742 research outputs found
Complexation of a polyelectrolyte with oppositely charged spherical macroions: Giant inversion of charge
Complexation of a long flexible polyelectrolyte (PE) molecule with oppositely
charged spherical particles such as colloids, micelles, or globular proteins in
a salty water solution is studied. PE binds spheres winding around them, while
spheres repel each other and form almost periodic necklace. If the total charge
of PE is larger than the total charge of spheres, repulsive correlations of PE
turns on a sphere lead to inversion of the net charge of each sphere. In the
opposite case, we predict another correlation effect: under-screened by PE
spheres bind to PE in such a great number that they invert charge of PE. The
inverted charge by absolute value can be larger than the bare charge of PE even
when screening by monovalent salt is weak. At larger concentrations of
monovalent salt, the inverted charge can reach giant proportions. Our theory is
in qualitative agreement with recent experiments on micelles-PE systems.Comment: Various additions and corrections to the text, Figures and
references. Accepted for publication in J. Chem. Phys. 200
Glassy phases in Random Heteropolymers with correlated sequences
We develop a new analytic approach for the study of lattice heteropolymers,
and apply it to copolymers with correlated Markovian sequences. According to
our analysis, heteropolymers present three different dense phases depending
upon the temperature, the nature of the monomer interactions, and the sequence
correlations: (i) a liquid phase, (ii) a ``soft glass'' phase, and (iii) a
``frozen glass'' phase. The presence of the new intermediate ``soft glass''
phase is predicted for instance in the case of polyampholytes with sequences
that favor the alternation of monomers.
Our approach is based on the cavity method, a refined Bethe Peierls
approximation adapted to frustrated systems. It amounts to a mean field
treatment in which the nearest neighbor correlations, which are crucial in the
dense phases of heteropolymers, are handled exactly. This approach is powerful
and versatile, it can be improved systematically and generalized to other
polymeric systems
Electrostatic Persistence Length of Semiflexible and Flexible Polyelectrolytes
ABSTRACT: I have calculated the dependence of the electrostatic persistence length on the Debye screening length for semiflexible, strongly and weakly charged flexible polyelectrolytes. For semiflexible and strongly charged flexible polyelectrolytes the electrostatic part of the chain persistence length is proportional to the Debye screening length. This result is obtained by evaluating the bending angle fluctuations and in the framework of the Gaussian variational principle. A polyelectrolyte chain with linear dependence of the electrostatic persistence length on the Debye screening length has a lower free energy than that of a chain with the Odijk-Skolnick-Fixman electrostatic persistence length. In the case of weakly charged chains the electrostatic persistence length has a sublinear dependence on the Debye screening length which is due to inverse logarithmic dependence of the linear charge density of the chain of electrostatic blobs on the Debye screening length. This result was derived by applying a coarse-graining procedure to an initially flexible polyelectrolyte chain by representing it as a chain of electrostatic blobs. The blob size and chain persistence length are then found self-consistently by minimizing the chain's variational free energy
Correlated disorder in random block-copolymers
We study the effect of a random Flory-Huggins parameter in a symmetric
diblock copolymer melt which is expected to occur in a copolymer where one
block is near its structural glass transition. In the clean limit the
microphase segregation between the two blocks causes a weak, fluctuation
induced first order transition to a lamellar state. Using a renormalization
group approach combined with the replica trick to treat the quenched disorder,
we show that beyond a critical disorder strength, that depends on the length of
the polymer chain, the character of the transition is changed. The system
becomes dominated by strong randomness and a glassy rather than an ordered
lamellar state occurs. A renormalization of the effective disorder distribution
leads to nonlocal disorder correlations that reflect strong compositional
fluctuation on the scale of the radius of gyration of the polymer chains. The
reason for this behavior is shown to be the chain length dependent role of
critical fluctuations, which are less important for shorter chains and become
increasingly more relevant as the polymer length increases and the clean first
order transition becomes weaker.Comment: 11 pages, 5 figures, submitted to PR
Deviations from the mean field predictions for the phase behaviour of random copolymers melts
We investigate the phase behaviour of random copolymers melts via large scale
Monte Carlo simulations. We observe macrophase separation into A and B--rich
phases as predicted by mean field theory only for systems with a very large
correlation lambda of blocks along the polymer chains, far away from the
Lifshitz point. For smaller values of lambda, we find that a locally
segregated, disordered microemulsion--like structure gradually forms as the
temperature decreases. As we increase the number of blocks in the polymers, the
region of macrophase separation further shrinks. The results of our Monte Carlo
simulation are in agreement with a Ginzburg criterium, which suggests that mean
field theory becomes worse as the number of blocks in polymers increases.Comment: 6 pages, 4 figures, Late
Increase in Arctic coastal erosion and its sensitivity to warming in the twenty-first century
Arctic coastal erosion damages infrastructure, threatens coastal communities and releases organic carbon from permafrost. However, the magnitude, timing and sensitivity of coastal erosion increase to global warming remain unknown. Here we project the Arctic-mean erosion rate to increase and very likely exceed its historical range of variability before the end of the century in a wide range of emission scenarios. The sensitivity of erosion to warming roughly doubles, reaching 0.4–0.8 m yr−1 °C−1 and 2.3–4.2 TgC yr−1 °C−1 by the end of the century. We develop a simplified semi-empirical model to produce twenty-first-century pan-Arctic coastal erosion rate projections. Our results will inform policymakers on coastal conservation and socioeconomic planning, and organic carbon flux projections lay out the path for future work to investigate the impact of Arctic coastal erosion on the changing Arctic Ocean, its role as a global carbon sink, and the permafrost–carbon feedback. © 2022, The Author(s)
A model of inversion of DNA charge by a positive polymer: fractionization of the polymer charge
Charge inversion of a DNA double helix by an oppositely charged flexible
polyelectrolyte (PE) is considered. We assume that, in the neutral state of the
DNA-PE complex, each of the DNA charges is locally compensated by a PE charge.
When an additional PE molecule is adsorbed by DNA, its charge gets fractionized
into monomer charges of defects (tails and arches) on the background of the
perfectly neutralized DNA. These charges spread all over the DNA eliminating
the self-energy of PE. This fractionization mechanism leads to a substantial
inversion of the DNA charge, a phenomenon which is widely used for gene
delivery.Comment: 4 pages, 2 figures. Improved figures and various corrections to tex
Chains Are More Flexible Under Tension
The mechanical response of networks, gels, and brush layers is a manifestation of the elastic properties of the individual macromolecules. Furthermore, the elastic response of macromolecules to an applied force is the foundation of the single-molecule force spectroscopy techniques. The two main classes of models describing chain elasticity include the worm-like and freely-jointed chain models. The selection between these two classes of models is based on the assumptions about chain flexibility. In many experimental situations the choice is not clear and a model describing the crossover between these two limiting classes is therefore in high demand. We are proposing a unified chain deformation model which describes the force-deformation curve in terms of the chain bending constant K and bond length b. This model demonstrates that the worm-like and freely-jointed chain models correspond to two different regimes of polymer deformation and the crossover between these two regimes depends on the chain bending rigidity and the magnitude of the applied force. Polymer chains with bending constant K>1 behave as a worm-like chain under tension in the interval of the applied forces f ≤ KkBT/b and as a freely-jointed chain for f ≥ KkBT/b (kB is the Boltzmann constant and T is the absolute temperature). The proposed crossover expression for chain deformation is in excellent agreement with the results of the molecular dynamics simulations of chain deformation and single-molecule deformation experiments of biological and synthetic macromolecules
Phase diagram of aggregation of oppositely charged colloids in salty water
Aggregation of two oppositely charged colloids in salty water is studied. We
focus on the role of Coulomb interaction in strongly asymmetric systems in
which the charge and size of one colloid is much larger than the other one. In
the solution, each large colloid (macroion) attracts certain number of
oppositely charged small colloids (-ion) to form a complex. If the
concentration ratio of the two colloids is such that complexes are not strongly
charged, they condense in a macroscopic aggregate. As a result, the phase
diagram in a plane of concentrations of two colloids consists of an aggregation
domain sandwiched between two domains of stable solutions of complexes. The
aggregation domain has a central part of total aggregation and two wings
corresponding to partial aggregation. A quantitative theory of the phase
diagram in the presence of monovalent salt is developed. It is shown that as
the Debye-H\"{u}ckel screening radius decreases, the aggregation domain
grows, but the relative size of the partial aggregation domains becomes much
smaller. As an important application of the theory, we consider solutions of
long double-helix DNA with strongly charged positive spheres (artificial
chromatin). We also consider implications of our theory for in vitro
experiments with the natural chromatin. Finally, the effect of different shapes
of macroions on the phase diagram is discussed.Comment: 10 pages, 9 figures. The text is rewritten, but results are not
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