386 research outputs found
A derivation of the Levinson algorithm for solving linear systems with symmetric positive definite Toeplitz matrix
AbstractBased on an orthogonalization technique, published earlier in this journal, a derivation is given of the Levinson algorithm for solving systems with a symmetric positive definite Toeplitz matrix
Nanomechanical and structural properties of native cellulose under compressive stress
Cellulose is an important biopolymer with applications ranging from its use as an additive in pharmaceutical products to the development of novel smart materials. This wide applicability arises in part from its interesting mechanical properties. Here we report on the use of high pressure X-ray diffraction and Raman spectroscopy in a diamond anvil cell to determine the bulk and local elastic moduli of native cellulose. The modulus values obtained are 20 GPa for the bulk modulus and 200-355 and 15 GPa for the crystalline parts and the overall elastic (Young's) modulus, respectively. These values are consistent with those calculated from tensile measurements. Above 8 GPa, the packing of the cellulose chains within the fibers undergoes significant structural distortion, whereas the chains themselves remain largely unaffected by compression
Tolerance of Artemia to static and shock pressure loading
Hydrostatic and hydrodynamic pressure loading has been applied to unicellular organisms for a number of years due to interest from food technology and extremophile communities. There is also an emerging interest in the response of multicellular organisms to high pressure conditions. Artemia salina is one such organism. Previous experiments have shown a marked difference in the hatching rate of these organisms after exposure to different magnitudes of pressure, with hydrostatic tests showing hatching rates at pressures up to several GPa, compared to dynamic loading that resulted in comparatively low survival rates at lower pressure magnitudes. In order to begin to investigate the origin of this difference, the work presented here has focussed on the response of Artemia salina to (quasi) one-dimensional shock loading. Such experiments were carried out using the plate-impact technique in order to create a planar shock front. Artemia cysts were investigated in this manner along with freshly hatched larvae (nauplii). The nauplii and cysts were observed post-shock using optical microscopy to detect motility or hatching, respectively. Hatching rates of 18% were recorded at pressures reaching 1.5 GPa, as determined with the aid of numerical models. Subjecting Artemia to quasi-one-dimensional shock loading offers a way to more thoroughly explore the shock pressure ranges these organisms can survive
Water Dynamics in Shewanella oneidensis at Ambient and High Pressure using Quasi-Elastic Neutron Scattering
Quasielastic neutron scattering (QENS) is an ideal technique for studying water transport and relaxation dynamics at pico-to nanosecond timescales and at length scales relevant to cellular dimensions. Studies of high pressure dynamic effects in live organisms are needed to understand Earth's deep biosphere and biotechnology applications. Here we applied QENS to study water transport in Shewanella oneidensis at ambient (0.1 MPa) and high (200 MPa) pressure using H/D isotopic contrast experiments for normal and perdeuterated bacteria and buffer solutions to distinguish intracellular and transmembrane processes. The results indicate that intracellular water dynamics are comparable with bulk diffusion rates in aqueous fluids at ambient conditions but a significant reduction occurs in high pressure mobility. We interpret this as due to enhanced interactions with macromolecules in the nanoconfined environment. Overall diffusion rates across the cell envelope also occur at similar rates but unexpected narrowing of the QENS signal appears between momentum transfer values Q = 0.7-1.1 Ã…-1 corresponding to real space dimensions of 6-9 Ã…. The relaxation time increase can be explained by correlated dynamics of molecules passing through Aquaporin water transport complexes located within the inner or outer membrane structures
On the Degree of Team Cooperation in CD Grammar Systems.
In this paper, we introduce a dynamical complexity measure, namely the degree of team cooperation, in the aim of investigating "how much" the components of a grammar system cooperate when forming a team in the process of generating terminal words. We present several results which strongly suggest that this measure is trivial in the sense that the degree of team cooperation of any language is bounded by a constant. Finally, we prove that the degree of team cooperation of a given cooperating/distributed grammar system cannot be algorithmically computed and discuss a decision problem
Generic Mechanism of Emergence of Amyloid Protofilaments from Disordered Oligomeric aggregates
The presence of oligomeric aggregates, which is often observed during the
process of amyloid formation, has recently attracted much attention since it
has been associated with neurodegenerative conditions such as Alzheimer's and
Parkinson's diseases. We provide a description of a sequence-indepedent
mechanism by which polypeptide chains aggregate by forming metastable
oligomeric intermediate states prior to converting into fibrillar structures.
Our results illustrate how the formation of ordered arrays of hydrogen bonds
drives the formation of beta-sheets within the disordered oligomeric aggregates
that form early under the effect of hydrophobic forces. Initially individual
beta-sheets form with random orientations, which subsequently tend to align
into protofilaments as their lengths increases. Our results suggest that
amyloid aggregation represents an example of the Ostwald step rule of first
order phase transitions by showing that ordered cross-beta structures emerge
preferentially from disordered compact dynamical intermediate assemblies.Comment: 14 pages, 4 figure
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