483 research outputs found
Where the linearized Poisson-Boltzmann cell model fails: (II) the planar case as a prototype study
The classical problem of two uniformly charged infinite planes in
electrochemical equilibrium with an infinite monovalent salt reservoir is
solved exactly at the mean-field nonlinear Poisson-Boltzmann (PB) level,
including an explicit expression of the associated nonlinear electrostatic
contribution to the semi-grand-canonical potential. A linearization of the
nonlinear functional is presented that leads to Debye-H\"uckel-like equations
agreeing asymptotically with the nonlinear PB results in the weak-coupling
(high-temperature) and counterionic ideal-gas limits. This linearization scheme
yields artifacts in the low-temperature, large-separation or high-surface
charge limits. In particular, the osmotic-pressure difference between the
interplane region and the salt reservoir becomes negative in the above limits,
in disagreement with the exact (at mean-field level) nonlinear PB solution. By
using explicitly gauge-invariant forms of the electrostatic potential we show
that these artifacts -- although thermodynamically consistent with quadratic
expansions of the nonlinear functional -- can be traced back to the
non-fulfillment of the underlying assumptions of the linearization. Explicit
comparison between the analytical expressions of the exact nonlinear solution
and the corresponding linearized equations allows us to show that the
linearized results are asymptotically exact in the weak-coupling and
counterionic ideal-gas limits, but always fail otherwise, predicting negative
osmotic-pressure differences.Comment: 24 pages, 3 PostScript figures, submitted to J. Chem. Phy
Apparent metamorphosis and related defense reactions of haemocytes in the lawn armyworm, Spodoptera mauritia acronyctoides (Guenホe)
Where the linearized Poisson-Boltzmann cell model fails: (I) spurious phase separation in charged colloidal suspensions
We perform a linearization of the Poisson-Boltzmann (PB) density functional
for spherical Wigner-Seitz cells that yields Debye-H\"uckel-like equations
agreeing asymptotically with the PB results in the weak-coupling
(high-temperature) limit. Both the canonical (fixed number of microions) as
well as the semi-grand-canonical (in contact with an infinite salt reservoir)
cases are considered and discussed in a unified linearized framework. In the
canonical case, for sufficiently large colloidal charges the linearized theory
predicts the occurrence of a thermodynamical instability with an associated
phase separation of the homogeneous suspension into dilute (gas) and dense
(liquid) phases. In the semi-grand-canonical case it is predicted that the
isothermal compressibility and the osmotic-pressure difference between the
colloidal suspension and the salt reservoir become negative in the
low-temperature, high-surface charge or infinite-dilution (of polyions) limits.
As already pointed out in the literature for the latter case, these features
are in disagreement with the exact nonlinear PB solution inside a Wigner-Seitz
cell and are thus artifacts of the linearization. By using explicitly
gauge-invariant forms of the electrostatic potential we show that these
artifacts, although thermodynamically consistent with quadratic expansions of
the nonlinear functional and osmotic pressure, may be traced back to the
non-fulfillment of the underlying assumptions of the linearization.Comment: 32 pages, 3 PostScript figures, submitted to J. Chem. Phy
Aging Reduces The Primary Humoral Response And The In Vitro Cytokine Production In Mice.
Aging is accompanied by a decrease in several physiological functions that make older individuals less responsive to environmental challenges. In the present study, we analyzed the immune response of female BALB/c mice (N = 6) of different ages (from 2 to 96 weeks) and identified significant age-related alterations. Immunization with hapten-protein (trinitrophenyl-bovine serum albumin) conjugates resulted in lower antibody levels in the primary and secondary responses of old mice (72 weeks old). Moreover, young mice (2, 16, and 32 weeks old) maintained specific antibodies in their sera for longer periods after primary immunization than did old mice. However, a secondary challenge efficiently induced memory in old mice, as shown by the increased antibody levels in their sera. The number of CD4+ and CD8+ T cells in the spleen increased until 8 weeks of age but there was no change in the CD4+/CD8+ ratio with aging. Splenic T cells from old mice that had or had not been immunized were less responsive to concanavalin-A and showed reduced cytokine production compared to young mice (IL-2: 57-127 vs 367-1104 pg/mL, IFN-gamma: 2344-12,836 vs 752-23,106 pg/mL and IL-10: 393-2172 vs 105-2869 pg/mL in old and young mice, respectively). These data suggest that there are significant changes in the organization of the immune system throughout life. However, the relevance of these alterations for the functioning of the immune system is unknown.401111-2
Polyelectrolyte Bundles
Using extensive Molecular Dynamics simulations we study the behavior of
polyelectrolytes with hydrophobic side chains, which are known to form
cylindrical micelles in aqueous solution. We investigate the stability of such
bundles with respect to hydrophobicity, the strength of the electrostatic
interaction, and the bundle size. We show that for the parameter range relevant
for sulfonated poly-para-phenylenes (PPP) one finds a stable finite bundle
size. In a more generic model we also show the influence of the length of the
precursor oligomer on the stability of the bundles. We also point out that our
model has close similarities to DNA solutions with added condensing agents,
hinting to the possibility that the size of DNA aggregates is under certain
circumstances thermodynamically limited.Comment: 10 pages, 8 figure
Dynamical coherent-potential approximation approach to excitation spectra in 3d transition metals
First-principles dynamical CPA (Coherent-Potential Approximation) for
electron correlations has been developed further by taking into account
higher-order dynamical corrections with use of the asymptotic approximation.
The theory is applied to the investigations of a systematic change of
excitation spectra in transition metals from Sc to Cu at finite
temperatures. It is shown that the dynamical effects damp main peaks in the
densities of states (DOS) obtained by the local density approximation to the
density functional theory, reduce the band broadening due to thermal spin
fluctuations, create the Mott-Hubbard type bands in the case of fcc Mn and fcc
Fe, and create a small hump corresponding to the `6 eV' satellite in the case
of Co, Ni, and Cu. Calculated DOS explain the X-ray photoelectron spectroscopy
data as well as the bremsstrahlung isochromat spectroscopy data. Moreover, it
is found that screening effects on the exchange energy parameters are
significant for understanding the spectra in magnetic transition metals.Comment: To be published in Phys. Rev.
Field theoretical representation of the Hohenberg-Kohn free energy for fluids
To go beyond Gaussian approximation to the Hohenberg-Kohn free energy playing
the key role in the density functional theory (DFT), the density functional
\textit{integral} representation would be relevant, because field theoretical
approach to perturbative calculations becomes available. Then the present
letter first derives the associated Hamiltonian of density functional,
explicitly including logarithmic entropy term, from the grand partition
function expressed by configurational integrals. Moreover, two things are done
so that the efficiency of the obtained form may be revealed: to demonstrate
that this representation facilitates the field theoretical treatment of the
perturbative calculation, and further to compare our perturbative formulation
with that of the DFT.Comment: 5 pages, revtex, modified on 13 April 2000 [see eqs. (3), (6), and
(13)
Extracting Structural Information of a Heteropolymer from Force-Extension Curves
We present a theory for the reverse analysis on the sequence information of a
single H/P two-letter random hetero-polymer (RHP) from its force-extension(f-z)
curves during quasi static stretching. Upon stretching of a self-assembled RHP,
it undergoes several structural transitions. The typical elastic response of a
hetero-polymeric globule is a set of overlapping saw-tooth patterns. With
consideration of the height and the position of the overlapping saw-tooth
shape, we analyze the possibility of extracting the binding energies of the
internal domains and the corresponding block sizes of the contributing
conformations.Comment: 5 figures 7 page
Poisson -- Boltzmann Brownian Dynamics of Charged Colloids in Suspension
We describe a method to simulate the dynamics of charged colloidal particles
suspended in a liquid containing dissociated ions and salt ions. Regimes of
prime current interest are those of large volume fraction of colloids, highly
charged particles and low salt concentrations. A description which is tractable
under these conditions is obtained by treating the small dissociated and salt
ions as continuous fields, while keeping the colloidal macroions as discrete
particles. For each spatial configuration of the macroions, the electrostatic
potential arising from all charges in the system is determined by solving the
nonlinear Poisson--Boltzmann equation. From the electrostatic potential, the
forces acting on the macroions are calculated and used in a Brownian dynamics
simulation to obtain the motion of the latter. The method is validated by
comparison to known results in a parameter regime where the effective
interaction between the macroions is of a pairwise Yukawa form
Reversible stretching of homopolymers and random heteropolymers
We have analyzed the equilibrium response of chain molecules to stretching.
For a homogeneous sequence of monomers, the induced transition from compact
globule to extended coil below the -temperature is predicted to be
sharp. For random sequences, however, the transition may be smoothed by a
prevalence of necklace-like structures, in which globular regions and coil
regions coexist in a single chain. As we show in the context of a random
copolymer, preferential solvation of one monomer type lends stability to such
structures. The range of stretching forces over which necklaces are stable is
sensitive to chain length as well as sequence statistics.Comment: 14 pages, 4 figure
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