13,047 research outputs found
Comment on "Chain Length Scaling of Protein Folding Time", PRL 77, 5433 (1996)
In a recent Letter, Gutin, Abkevich, and Shakhnovich (GAS) reported on a
series of dynamical Monte Carlo simulations on lattice models of proteins.
Based on these highly simplified models, they found that four different
potential energies lead to four different folding time scales tau_f, where
tau_f scales with chain length as N^lambda (see, also, Refs. [2-4]), with
lambda varying from 2.7 to 6.0. However, due to the lack of microscopic models
of protein folding dynamics, the interpretation and origin of the data have
remained somewhat speculative. It is the purpose of this Comment to point out
that the application of a simple "mesoscopic" model (cond-mat/9512019, PRL 77,
2324, 1996) of protein folding provides a full account of the data presented in
their paper. Moreover, we find a major qualitative disagreement with the
argumentative interpretation of GAS. Including, the origin of the dynamics, and
size of the critical folding nucleus.Comment: 1 page Revtex, 1 fig. upon request. Submitted to PR
Determining distributions of weakly bound nuclei from breakup cross sections using Continuum Discretized Coupled Channels calculations. Application to Be
A novel method to extract the strength of a weakly bound nucleus from
experimental Coulomb dissociation data is proposed. The method makes use of
continuum discretized coupled channels (CDCC) calculations, in which both
nuclear and Coulomb forces are taken into account to all orders. This is a
crucial advantage with respect to the standard procedure based on the
Equivalent Photon Method (EPM) which does not properly take into account
nuclear distortion, higher order coupling effects, or Coulomb-nuclear
interference terms. The procedure is applied to the Be nucleus using two
sets of available experimental data at different energies, for which seemingly
incompatible have been reported using the EPM. We show that the present
procedure gives consistent strengths, thus solving the aforementioned
long-standing discrepancy between the two measurements.Comment: Submitted for publicatio
From Collapse to Freezing in Random Heteropolymers
We consider a two-letter self-avoiding (square) lattice heteropolymer model
of N_H (out ofN) attracting sites. At zero temperature, permanent links are
formed leading to collapse structures for any fraction rho_H=N_H/N. The average
chain size scales as R = N^{1/d}F(rho_H) (d is space dimension). As rho_H -->
0, F(rho_H) ~ rho_H^z with z={1/d-nu}=-1/4 for d=2. Moreover, for 0 < rho_H <
1, entropy approaches zero as N --> infty (being finite for a homopolymer). An
abrupt decrease in entropy occurs at the phase boundary between the swollen (R
~ N^nu) and collapsed region. Scaling arguments predict different regimes
depending on the ensemble of crosslinks. Some implications to the protein
folding problem are discussed.Comment: 4 pages, Revtex, figs upon request. New interpretation and emphasis.
Submitted to Europhys.Let
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