468 research outputs found
DNA Confinement drives uncoating of the HIV Virus
We present a model for the uncoating of the HIV virus driven by forces
exerted on the protein shell of HIV generated by DNA confinement
Reentrant Condensation of DNA induced by Multivalent Counterions
A theory of condensation and resolubilization of a dilute DNA solution with
growing concentration of multivalent cations, N is suggested. It is based on a
new theory of screening of a macroion by multivalent cations, which shows that
due to strong cation correlations at the surface of DNA the net charge of DNA
changes sign at some small concentration of cations N_0. DNA condensation takes
place in the vicinity of N_0, where absolute value of the DNA net charge is
small and the correlation induced short range attraction dominates the Coulomb
repulsion. At N > N_0 positive DNA should move in the oppisite direction in an
electrophoresis experiment. From comparison of our theory with experimental
values of condensation and resolubilization thresholds for DNA solution
containing Spe, we obtain that N_0 = 3.2 mM and that the energy of DNA
condensation per nucleotide is .Comment: 8 pages, 4 figures, references correcte
Competition between condensation of monovalent and multivalent ions in DNA aggregation
We discuss the distribution of ions around highly charged PEs when there is
competition between monovalent and multivalent ions, pointing out that in this
case the number of condensed ions is sensitive to short-range interactions,
salt, and model-dependent approximations. This sensitivity is discussed in the
context of recent experiments on DNA aggregation, induced by multivalent
counterions such as spermine and spermidine.Comment: 6 pages, 1 figur
Effects of mechanical strain on thermal denaturation of DNA
As sections of a strand duplexed DNA denature when exposed to high
temperature, the excess linking number is taken up by the undenatured portions
of the molecule. The mechanical energy that arises because of the overwinding
of the undenatured sections can, in principle, alter the nature of the thermal
denaturation process. Assuming that the strains associated with this
overwinding are not relieved, we find that a simple model of strain-altered
melting leads to a suppression of the melting transition when the unaltered
transition is continuous. When the melting transition is first order in the
absence of strain associated with overwinding, the modification is to a third
order phase transition.Comment: 4 pages, 5 figures, RevTe
Unzipping Kinetics of Double-Stranded DNA in a Nanopore
We studied the unzipping kinetics of single molecules of double-stranded DNA
by pulling one of their two strands through a narrow protein pore. PCR analysis
yielded the first direct proof of DNA unzipping in such a system. The time to
unzip each molecule was inferred from the ionic current signature of DNA
traversal. The distribution of times to unzip under various experimental
conditions fit a simple kinetic model. Using this model, we estimated the
enthalpy barriers to unzipping and the effective charge of a nucleotide in the
pore, which was considerably smaller than previously assumed.Comment: 10 pages, 5 figures, Accepted: Physics Review Letter
Screening of a macroion by multivalent ions: A new boundary condition for Poisson-Boltzmann equation and charge inversion
Screening of a macroion by multivalent counterions is considered. It is shown
that ions form strongly correlated liquid at the macroion surface. Cohesive
energy of this liquid leads to strong additional attraction of counterions to
the surface. Away from the surface this attraction is taken into account by a
new boundary condition for the Poisson-Boltzmann equation. This equation is
solved with the new boundary condition for a charged flat surface and a long
cylinder. For a cylinder Onsager-Manning theory looses its universality so that
apparent charge of the cylinder is smaller than their theory predicts and
depends on its bare charge. It can also vanish or change sign.Comment: 4 pages, no figure
The pulling force of a single DNA molecule condensed by spermidine
In a recent experiment, a single DNA double helix is stretched and relaxed in
the presence of spermidine, a short positive polyelectrolyte, and the pulling
force is measured as a function of DNA extension. In a certain range of
spermidine concentration, a force plateau appears whose value shows maximum as
a function of spermidine concentration. We present a quantitative theory of
this plateau force based on the theory of reentrant condensation and derive
almost parabolic behavior of the plateau force as a function of the logarithm
of the spermidine concentration in the range of condensation. Our result is in
good agreement with experimental data.Comment: 4 pages, 4 figures. Small change in the text, no change in result
The bend stiffness of S-DNA
We formulate and solve a two-state model for the elasticity of nicked,
double-stranded DNA that borrows features from both the Worm Like Chain and the
Bragg--Zimm model. Our model is computationally simple, and gives an excellent
fit to recent experimental data through the entire overstretching transition.
The fit gives the first value for the bending stiffness of the overstretched
state as about 10 nm*kbt, a value quite different from either B-form or
single-stranded DNA.Comment: 7 pages, 1 figur
Electrostatic image effects for counter-ions between charged planar walls
We study the effect of dielectric inhomogeneities on the interaction between
two planparallel charged surfaces with oppositely charged mobile charges in
between. The dielectric constant between the surfaces is assumed to be
different from the dielectric constant of the two semiinfinite regions bounded
by the surfaces, giving rise to electrostatic image interactions. We show that
on the weak coupling level the image charge effects are generally small, making
their mark only in the second order fluctuation term. However, in the strong
coupling limit, the image effects are large and fundamental. They modify the
interactions between the two surfaces in an essential way. Our calculations are
particularly useful in the regime of parameters where computer simulations
would be difficult and extremely time consuming due to the complicated nature
of the long range image potentials.Comment: 21 pages, 8 figure
Mechanisms of Small Molecule-DNA Interactions Probed by Single-molecule Force Spectroscopy
There is a wide range of applications for non-covalent DNA binding ligands, and optimization of such interactions requires detailed understanding of the binding mechanisms. One important class of these ligands is that of intercalators, which bind DNA by inserting aromatic moieties between adjacent DNA base pairs. Characterizing the dynamic and equilibrium aspects of DNA-intercalator complex assembly may allow optimization of DNA binding for specific functions. Single-molecule force spectroscopy studies have recently revealed new details about the molecular mechanisms governing DNA intercalation. These studies can provide the binding kinetics and affinity as well as determining the magnitude of the double helix structural deformations during the dynamic assembly of DNA–ligand complexes. These results may in turn guide the rational design of intercalators synthesized for DNA-targeted drugs, optical probes, or integrated biological self-assembly processes. Herein, we survey the progress in experimental methods as well as the corresponding analysis framework for understanding single molecule DNA binding mechanisms. We discuss briefly minor and major groove binding ligands, and then focus on intercalators, which have been probed extensively with these methods. Conventional mono-intercalators and bis-intercalators are discussed, followed by unconventional DNA intercalation. We then consider the prospects for using these methods in optimizing conventional and unconventional DNA-intercalating small molecules
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