23,575 research outputs found
Helix Formation and Folding in an Artificial Peptide
We study the relation between -helix formation and folding for a
simple artificial peptide, Ala-Gly-Ala. Our data rely on
multicanonical Monte Carlo simulations where the interactions among all atoms
are taken into account. The free-energy landscape of the peptide is evaluated
for various temperatures. Our data indicate that folding of this peptide is a
two-step process: in a first step two -helices are formed which
afterwards re-arrange themselves into a U-like structure.Comment: 15 pages, with 9 eps figure
On the relation between mass of pion, fundamental physical constants and cosmological parameters
In this article we reconsider the old mysterious relation, advocated by Dirac
and Weinberg, between the mass of the pion, the fundamental physical constants,
and the Hubble parameter. By introducing the cosmological density parameters,
we show how the corresponding equation may be written in a form that is
invariant with respect to the expansion of the Universe and without invoking a
varying gravitational "constant", as was originaly proposed by Dirac. It is
suggest that, through this relation, Nature gives a hint that virtual pions
dominante the "content" of the quantum vacuum
Aggregation in a mixture of Brownian and ballistic wandering particles
In this paper, we analyze the scaling properties of a model that has as
limiting cases the diffusion-limited aggregation (DLA) and the ballistic
aggregation (BA) models. This model allows us to control the radial and angular
scaling of the patterns, as well as, their gap distributions. The particles
added to the cluster can follow either ballistic trajectories, with probability
, or random ones, with probability . The patterns were
characterized through several quantities, including those related to the radial
and angular scaling. The fractal dimension as a function of
continuously increases from (DLA dimensionality) for
to (BA dimensionality) for . However, the
lacunarity and the active zone width exhibt a distinct behavior: they are
convex functions of with a maximum at . Through the
analysis of the angular correlation function, we found that the difference
between the radial and angular exponents decreases continuously with increasing
and rapidly vanishes for , in agreement with recent
results concerning the asymptotic scaling of DLA clusters.Comment: 7 pages, 6 figures. accepted for publication on PR
Market imperfections and income concentration: global and regional perspectives on Brazilian agricultural production performance.
We measure performance for the Brazilian agriculture by means of free disposal hull (FDH) measures of technical efficiency. Measurements are conditional on contextual variables that may be responsible for market imperfection variables. The production frontier is generated by a product probability measure. Production observations are aggre gated by county and analyzed by region. The efficiency measure is output oriented and assumes variable returns to scale. Output is rural gross income and inputs are land expenses, labor expenses and expenses on other technological inputs. The covariates for production are credit, technical assistance, social, environmental, and demographic indicators and income concentration, measured by the Gini index. Overall Brazilian rural production performance responds favorably to credit, income concentration and environment score and unfavorably to technical assistance, at the 95% level. Results differ by region. Agricultural public policies envisaging inclusion of small farmers into the main stream of production should be regionally oriented
Simulated Tempering: A New Monte Carlo Scheme
We propose a new global optimization method ({\em Simulated Tempering}) for
simulating effectively a system with a rough free energy landscape (i.e. many
coexisting states) at finite non-zero temperature. This method is related to
simulated annealing, but here the temperature becomes a dynamic variable, and
the system is always kept at equilibrium. We analyze the method on the Random
Field Ising Model, and we find a dramatic improvement over conventional
Metropolis and cluster methods. We analyze and discuss the conditions under
which the method has optimal performances.Comment: 12 pages, very simple LaTeX file, figures are not included, sorr
Slow down of a globally neutral relativistic beam shearing the vacuum
The microphysics of relativistic collisionless sheared flows is investigated
in a configuration consisting of a globally neutral, relativistic beam
streaming through a hollow plasma/dielectric channel. We show through
multidimensional PIC simulations that this scenario excites the Mushroom
instability (MI), a transverse shear instability on the electron-scale, when
there is no overlap (no contact) between the beam and the walls of the
hollow plasma channel. The onset of the MI leads to the conversion of the
beam's kinetic energy into magnetic (and electric) field energy, effectively
slowing down a globally neutral body in the absence of contact. The
collisionless shear physics explored in this configuration may operate in
astrophysical environments, particularly in highly relativistic and supersonic
settings where macroscopic shear processes are stable
Déficit hÃdrico severo afeta as reservas orgânicas do cafeeiro.
Os fotoassimilados, produzidos na parte aérea durante o processo de fotossÃntese, são translocados para toda a planta a fim de suprir a demanda de energia que irá ser responsável pela formação de novos tecidos. A intensidade da produção, translocação e uso desses carboidratos depende entre outros fatores, da carga pendente de frutos e do status hÃdrico das plantas. Esse trabalho teve por objetivo verificar ao efeito do déficit hÃdrico nas reservas orgânicas de cafeeiros com diferentes nÃveis de produtividade. Os resultados mostram que os efeitos da seca afetaram em maior intensidade os nÃveis de reservas orgânicas das plantas e o crescimento de radicelas do que a carga pendente da cultura
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