Deformations of rational curves on primitive symplectic varieties and applications

We study the deformation theory of rational curves on primitive symplectic varieties and show that if the rational curves cover a divisor, then, as in the smooth case, they deform along their Hodge locus in the universal locally trivial deformation. As applica-tions, we extend Markman's deformation invariance of prime exceptional divisors along their Hodge locus to this singular framework and provide existence results for uniruled ample divisors on primitive symplectic varieties that are locally trivial deformations of any moduli space of semistable objects on a projective K3 or fibers of the Albanese map of those on an abelian surface. We also present an application to the existence of prime exceptional divisors

Proceedings of the Spacecraft Charging Technology Conference: Executive Summary

Aerospace environments are reviewed in reference to spacecraft charging. Modelling, a theoretical scheme which can be used to describe the structure of the sheath around the spacecraft and to calculate the charging currents within, is discussed. Materials characterization is considered for experimental determination of the behavior of typical spacecraft materials when exposed to simulated geomagnetic substorm conditions. Materials development is also examined for controlling and minimizing spacecraft charging or at least for distributing the charge in an equipotential manner, using electrical conductive surfaces for materials exposed to space environment

Pathway for insertion of amphiphilic nanoparticles into defect-free lipid bilayers from atomistic molecular dynamics simulations

Gold nanoparticles (NPs) have been increasingly used in biological applications that involve potential contact with cellular membranes. As a result, it is essential to gain a physical understanding of NP-membrane interactions to guide the design of next-generation bioactive nanoparticles. In previous work, we showed that charged, amphiphilic NPs can fuse with lipid bilayers after contact between protruding solvent-exposed lipid tails and the NP monolayer. Fusion was only observed at the high-curvature edges of large bilayer defects, but not in low-curvature regions where protrusions are rarely observed. Here, we use atomistic molecular dynamics simulations to show that the same NPs can also fuse with low-curvature bilayers in the absence of defects if NP-protrusion contact occurs, generalizing the results of our previous work. Insertion proceeds without applying biasing forces to the NP, driven by the hydrophobic effect, and involves the transient generation of bilayer curvature. We further find that NPs with long hydrophobic ligands can insert a single ligand into the bilayer core in a manner similar to the binding of peripheral proteins. Such anchoring may precede insertion, revealing potential methods for engineering NP monolayers to enhance NP-bilayer fusion in systems with a low likelihood of lipid tail protrusions. These results reveal new pathways for NP-bilayer fusion and provide fundamental insight into behavior at the nano-bio interface.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-0819762)National Science Foundation (U.S.) (CAREER Award DMR-1054671

Regulation of multispanning membrane protein topology via post-translational annealing

The canonical mechanism for multispanning membrane protein topogenesis suggests that protein topology is established during cotranslational membrane integration. However, this mechanism is inconsistent with the behavior of EmrE, a dual-topology protein for which the mutation of positively charged loop residues, even close to the C-terminus, leads to dramatic shifts in its topology. We use coarse-grained simulations to investigate the Sec-facilitated membrane integration of EmrE and its mutants on realistic biological timescales. This work reveals a mechanism for regulating membrane-protein topogenesis, in which initially misintegrated configurations of the proteins undergo post-translational annealing to reach fully integrated multispanning topologies. The energetic barriers associated with this post-translational annealing process enforce kinetic pathways that dictate the topology of the fully integrated proteins. The proposed mechanism agrees well with the experimentally observed features of EmrE topogenesis and provides a range of experimentally testable predictions regarding the effect of translocon mutations on membrane protein topogenesis

Ecological aspects of Langsdorffia hypogaea (Balanophoraceae) parasitism in the Pantanal wetlands.

Most studies on holoparasitic plants have focused on taxonomic or systematic issues. The objective of this study was to examine the ecological aspects of parasitism of Langsdorffia hypogaea (Balanophoraceae) in the Pantanal wetlands. Individuals of L. hypogaea were dug out by hand and the host trees were identified. Eighty-eight percent of host trees exhibited zoochory dispersal syndrome. Protium heptaphyllum and Cordiera sessilis represented 50% of the sampled trees. Both species are evergreen and are preferentially shade-tolerant species growing under the canopy of other trees. Fecal sample of collared peccaries (Pecari tajacu) had seeds from both the host tree (Protium sp.) and the parasite L. hypogaea. We therefore propose the hypothesis that P. tajacu play a role as a seed disperser and may affect root parasitism

Solvent-exposed lipid tail protrusions depend on lipid membrane composition and curvature

The stochastic protrusion of hydrophobic lipid tails into solution, a subclass of hydrophobic membrane defects, has recently been shown to be a critical step in a number of biological processes like membrane fusion. Understanding the factors that govern the appearance of lipid tail protrusions is critical for identifying membrane features that affect the rate of fusion or other processes that depend on contact with solvent-exposed lipid tails. In this work, we utilize atomistic molecular dynamics simulations to characterize the likelihood of tail protrusions in phosphotidylcholine lipid bilayers of varying composition, curvature, and hydration. We distinguish two protrusion modes corresponding to atoms near the end of the lipid tail or near the glycerol group. Through potential of mean force calculations, we demonstrate that the thermodynamic cost for inducing a protrusion depends on tail saturation but is insensitive to other bilayer structural properties or hydration above a threshold value. Similarly, highly curved vesicles or micelles increase both the overall frequency of lipid tail protrusions as well as the preference for splay protrusions, both of which play an important role in driving membrane fusion. In multi-component bilayers, however, the incidence of protrusion events does not clearly depend on the mismatch between tail length or tail saturation of the constituent lipids. Together, these results provide significant physical insight into how system components might affect the appearance of protrusions in biological membranes, and help explain the roles of composition or curvature-modifying proteins in membrane fusion.National Science Foundation (U.S.). MRSEC Program (award number DMR-0819762)National Science Foundation (U.S.). Faculty Early Career Development Program (Award No. DMR-1054671)United States. Department of Energy. Computational Science Graduate Fellowship Program (grant number DE-FG02-97ER25308)National Science Foundation (U.S.) (grant number OCI-1053575

Changes in the structure due to strong winds in forest areas in the Pantanal, Brazil.

The Pantanal climate presents marked seasonality and eventually strong winds occur, especially in the beginning of the rainy season, which may last from September or October until April. A phytosociological study was conducted to evaluate the effects of a strong wind on the composition and structure of two forest formations in Pantanal wetland, a semideciduous forest (19º 15’ 32’’S and 55º 45’ 23.7’’W) and a forested savanna - “cerradão” (19° 17’ 21’’S and 55º 45’ 8.9’’W), with trees with diameter at breast height (DBH) ? 5 cm. After the strong wind, a reduction of 6% of the basal area and volume in the semideciduous forest was observed, mainly due to the uprooting of Xylopia aromatica trees. In the forested savanna, the basal area and volume reduction was even higher; an estimated 10%, representing 69 uprooted trees per hectare, mainly of Copaifera martii trees. In both areas it was observed that the uprooted trees presented an average height and diameter bigger than the trees that remained intact. Usually, the trees that were uprooted presented higher wood density and the species that had broken branches had a lower density