Showing papers by "David A. Kessler published in 2000"
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TL;DR: Bestselling books perpetuate the myth that natural products such as herbs and other “dietary supplements” tend to be safer than conventional medicines, research suggests.
Abstract: Bestselling books perpetuate the myth that natural products such as herbs and other “dietary supplements” tend to be safer than conventional medicines.1 Once relegated to health food stores, these products now fill pharmacy and supermarket shelves. So-called natural substances are more popular than ever. Fueled by congressional passage of the Dietary Supplement Health and Education Act of 1994,2 which deregulated the industry by limiting the role of the Food and Drug Administration (FDA), the popularity of dietary supplements has created a $15-billion-a-year industry. In this issue of the Journal, Nortier et al. present evidence of an association between the use . . .
101 citations
TL;DR: It is shown that the hysteretic behavior at small driving is very sensitive to the smoothness of the force law, and at large driving, a Hopf bifurcation to a straight crack whose velocity is periodic in time is found.
Abstract: We study the effect of general nonlinear force laws in viscoelastic lattice models of fracture, focusing on the existence and stability of steady-state mode III cracks. We show that the hysteretic behavior at small driving is very sensitive to the smoothness of the force law. At large driving, we find a Hopf bifurcation to a straight crack whose velocity is periodic in time. The frequency of the unstable bifurcating mode depends on the smoothness of the potential, but is very close to an exact period-doubling instability. Slightly above the onset of the instability, the system settles into a exactly period-doubled state, presumably connected to the aforementioned bifurcation structure. We explicitly solve for this new state and map out its velocity-driving relation.
23 citations
TL;DR: The competition between topological effects and sequence inhomogeneities in determining the thermodynamics and the un/folding kinetics of a beta-hairpin is studied using a new exactly solvable model that allows for arbitrary configurations of native contacts.
Abstract: We study the competition between topological effects and sequence inhomogeneities in determining the thermodynamics and the un/folding kinetics of a β-hairpin. Our work utilizes a new exactly solvable model that allows for arbitrary configurations of native contacts. In general, the competition between heterogeneity and topology results in a crossover of the dominant transition state. Interestingly, near this crossover, the single reaction coordinate picture can be seriously misleading. Our results also suggest that inferring the folding pathway from unfolding simulations is not always justified.
18 citations
TL;DR: A simple model for the formation of a Beta hairpin is described, motivated by the fact that folding of a beta hairpin captures much of the basic physics of protein folding.
Abstract: Understanding the mechanism of protein secondary structure formation is an essential part of the protein-folding puzzle. Here we describe a simple model for the formation of a beta hairpin, motivated by the fact that folding of a beta hairpin captures much of the basic physics of protein folding. The modeled hairpin is composed of two interacting Gaussian chains with one pairwise (two-body) and two many-body interactions. We show that these many-body interactions, arising from side chain packing effects, are responsible for producing an "all-or-none" folding transition. We also estimate the (single exponential) folding/unfolding rate via calculating the thermodynamic weight of the "critical" droplet/bubble.
9 citations
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TL;DR: In this paper, the authors extend the Slepyan solution of the problem of a steady-state crack in an infinite ideally brittle lattice model to include dissipation in the form of Kelvin viscosity.
Abstract: We extend the Slepyan solution of the problem of a steady-state crack in an infinite ideally brittle lattice model to include dissipation in the form of Kelvin viscosity. As a demonstration of this technique, based on the Wiener-Hopf method, we apply the method to mode III cracks in a square lattice. We use this solution to find the critical velocity at which the steady-state solution becomes inconsistent due to additional bond-breaking; this point signaling the onset of complex dynamical behavior.
4 citations
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TL;DR: In this article, the exact solutions for steady-state cracks in ideally brittle viscoelastic lattice models were studied by focusing on mode I in a triangular system, and the issues were addressed include the crack velocity versus driving curve as well as the onset of additional bond-breaking, signaling the emergence of complex spatio-temporal behavior.
Abstract: We continue our study of the exact solutions for steady-state cracks in ideally brittle viscoelastic lattice models by focusing on mode I in a triangular system. The issues we address include the crack velocity versus driving curve as well as the onset of additional bond-breaking, signaling the emergence of complex spatio-temporal behavior. Somewhat surprisingly, the critical velocity for this transition becomes a decreasing function of the dissipation for sufficiently large values thereof. At the end, we briefly discuss the possible relevance of our findings for experiments on mode I crack instabilities.
2 citations