191 research outputs found
Potential for modulation of the hydrophobic effect inside chaperonins
Despite the spontaneity of some in vitro protein folding reactions, native
folding in vivo often requires the participation of barrel-shaped multimeric
complexes known as chaperonins. Although it has long been known that chaperonin
substrates fold upon sequestration inside the chaperonin barrel, the precise
mechanism by which confinement within this space facilitates folding remains
unknown. In this study, we examine the possibility that the chaperonin mediates
a favorable reorganization of the solvent for the folding reaction. We begin by
discussing the effect of electrostatic charge on solvent-mediated hydrophobic
forces in an aqueous environment. Based on these initial physical arguments, we
construct a simple, phenomenological theory for the thermodynamics of density
and hydrogen bond order fluctuations in liquid water. Within the framework of
this model, we investigate the effect of confinement within a chaperonin-like
cavity on the configurational free energy of water by calculating solvent free
energies for cavities corresponding to the different conformational states in
the ATP- driven catalytic cycle of the prokaryotic chaperonin GroEL. Our
findings suggest that one function of chaperonins may be to trap unfolded
proteins and subsequently expose them to a micro-environment in which the
hydrophobic effect, a crucial thermodynamic driving force for folding, is
enhanced
Surface Analysis of OFE-Copper X-Band Accelerating Structures and Possible Correlation to RF Breakdown Events
X-band accelerator structures meeting the Next Linear Collider (NLC) design
requirements have been found to suffer vacuum surface damage caused by radio
frequency (RF) breakdown, when processed to high electric-field gradients.
Improved understanding of these breakdown events is desirable for the
development of structure designs, fabrication procedures, and processing
techniques that minimize structure damage. RF reflected wave analysis and
acoustic sensor pickup have provided breakdowns localization in RF structures.
Particle contaminations found following clean autopsy of four RF-processed
travelling wave structures, have been catalogued and analyzed. Their influence
on RF breakdown, as well as that of several other material-based properties,
will be discussed.Comment: 21 pages, 8 figures, 4 tables, Submitted to JVST A as a proceeding of
the 50th AVS conference (Baltimore, MD, 2-7 Nov 2003
Electron-fluctuation interaction in a non-Fermi superconductor
We studied the influence of the amplitude fluctuations of a non-Fermi
superconductor on the energy spectrum of the 2D Anderson non-Fermi system. The
classical fluctuations give a temperature dependence in the pseudogap induced
in the fermionic excitations.Comment: revtex fil
Many-body Theory vs Simulations for the pseudogap in the Hubbard model
The opening of a critical-fluctuation induced pseudogap (or precursor
pseudogap) in the one-particle spectral weight of the half-filled
two-dimensional Hubbard model is discussed. This pseudogap, appearing in our
Monte Carlo simulations, may be obtained from many-body techniques that use
Green functions and vertex corrections that are at the same level of
approximation. Self-consistent theories of the Eliashberg type (such as the
Fluctuation Exchange Approximation) use renormalized Green functions and bare
vertices in a context where there is no Migdal theorem. They do not find the
pseudogap, in quantitative and qualitative disagreement with simulations,
suggesting these methods are inadequate for this problem. Differences between
precursor pseudogaps and strong-coupling pseudogaps are also discussed.Comment: Accepted, Phys. Rev. B15 15Mar00. Expanded version of original
submission, Latex, 8 pages, epsfig, 5 eps figures (Last one new). Discussion
on fluctuation and strong coupling induced pseudogaps expande
Charge Fluctuations on Membrane Surfaces in Water
We generalize the predictions for attractions between over-all neutral
surfaces induced by charge fluctuations/correlations to non-uniform systems
that include dielectric discontinuities, as is the case for mixed charged lipid
membranes in an aqueous solution. We show that the induced interactions depend
in a non-trivial way on the dielectric constants of membrane and water and show
different scaling with distance depending on these properties. The generality
of the calculations also allows us to predict under which dielectric conditions
the interaction will change sign and become repulsive
Optimal measurement of visual motion across spatial and temporal scales
Sensory systems use limited resources to mediate the perception of a great
variety of objects and events. Here a normative framework is presented for
exploring how the problem of efficient allocation of resources can be solved in
visual perception. Starting with a basic property of every measurement,
captured by Gabor's uncertainty relation about the location and frequency
content of signals, prescriptions are developed for optimal allocation of
sensors for reliable perception of visual motion. This study reveals that a
large-scale characteristic of human vision (the spatiotemporal contrast
sensitivity function) is similar to the optimal prescription, and it suggests
that some previously puzzling phenomena of visual sensitivity, adaptation, and
perceptual organization have simple principled explanations.Comment: 28 pages, 10 figures, 2 appendices; in press in Favorskaya MN and
Jain LC (Eds), Computer Vision in Advanced Control Systems using Conventional
and Intelligent Paradigms, Intelligent Systems Reference Library,
Springer-Verlag, Berli
Charge-Fluctuation-Induced Non-analytic Bending Rigidity
In this Letter, we consider a neutral system of mobile positive and negative
charges confined on the surface of curved films. This may be an appropriate
model for: i) a highly charged membrane whose counterions are confined to a
sheath near its surface; ii) a membrane composed of an equimolar mixture of
anionic and cationic surfactants in aqueous solution. We find that the charge
fluctuations contribute a non-analytic term to the bending rigidity that varies
logarithmically with the radius of curvature. This may lead to spontaneous
vesicle formation, which is indeed observed in similar systems.Comment: Revtex, 9 pages, no figures, submitted to PR
Pseudogap effects induced by resonant pair scattering
We demonstrate how resonant pair scattering of correlated electrons above T_c
can give rise to pseudogap behavior. This resonance in the scattering T-matrix
appears for superconducting interactions of intermediate strength, within the
framework of a simple fermionic model. It is associated with a splitting of the
single peak in the spectral function into a pair of peaks separated by an
energy gap. Our physical picture is contrasted with that derived from other
T-matrix schemes, with superconducting fluctuation effects, and with preformed
pair (boson-fermion) models. Implications for photoemission and tunneling
experiments in the cuprates are discussed.Comment: REVTeX3.0; 4 pages, 4 EPS figures (included
Influence of solvent granularity on the effective interaction between charged colloidal suspensions
We study the effect of solvent granularity on the effective force between two
charged colloidal particles by computer simulations of the primitive model of
strongly asymmetric electrolytes with an explicitly added hard sphere solvent.
Apart from molecular oscillating forces for nearly touching colloids which
arise from solvent and counterion layering, the counterions are attracted
towards the colloidal surfaces by solvent depletion providing a simple
statistical description of hydration. This, in turn, has an important influence
on the effective forces for larger distances which are considerably reduced as
compared to the prediction based on the primitive model. When these forces are
repulsive, the long-distance behaviour can be described by an effective Yukawa
pair potential with a solvent-renormalized charge. As a function of colloidal
volume fraction and added salt concentration, this solvent-renormalized charge
behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell
model but there are quantitative differences. For divalent counterions and
nano-sized colloids, on the other hand, the hydration may lead to overscreened
colloids with mutual attraction while the primitive model yields repulsive
forces. All these new effects can be accounted for through a solvent-averaged
primitive model (SPM) which is obtained from the full model by integrating out
the solvent degrees of freedom. The SPM was used to access larger colloidal
particles without simulating the solvent explicitly.Comment: 14 pages, 16 craphic
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