyroid minimal surface. This is achieved by the use of Voronoi tessellation, whereby both the shape and volume of Voronoi cells can be assessed in regard to the local Gauss curvature of the gyroid minimal surface. Through this, we show that the mechanisms prevalent in this entropy-driven system differ from those found in systems which form gyroid structures in nature (lipid bilayers) and from synthesized materials (di-block copolymers) and where the formation of the gyroid is enthalpically driven. We further argue that the gyroid phase formed in these systems is a realization of a modulated splay-bend phase in which the conventional nematic has been predicted to be destabilized at the mesoscale due to molecular-scale coupling of polar and orientational degrees of freedo

A viscoelastic deadly fluid in carnivorous pitcher plants

Background : The carnivorous plants of the genus Nepenthes, widely distributed in the Asian tropics, rely mostly on nutrients derived from arthropods trapped in their pitcher-shaped leaves and digested by their enzymatic fluid. The genus exhibits a great diversity of prey and pitcher forms and its mechanism of trapping has long intrigued scientists. The slippery inner surfaces of the pitchers, which can be waxy or highly wettable, have so far been considered as the key trapping devices. However, the occurrence of species lacking such epidermal specializations but still effective at trapping insects suggests the possible implication of other mechanisms. Methodology/Principal Findings : Using a combination of insect bioassays, high-speed video and rheological measurements, we show that the digestive fluid of Nepenthes rafflesiana is highly viscoelastic and that this physical property is crucial for the retention of insects in its traps. Trapping efficiency is shown to remain strong even when the fluid is highly diluted by water, as long as the elastic relaxation time of the fluid is higher than the typical time scale of insect movements. Conclusions/Significance : This finding challenges the common classification of Nepenthes pitchers as simple passive traps and is of great adaptive significance for these tropical plants, which are often submitted to high rainfalls and variations in fluid concentration. The viscoelastic trap constitutes a cryptic but potentially widespread adaptation of Nepenthes species and could be a homologous trait shared through common ancestry with the sundew (Drosera) flypaper plants. Such large production of a highly viscoelastic biopolymer fluid in permanent pools is nevertheless unique in the plant kingdom and suggests novel applications for pest control

Bridging Alone: Religious Conservatism, Marital Homogamy, and Voluntary Association Membership

This study characterizes social insularity of religiously conservative American married couples by examining patterns of voluntary associationmembership. Constructing a dataset of 3938 marital dyads from the second wave of the National Survey of Families and Households, the author investigates whether conservative religious homogamy encourages membership in religious voluntary groups and discourages membership in secular voluntary groups. Results indicate that couples’ shared affiliation with conservative denominations, paired with beliefs in biblical authority and inerrancy, increases the likelihood of religious group membership for husbands and wives and reduces the likelihood of secular group membership for wives, but not for husbands. The social insularity of conservative religious groups appears to be reinforced by homogamy—particularly by wives who share faith with husbands

GMO detection using a bioluminescent real time reporter (BART) of loop mediated isothermal amplification (LAMP) suitable for field use

<p>Abstract</p> <p>Background</p> <p>There is an increasing need for quantitative technologies suitable for molecular detection in a variety of settings for applications including food traceability and monitoring of genetically modified (GM) crops and their products through the food processing chain. Conventional molecular diagnostics utilising real-time polymerase chain reaction (RT-PCR) and fluorescence-based determination of amplification require temperature cycling and relatively complex optics. In contrast, isothermal amplification coupled to a bioluminescent output produced in real-time (BART) occurs at a constant temperature and only requires a simple light detection and integration device.</p> <p>Results</p> <p>Loop mediated isothermal amplification (LAMP) shows robustness to sample-derived inhibitors. Here we show the applicability of coupled LAMP and BART reactions (LAMP-BART) for determination of genetically modified (GM) maize target DNA at low levels of contamination (0.1-5.0% GM) using certified reference material, and compare this to RT-PCR. Results show that conventional DNA extraction methods developed for PCR may not be optimal for LAMP-BART quantification. Additionally, we demonstrate that LAMP is more tolerant to plant sample-derived inhibitors, and show this can be exploited to develop rapid extraction techniques suitable for simple field-based qualitative tests for GM status determination. We also assess the effect of total DNA assay load on LAMP-BART quantitation.</p> <p>Conclusions</p> <p>LAMP-BART is an effective and sensitive technique for GM detection with significant potential for quantification even at low levels of contamination and in samples derived from crops such as maize with a large genome size. The resilience of LAMP-BART to acidic polysaccharides makes it well suited to rapid sample preparation techniques and hence to both high throughput laboratory settings and to portable GM detection applications. The impact of the plant sample matrix and genome loading within a reaction must be controlled to ensure quantification at low target concentrations.</p

Clamp loader ATPases and the evolution of DNA replication machinery

Clamp loaders are pentameric ATPases of the AAA+ family that operate to ensure processive DNA replication. They do so by loading onto DNA the ring-shaped sliding clamps that tether the polymerase to the DNA. Structural and biochemical analysis of clamp loaders has shown how, despite differences in composition across different branches of life, all clamp loaders undergo the same concerted conformational transformations, which generate a binding surface for the open clamp and an internal spiral chamber into which the DNA at the replication fork can slide, triggering ATP hydrolysis, release of the clamp loader, and closure of the clamp round the DNA. We review here the current understanding of the clamp loader mechanism and discuss the implications of the differences between clamp loaders from the different branches of life

Lawson criterion for ignition exceeded in an inertial fusion experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion

Computer simulations of tapered particles

We present here the results of a series of molecular dynamics (MD) simulations of systems of soft repulsive tapered particles. Essentially, the objective of the project was to investigate the effect that changing the degree of taper of these particles has on the collective behaviour of the system. The particle shapes were modelled using the parameterised Gaussian overlap (PHGO) contact function, which had previously been used in Monte Carlo (MC) studies of systems of hard particles with the same range of shapes [Phys. Rev. E 68,021708 (2003)]. The work carried out falls into three main categories. Firstly we calculated the splay and bend reduced flexoelectric coefficients, e11 and e33, for a number of systems in the nematic phase. This was done using the linear response approach developed by Nemtsov and Osipov [Kristallografiya 31 2,213-218 (1986)]. The values of e11 measured for the tapered systems studied were all positive and of the order of +0.1, whilst the e33 values were of a similar magnitude but negative in all cases. These numbers correspond to values of the order of pCm-1, which is consistent with typical values measured experimentally for the flexoelectric coefficients. The reduced coefficients for systems of uniaxial particles were also calculated and found to be approximately zero, as they should be for particles with this type of symmetry. The second major theme in the project was the mapping out of the shape-density phase diagram, through both compression and decompression sequences, for tapered particles having a constant length to breadth ratio of 3 but different degrees of tapering, ranging from an extreme tear-drop shape to the uniaxial Gaussian ellipsoid. The results of our MD simulations broadly agreed with those obtained by the MC route [Phys. Rev. E 68,021708 (2003)]. Isotropic, nematic, smectic and ordered solid phases were clearly identified. In addition a so-called `curvy-bilayer' (CB) phase was observed, which locally possessed similar order to the smectics but did not exhibit any clear long range order. The structure of the CB phase was investigated further and found to be a type of bicontinuous cubic phase, specifically the Ia3d or gyroid (G) as it is also known -a phase never before obtained from this type of simulation. Characterisation of the I-G transition was undertaken, which indicated that the gyroid freely selfassembled from precursors present in the isotropic fluid. The Sm-G transition was also characterised and found to take place via the formation, firstly of stalks joining two adjacent smectic bilayers and then subsequently pores which bisect these bilayers and initiate the emergence of the gyroid morphology.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Computer simulations of tapered particles

We present here the results of a series of molecular dynamics (MD) simulations of systems of soft repulsive tapered particles. Essentially, the objective of the project was to investigate the effect that changing the degree of taper of these particles has on the collective behaviour of the system. The particle shapes were modelled using the parameterised Gaussian overlap (PHGO) contact function, which had previously been used in Monte Carlo (MC) studies of systems of hard particles with the same range of shapes [Phys. Rev. E 68,021708 (2003)]. The work carried out falls into three main categories. Firstly we calculated the splay and bend reduced flexoelectric coefficients, e11 and e33, for a number of systems in the nematic phase. This was done using the linear response approach developed by Nemtsov and Osipov [Kristallografiya 31 2,213-218 (1986)]. The values of e11 measured for the tapered systems studied were all positive and of the order of +0.1, whilst the e33 values were of a similar magnitude but negative in all cases. These numbers correspond to values of the order of pCm-1, which is consistent with typical values measured experimentally for the flexoelectric coefficients. The reduced coefficients for systems of uniaxial particles were also calculated and found to be approximately zero, as they should be for particles with this type of symmetry. The second major theme in the project was the mapping out of the shape-density phase diagram, through both compression and decompression sequences, for tapered particles having a constant length to breadth ratio of 3 but different degrees of tapering, ranging from an extreme tear-drop shape to the uniaxial Gaussian ellipsoid. The results of our MD simulations broadly agreed with those obtained by the MC route [Phys. Rev. E 68,021708 (2003)]. Isotropic, nematic, smectic and ordered solid phases were clearly identified. In addition a so-called `curvy-bilayer' (CB) phase was observed, which locally possessed similar order to the smectics but did not exhibit any clear long range order. The structure of the CB phase was investigated further and found to be a type of bicontinuous cubic phase, specifically the Ia3d or gyroid (G) as it is also known -a phase never before obtained from this type of simulation. Characterisation of the I-G transition was undertaken, which indicated that the gyroid freely selfassembled from precursors present in the isotropic fluid. The Sm-G transition was also characterised and found to take place via the formation, firstly of stalks joining two adjacent smectic bilayers and then subsequently pores which bisect these bilayers and initiate the emergence of the gyroid morphology.</p