593 research outputs found
The Dynamic Transition of Protein Hydration Water
Thin layers of water on biomolecular and other nanostructured surfaces can be
supercooled to temperatures not accessible with bulk water. Chen et al. [PNAS
103, 9012 (2006)] suggested that anomalies near 220 K observed by quasi-elastic
neutron scattering can be explained by a hidden critical point of bulk water.
Based on more sensitive measurements of water on perdeuterated phycocyanin,
using the new neutron backscattering spectrometer SPHERES, and an improved data
analysis, we present results that show no sign of such a fragile-to-strong
transition. The inflection of the elastic intensity at 220 K has a dynamic
origin that is compatible with a calorimetric glass transition at 170 K. The
temperature dependence of the relaxation times is highly sensitive to data
evaluation; it can be brought into perfect agreement with the results of other
techniques, without any anomaly.Comment: 4 pages, 3 figures. Phys. Rev. Lett. (in press
Observation of Fragile-to-Strong Dynamic Crossover in Protein Hydration Water
At low temperatures proteins exist in a glassy state, a state which has no
conformational flexibility and shows no biological functions. In a hydrated
protein, at and above 220 K, this flexibility is restored and the protein is
able to sample more conformational sub-states, thus becomes biologically
functional. This 'dynamical' transition of protein is believed to be triggered
by its strong coupling with the hydration water, which also shows a similar
dynamic transition. Here we demonstrate experimentally that this sudden switch
in dynamic behavior of the hydration water on lysozyme occurs precisely at 220
K and can be described as a Fragile-to-Strong dynamic crossover (FSC). At FSC,
the structure of hydration water makes a transition from predominantly
high-density (more fluid state) to low-density (less fluid state) forms derived
from existence of the second critical point at an elevated pressure.Comment: 6 pages (Latex), 4 figures (Postscript
Access Control in a Workstation-Based Distributed Computing Environment
This paper describes the mechanisms employed to control access to system services on the IFS project. We base our distributed computing environment on systems that we trust, and run those systems in physically secure rooms. From that base, we add services, modifying them to interoperate with existing access control mechanisms. Some weaknesses remain in our environment; we conclude with a description of present vulnerabilities and future plans.http://deepblue.lib.umich.edu/bitstream/2027.42/107869/1/citi-tr-90-2.pd
Picosecond fluctuating protein energy landscape mapped by pressure–temperature molecular dynamics simulation
Microscopic statistical pressure fluctuations can, in principle, lead to corresponding fluctuations in the shape of a protein energy landscape. To examine this, nanosecond molecular dynamics simulations of lysozyme are performed covering a range of temperatures and pressures. The well known dynamical transition with temperature is found to be pressure-independent, indicating that the effective energy barriers separating conformational substates are not significantly influenced by pressure. In contrast, vibrations within substates stiffen with pressure, due to increased curvature of the local harmonic potential in which the atoms vibrate. The application of pressure is also shown to selectively increase the damping of the anharmonic, low-frequency collective modes in the protein, leaving the more local modes relatively unaffected. The critical damping frequency, i.e., the frequency at which energy is most efficiently dissipated, increases linearly with pressure. The results suggest that an invariant description of protein energy landscapes should be subsumed by a fluctuating picture and that this may have repercussions in, for example, mechanisms of energy dissipation accompanying functional, structural, and chemical relaxation
Mesenchymal Stromal Cell Therapy for the Treatment of Intestinal Ischemia: Defining the Optimal Cell Isolate for Maximum Therapeutic Benefit
Intestinal ischemia is a devastating intraabdominal emergency that often necessitates surgical intervention. Mortality rates can be high, and patients who survive often have significant long-term morbidity. The implementation of traditional medical therapies to prevent or treat intestinal ischemia have been sparse over the last decade, and therefore, the use of novel therapies are becoming more prevalent. Cellular therapy using mesenchymal stromal cells is one such treatment modality that is attracting noteworthy attention in the scientific community. Several groups have seen benefit with cellular therapy, but the optimal cell line has not been identified. The purpose of this review is to: 1) Review the mechanism of intestinal ischemia and reperfusion injury, 2) Identify the mechanisms of how cellular therapy may be therapeutic for this disease, and 3) Compare various MSC tissue sources to maximize potential therapeutic efficacy in the treatment of intestinal I/R diseases
Glass transition in biomolecules and the liquid-liquid critical point of water
Using molecular dynamics simulations, we investigate the relation between the
dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and
thermodynamic properties of hydration water. We find that the dynamic
transition of the macromolecules, sometimes called a ``protein glass
transition'', occurs at the temperature of dynamic crossover in the diffusivity
of hydration water, and also coincides with the maxima of the isobaric specific
heat and the temperature derivative of the orientational order parameter.
We relate these findings to the hypothesis of a liquid-liquid critical point in
water. Our simulations are consistent with the possibility that the protein
glass transition results from crossing the Widom line, which is defined as the
locus of correlation length maxima emanating from the hypothesized second
critical point of water.Comment: 10 Pages, 12 figure
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