Investigation of a square-based pyramidal-sheet roof

Imperial Users onl

Effective dynamics of microorganisms that interact with their own trail

Like ants, some microorganisms are known to leave trails on surfaces to communicate. We explore how trail-mediated self-interaction could affect the behavior of individual microorganisms when diffusive spreading of the trail is negligible on the timescale of the microorganism using a simple phenomenological model for an actively moving particle and a finite-width trail. The effective dynamics of each microorganism takes on the form of a stochastic integral equation with the trail interaction appearing in the form of short-term memory. For moderate coupling strength below an emergent critical value, the dynamics exhibits effective diffusion in both orientation and position after a phase of superdiffusive reorientation. We report experimental verification of a seemingly counterintuitive perpendicular alignment mechanism that emerges from the model.Comment: new figure with experimental results; expanded appendi

Dynamics of confined water reconstructed from inelastic x-ray scattering measurements of bulk response functions

Nanoconfined water and surface-structured water impacts a broad range of fields. For water confined between hydrophilic surfaces, measurements and simulations have shown conflicting results ranging from “liquidlike” to “solidlike” behavior, from bulklike water viscosity to viscosity orders of magnitude higher. Here, we investigate how a homogeneous fluid behaves under nanoconfinement using its bulk response function: The Green's function of water extracted from a library of S(q,ω) inelastic x-ray scattering data is used to make femtosecond movies of nanoconfined water. Between two confining surfaces, the structure undergoes drastic changes as a function of surface separation. For surface separations of ≈9 Å, although the surface-associated hydration layers are highly deformed, they are separated by a layer of bulklike water. For separations of ≈6 Å, the two surface-associated hydration layers are forced to reconstruct into a single layer that modulates between localized “frozen’ and delocalized “melted” structures due to interference of density fields. These results potentially reconcile recent conflicting experiments. Importantly, we find a different delocalized wetting regime for nanoconfined water between surfaces with high spatial frequency charge densities, where water is organized into delocalized hydration layers instead of localized hydration shells, and are strongly resistant to `freezing' down to molecular distances (<6 Å)

Perceived barriers and enablers to physical activity participation in people with alopecia areata : a constructivist grounded theory study

Background: Alopecia Areata (AA) is an autoimmune disease that is characterised by hair loss. Individuals diagnosed with it often describe feelings of trauma and social rejection due to cosmetic repercussions and are at high risk of experiencing psychological distress. Physical activity (PA) participation has been associated with better mental health outcomes in diverse populations. A preliminary study of individuals with AA indicated that severe hair loss is associated with symptomatic depression, anxiety and stress, which negatively impacted PA participation. While strategies to increase PA participation in the general population have been established, little is known about PA participation in people with AA. This study aimed to understand barriers and enablers to PA participation in people with AA to inform the development of evidence-based interventions. Methods: The study used a grounded theory (GT) methodology, relying on an iterative and simultaneous process of data collection, coding, theory development, and data comparisons to explore the perceived barriers and enablers to PA. Data were collected through a focus group (8 participants [33.38 ± 10.81 years]) and individual telephone interviews (8 participants [33.89 ± 11.87 years]). The study was conducted in Melbourne, Australia. Interview data were recorded digitally, transcribed verbatim and analysed. Recruitment continued until theoretical saturation was achieved. Results: The constructivist grounded theory method used has assisted to develop an explanatory model which is used to explain the themes for barriers and enablers to PA participation. The four phases in the explanatory model are as follows (1) onset of AA; (2) reaction towards the condition; (3) adjustment; and (4) acceptance. Conclusion: The findings highlighted perceived barriers and enablers to PA participation in people with AA. Future interventions could consider addressing these barriers specifically to maximise effectiveness and to improve mental health status based on the phases of the explanatory model. © 2020, The Author(s)

Cooperativity and Frustration in Protein-Mediated Parallel Actin Bundles

We examine the mechanism of bundling of cytoskeletal actin filaments by two representative bundling proteins, fascin and espin. Small-angle X-ray studies show that increased binding from linkers drives a systematic \textit{overtwist} of actin filaments from their native state, which occurs in a linker-dependent fashion. Fascin bundles actin into a continuous spectrum of intermediate twist states, while espin only allows for untwisted actin filaments and fully-overtwisted bundles. Based on a coarse-grained, statistical model of protein binding, we show that the interplay between binding geometry and the intrinsic \textit{flexibility} of linkers mediates cooperative binding in the bundle. We attribute the respective continuous/discontinous bundling mechanisms of fascin/espin to differences in the stiffness of linker bonds themselves.Comment: 5 pages, 3 figures, figure file has been corrected in v

Scanning tunnelling miscroscopy/spectroscopy and X-ray absorption spectroscopy studies of Co adatoms and anoislands on highly oriented pyrolytic graphite

In this paper, the scanning tunneling microscopy, scanning tunneling spectroscopy and X-ray absorption spectroscopy of cobalt adatoms and nanoislands were studied on a highly oriented pyrolytic graphite. Local electronic structure were observed by STS.\ud \u

Stresses and flexibilities for pressure vessel attachments

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1984.MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE.Includes bibliographical references.by Frank Michael Gerard Wong.M.S

Electrostatics of Rigid Polyelectrolytes

Abstract The organization of rigid biological polyelectrolytes by multivalent ions and macroions are important for many fundamental problems in biology and biomedicine, such as cytoskeletal regulation and antimicrobial sequestration in cystic fibrosis. These polyelectrolytes have been used as model systems for understanding electrostatics in complex fluids. Here, we review some recent results in theory, simulations, and experiments. © 2007 Elsevier Ltd. All rights reserved. Electrostatics is important in biology because all nucleic acids and virtually all proteins and membranes are charged. Electrostatic interactions in water however are qualitatively different from those in a vacuum or a dielectric. The starting point for understanding these interactions is usually mean-field theories like the Poisson-Boltzmann (PB) formalism and their approximations, which are routinely employed in colloid science and computational biology. It can for example be used to computationally estimate the distribution of counterions around realistic models of biological macromolecules. This approximate approach, however, systematically ignores counterion correlations and finite ion sizes. (Recent work suggests that a partial cancellation between these two effects result in approximate agreement with experiments [1 •• ].) Within the PB description, like-charged objects such as polyelectrolytes always repel, in accord with intuition. In systems with strong electrostatic interactions (ex: high surface charge densities, multivalent ions), interactions between polyelectrolytes are controlled by the organization and dynamics of the condensed ions surrounding the polyelectrolyte. For example, using the hypernetted chain approximation, Kjellander and Marcelja •• ]. It is generally agreed that condensation of polyelectrolytes by multivalent ions or macroions is important for many fundamental problems in biology and biomedicine. One wellknown series of examples is nucleic acid packaging in viruses, bacteria, chromosomes, and artificial gene delivery systems [5 • ] In the last 10 years, theoretical investigations have focused on the electrostatic behavior of highly-charged polyelectrolytes that cannot be explained by mean-fields, such as overcharging, the collapse behavior of the polyelectrolyte itself, as well as the existence and form of multivalent ion induced like-charge attraction, using approaches such as density functional theories, integral equations, field theoretical calculations, as well as others. A number of excellent reviews with comprehensive references have been published [21 recent work on the structure, interactions, and phase behavior of primarily rigid polyelectrolytes. Rigid polyelectrolytes Anionic rigid biopolymers have been recently used as experimental systems for polyelectrolyte electrostatics. Due to their large persistence lengths (Nμm), they can be thought of as idealized charged rods, and are well-suited for comparison with theory. For example, the addition of divalent cations to a solution of anionic F-actin filaments will drive the ordering of close-packed bundles of twisted filaments [25 • ]. In the case of microtubules, a polymorphism of bundle structures can be formed via multivalent ions. Hexagonal bundles with controllable diameters are formed via interactions with trivalent and higher valence ions, whereas &apos;living necklace&apos; bundles with linear, branched, and looped morphologies are formed with divalent ions [26 • ] •• ]. Using a family of homologous diamine ions, &apos;dumbbell&apos;-shaped molecules with two cationic amine (+1) groups connected by a spacer of variable length, it was experimentally shown that the small divalent diamines condense M13 virus polyelectrolyte rods while the larger divalent diamines cannot. Moreover, the addition of a single CH 2 group to the spacer can enforce a transition between condensing and non-condensing behavior [34 • ]. A rough empirical criterion for condensation using these prototypical &apos;dumbbell&apos; ionic linkers was proposed, but clearly a rigorous microscopic understanding is still lacking. Phase behavior of polyelectrolytes Counterion-induced attractions strongly impinge on polyelectrolyte phase behavior. For example, the mechanisms described above for multivalent ion induced polyelectrolyte attraction are all relatively short-ranged, while the imperfectly compensated polyelectrolyte rods are mutually repulsive at long-range. The competition between long-range electrostatic repulsion and shortrange attractions can drive the formation of a rich polymorphism of structures. As a function of increasing divalent ion concentration, the global organization of cytoskeletal F-actin rods can convert between different liquid crystalline phases with different symmetries and packing densities. F-actin rods in isotropic or nematic phases will both organize into a lamellar liquid crystalline network phase of crosslinked rafts before fully condensing into bundles composed of close-packed F-actin rods [35 • ]. Interestingly, this liquid crystalline network phase can exist at physiological concentrations of divalent ions, which suggests that it may play a role in cytoskeletal organization. The physics of multi-axial liquid crystalline gels and networks have been engaged theoretically in a number of studies [36 •• 37-40]. In the presence of strong linkers such as multivalent ions that can form crosslinks between polyelectrolyte rods, where it is possible to effectively maintain a highdensity of repulsive rods, the repulsive interactions responsible for the orientational behavior are expected to be strong since the rods forced into proximity via crosslinking. Bruinsma extended the Onsager theory of nematic liquid crystals to rod-like polyelectrolytes crosslinked by multivalent ions, and found that a range of exotic multi-axial liquid crystalline phases can exist near regions of phase coexistence between the isotropic phase and dense bundle phase Since counterions play a central role in the generation of inter-polyelectrolyte forces, it is important to be able to probe their spatial distribution, correlations, and dynamics. In the last few years, progress has been made in this area. Role of counterions Although experiments show unambiguously that an attractive interaction exists, there has been little done on measuring actual counterion correlations, which are necessary for generating attractions. The organization of divalent Ba ions on actin filaments was studied using synchrotron x-ray diffraction [25 • ]. Interestingly, the counterions do not form a lattice that simply follows actin&apos;s helical symmetry; rather, they organize into onedimensional (1-D) counterion density waves (CDW) parallel to the actin filaments. Each monomer has a heterogeneous charge distribution, which is repeated along the symmetry a 13/6 helix, or 13 monomers in 6 full helical turns. The 1-D counterion density wave is coupled to torsional deformations of the oppositely charged actin polyelectrolyte, so that attractions are optimized via charge alignment with the counterion domains. It will be interesting to see how this counterion organization is modified under different conditions. For example, it has been shown that the structure of a counterion lattice within a condensed polyelectrolyte rod phase undergoes a series of shearing transitions as the spacing between the rods decreases [44 • ]. This general theme of a coupled mode between counterion density changes and polyelectrolyte distortions can also be seen in recent work on DNA. Kornyshev and Leikin developed a theory of interactions between DNA with explicitly defined helical charge patterns [45 •• ]. It was proposed that an &apos;electrostatic zipper&apos; can form when multivalent cations adsorb in the major groove of one DNA chain, and interact with the protrusive anionic ridges of the sugar-phosphate backbone on an adjacent DNA chain. Within this theoretical framework, it was shown that counterion-induced aggregation of DNA can be accompanied by significant torsional deformations of the DNA helix, in order to maintain registry between opposing DN