125 research outputs found
Effects of lengthscales and attractions on the collapse of hydrophobic polymers in water
We present results from extensive molecular dynamics simulations of collapse
transitions of hydrophobic polymers in explicit water focused on understanding
effects of lengthscale of the hydrophobic surface and of attractive
interactions on folding. Hydrophobic polymers display parabolic, protein-like,
temperature-dependent free energy of unfolding. Folded states of small
attractive polymers are marginally stable at 300 K, and can be unfolded by
heating or cooling. Increasing the lengthscale or decreasing the polymer-water
attractions stabilizes folded states significantly, the former dominated by the
hydration contribution. That hydration contribution can be described by the
surface tension model, , where the surface
tension, , is lengthscale dependent and decreases monotonically with
temperature. The resulting variation of the hydration entropy with polymer
lengthscale is consistent with theoretical predictions of Huang and Chandler
(Proc. Natl. Acad. Sci.,97, 8324-8327, 2000) that explain the blurring of
entropy convergence observed in protein folding thermodynamics. Analysis of
water structure shows that the polymer-water hydrophobic interface is soft and
weakly dewetted, and is characterized by enhanced interfacial density
fluctuations. Formation of this interface, which induces polymer folding, is
strongly opposed by enthalpy and favored by entropy, similar to the
vapor-liquid interface.Comment: 24 pages, 5 figure
An Improved Intrusion Prevention Sytem for WLAN
The volatile growth in wireless networks over the last few years resembles the rapid growth of the Internet within the last decade. The current IPS presents a less security. Unfortunately, our work combined with the work of others show that each of these mechanisms are completely futile. As a result, organizations with deployed wireless networks are vulnerable to illegal use of, and access to, their internal communications
The Ternary System Cu2HIO6 â 2H2O-H5IO6-H2O at 303 K
Copper hexaoxoiodates 4CuO â
I2O7 â
5H2O, 3CuO â
I2O7 â
3H2O and 3CuO â
2I2O7 â
7H2O have been isolated from the ternary system Cu2HIO6 â
2H2O-H5IO6-H2O at 303 K and studied by chemical, thermal X-ray diffraction and infrared spectroscopic methods. Some of the intermediates obtained on isothermal heating of the parent compounds have been characterised
Surface modification of starch based blends using potassium permanganate-nitric acid system and its effect on the adherence and proliferation of osteoblastic-like cells
The surface modification of three starch based polymeric biomaterials, using a KMnO4/NHO3 oxidizing system, and the effect of that modification on the osteoblastic cell adhesion has been investigated. The rationale of this work is as followsâstarch based polymers have been proposed for use as tissue engineering scaffolds in several publications. It is known that in biodegradable systems it is quite difficult to have both cell adhesion and proliferation. Starch based polymers have shown to perform better than poly-lactic acid based materials but there is still room for improvement. This particular work is aimed at enhancing cell adhesion and proliferation on the surface of several starch based polymer blends that are being proposed as tissue engineering scaffolds.
The surface of the polymeric biomaterials was chemically modified using a KMnO4/HNO3 system. This treatment resulted in more hydrophilic surfaces, which was confirmed by contact angle measurements. The effect of the treatment on the bioactivity of the surface modified biomaterials was also studied. The bioactivity tests, performed in simulated body fluid after biomimetic coating, showed that a dense film of calcium phosphate was formed after 30 days. Finally, human osteoblast-like cells were cultured on unmodified (control) and modified materials in order to observe the effect of the presence of higher numbers of polar groups on the adhesion and proliferation of those cells. Two of the modified polymers presented changes in the adhesion behavior and a significant increase in the proliferation rate kinetics when compared to the unmodified controls.FCT (Portugal) for providing the postdoctoral grant (BPD/8491/2002)
Fluctuations of water near extended hydrophobic and hydrophilic surfaces
We use molecular dynamics simulations of the SPC-E model of liquid water to
derive probability distributions for water density fluctuations in probe
volumes of different shapes and sizes, both in the bulk as well as near
hydrophobic and hydrophilic surfaces. To obtain our results, we introduce a
biased sampling of coarse-grained densities, which in turn biases the actual
solvent density. The technique is easily combined with molecular dynamics
integration algorithms. Our principal result is that the probability for
density fluctuations of water near a hydrophobic surface, with or without
surface-water attractions, is akin to density fluctuations at the water-vapor
interface. Specifically, the probability of density depletion near the surface
is significantly larger than that in bulk. In contrast, we find that the
statistics of water density fluctuations near a model hydrophilic surface are
similar to that in the bulk
Comparison of phase structures and surface free energy values for the coatings synthesised from linear polyurethanes and from waterborne polyurethane cationomers
WAXS, DSC and AFM methods were employed to compare phase structures of the coatings obtained from waterborne polyurethane cationomers which had been synthesised in the reaction of some diisocyanates (MDI, IPDI, TDI and HDI) with polyoxyethylene glycols (Mâ=â600 and 2,000) and butane1,4-diol or N-methyl- or N-butyldiethanolamine and 2,2,3,3-tetrafluoro-1,4-butanediol. The structures were also analysed of the coatings derived from linear polyurethanes which had been synthesised on the basis of similar raw materials. Better rigidity was found for generally amorphous cationomer coats. Changes were discussed in the surface free energy (SFE) values and in their components, as calculated independently with the use of the van OssâGood and OwensâWendt methods. Polyurethane coats turned out more hydrophobic as compared to cationomer ones. In both coat types, fluorine incorporated into cationomers contributed to lower SFE values: from 50 down to about 30Â mJ/m2
A Rapid Synthesis of Oriented Palladium Nanoparticles by UV Irradiation
Palladium nanoparticles of average size around 8 nm have been synthesized rapidly by UV irradiation of mixture of palladium chloride and potassium oxalate solutions. A rod-shaped palladium oxalate complex has been observed as an intermediate. In the absence of potassium oxalate, no Pd nanoparticles have been observed. The synthesized Pd nanoparticles have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selective area electron diffraction and energy dispersive analysis by X-rays (EDAX) analyses. XRD analysis indicates the preferential orientation of catalytically active {111} planes in Pd nanoparticles. A plausible mechanism has been proposed for the formation of anisotropic Pd nanoparticles
DeterminaciĂłn de la densidad de entrecruzamiento de poliuretanos obtenidos a partir de aceite de ricino modificado por transesterificaciĂłn
Disaccharide topology induces slow down in local water dynamics
Molecular level insight into water structure and structural dynamics near proteins, lipids and
nucleic acids is critical to the quantitative understanding of many biophysical processes. Un-
fortunately, understanding hydration and hydration dynamics around such large molecules is challenging because of the necessity of deconvoluting the effects of topography and chemical heterogeneity. Here we study, via classical all atom simulation, water structure and structural dynamics around two biologically relevant solutes large enough to have significant chemical and topological heterogeneity but small enough to be computationally tractable: the disaccharides Kojibiose and Trehalose. We find both molecules to be strongly amphiphilic (as quantified from normalized local density fluctuations) and to induce nonuniform local slowdown in water translational and rotational motion. Detailed analysis of the rotational slowdown shows that while the rotational mechanism is similar to that previously identified in other aqueous systems by Laage, Hynes and coworkers, two novel characteristics are observed: broadening of the transition state during hydrogen bond exchange (water rotation) and a subpopulation of water for which rotation is slowed because of hindered access of the new accepting water molecule to the transition state. Both of these characteristics are expected to be generic features of water rotation around larger biomolecules and, taken together, emphasize the difficulty in transferring insight into water rotation around small molecules to much larger amphiphilic
solutes.This work is part of the research program of the âStichting voor Fundamenteel Onderzoek der
Materie (FOM)â which is financially supported by the âNederlandse organisatie voor Wetenschap-
pelijk Onderzoek (NWO)â. Further financial support was provided by a Marie Curie Incoming
International Fellowship (RKC). We gratefully acknowledge SARA, the Dutch center for high-
performance computing, for computational time and Huib Bakker and Daan Frenkel for useful
critical reviews on an earlier version of this work. We thank two anonymous reviewers for their
excellent work, especially for bringing to our attention calculations done on the transition state geometry of dimers and the overstructuring of the O-O radial distribution function of SPC/E water
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