231,069 research outputs found
Pulling hairpinned polynucleotide chains: Does base-pair stacking interaction matter?
Force-induced structural transitions both in relatively random and in
designed single-stranded DNA (ssDNA) chains are studied theoretically. At high
salt conditions, ssDNA forms compacted hairpin patterns stabilized by
base-pairing and base-pair stacking interactions, and a threshold external
force is needed to pull the hairpinned structure into a random coiled one. The
base-pair stacking interaction in the ssDNA chain makes this hairpin-coil
conversion a discontinuous (first-order) phase transition process characterized
by a force plateau in the force-extension curve, while lowering this potential
below some critical level turns this transition into continuous (second-order)
type, no matter how strong the base-pairing interaction is. The phase diagram
(including hairpin-I, -II, and random coil) is discussed as a function of
stacking potential and external force. These results are in quantitative
agreement with recent experimental observations of different ssDNA sequences,
and they reveal the necessity to consider the base-pair stacking interactions
in order to understand the structural formation of RNA, a polymer designed by
nature itself. The theoretical method used may be extended to study the
long-range interaction along double-stranded DNA caused by the topological
constraint of fixed linking number.Comment: 8 pages using Revte
Physical implementation of holonomic quantum computation in decoherence-free subspaces with trapped ions
We propose a feasible scheme to achieve holonomic quantum computation in a
decoherence-free subspace (DFS) with trapped ions. By the application of
appropriate bichromatic laser fields on the designated ions, we are able to
construct two noncommutable single-qubit gates and one controlled-phase gate
using the holonomic scenario in the encoded DFS.Comment: 4 pages, 3 figures. To appear in Phys. Rev. A 74 (2006
Modelling Time-varying Dark Energy with Constraints from Latest Observations
We introduce a set of two-parameter models for the dark energy equation of
state (EOS) to investigate time-varying dark energy. The models are
classified into two types according to their boundary behaviors at the redshift
and their local extremum properties. A joint analysis based on
four observations (SNe + BAO + CMB + ) is carried out to constrain all the
models. It is shown that all models get almost the same and the cosmological parameters with the
best-fit results , although the constraint results on two
parameters and the allowed regions for the EOS are
sensitive to different models and a given extra model parameter. For three of
Type I models which have similar functional behaviors with the so-called CPL
model, the constrained two parameters and have negative correlation
and are compatible with the ones in CPL model, and the allowed regions of
get a narrow node at . The best-fit results from the most
stringent constraints in Model Ia give which may compare with the best-fit results in the CPL model. For four of
Type II models which have logarithmic function forms and an extremum point, the
allowed regions of are found to be sensitive to different models and a
given extra parameter. It is interesting to obtain two models in which two
parameters and are strongly correlative and appropriately reduced
to one parameter by a linear relation .Comment: 30 pages, 7 figure
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