97 research outputs found
DNA denaturation bubbles: free-energy landscape and nucleation/closure rates
The issue of the nucleation and slow closure mechanisms of non superhelical
stress-induced denaturation bubbles in DNA is tackled using coarse-grained
MetaDynamics and Brownian simulations. A minimal mesoscopic model is used where
the double helix is made of two interacting bead-spring rotating strands with a
prescribed torsional modulus in the duplex state. We demonstrate that
timescales for the nucleation (resp. closure) of an approximately 10 base-pair
bubble, in agreement with experiments, are associated with the crossing of a
free-energy barrier of (resp. ) at room
temperature . MetaDynamics allows us to reconstruct accurately the
free-energy landscape, to show that the free-energy barriers come from the
difference in torsional energy between the bubble and duplex states, and thus
to highlight the limiting step, a collective twisting, that controls the
nucleation/closure mechanism, and to access opening time scales on the
millisecond range. Contrary to small breathing bubbles, these more than
4~base-pair bubbles are of biological relevance, for example when a preexisting
state of denaturation is required by specific DNA-binding proteins.Comment: 11 pages (5 pages and Appendix), 13 figures, published in Journal of
Chemical Physic
Energy required to pinch a DNA plectoneme
DNA supercoiling plays an important role on a biological point of view. One
of its consequences at the supra-molecular level is the formation of DNA
superhelices named plectonemes. Normally separated by a distance on the order
of 10 nm, the two opposite double-strands of a DNA plectoneme must be brought
closer if a protein or protein complex implicated in genetic regulation is to
be bound simultaneously to both strands, as if the plectoneme was locally
pinched. We propose an analytic calculation of the energetic barrier, of
elastic nature, required to bring closer the two loci situated on the opposed
double-strands. We examine how this energy barrier scales with the DNA
supercoiling. For physically relevant values of elastic parameters and of
supercoiling density, we show that the energy barrier is in the
range under physiological conditions, thus demonstrating that the limiting step
to loci encounter is more likely the preceding plectoneme slithering bringing
the two loci side by side.Comment: Published version (new title to conform to editorial policy
Mixed lipid bilayers with locally varying spontaneous curvature and bending
A model of lipid bilayers made of a mixture of two lipids with different
average compositions on both leaflets, is developed. A Landau hamiltonian
describing the lipid-lipid interactions on each leaflet, with two lipidic
fields and , is coupled to a Helfrich one, accounting for the
membrane elasticity, via both a local spontaneous curvature, which varies as
, and a bending modulus equal to
. This model allows us to define curved
patches as membrane domains where the asymmetry in composition,
, is large, and thick and stiff patches where is
large. These thick patches are good candidates for being lipidic rafts, as
observed in cell membranes, which are composed primarily of saturated lipids
forming a liquid-ordered domain and are known to be thick and flat
nano-domains. The lipid-lipid structure factors and correlation functions are
computed for globally spherical membranes and planar ones. Phase diagrams are
established, within a Gaussian approximation, showing the occurrence of two
types of Structure Disordered phases, with correlations between either curved
or thick patches, and an Ordered phase, corresponding to the divergence of the
structure factor at a finite wave vector. The varying bending modulus plays a
central role for curved membranes, where the driving force is
balanced by the line tension, to form raft domains of size ranging from 10 to
100~nm. For planar membranes, raft domains emerge via the cross-correlation
with curved domains. A global picture emerges from curvature-induced
mechanisms, described in the literature for planar membranes, to coupled
curvature- and bending-induced mechanisms in curved membranes forming a closed
vesicle
Inwardly curved polymer brushes : Concave is not like Convex
Inwardly curved polymer brushes are present in cylindrical and spherical
micelles or in membranes tubes and vesicles decorated with anchored polymers,
and influence their stability. We consider such polymer brushes in good solvent
and show that previous works, based on a self-similar concentric structure of
the brush, are physically inconsistent. We use scaling laws to derive very
simply the leading term of the free energy in the high curvature limit, where
the osmotic pressure is the relevant physical ingredient. We also derive the
complete conformation at all curvatures using a self-consistent field approach.
The free energy is computed therefrom using a local scaling description.Comment: Subm. to Eur. Phys. J. E., rev. version, 12 pages plus 9 figures,
PACS : 36.20.Ey / 82.35.Gh / 82.70.-y. Figure 1 modified. In introduction,
discussion added on concentration gradients near the edge of the brush.
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