Binding of Oppositely Charged Membranes and Membrane Reorganization

We consider the electrostatic interaction between two rigid membranes, with different surface charge densities of opposite sign, across an aqueous solution without added salt. Exact solutions to the nonlinear Poisson-Boltzmann equation are obtained and their physical meaning discussed. We also calculate the electrostatic contribution to the free energy and discuss the renormalization of the area per head group of the charged lipids arising from the Coulomb interaction.Comment: 13 pages, 6 figures, to be published in EJP

Polyelectrolyte-colloid complexes: polarizability and effective interaction

We theoretically study the polarizability and the interactions of neutral complexes consisting of a semi-flexible polyelectrolyte adsorbed onto an oppositely charged spherical colloid. In the systems we studied, the bending energy of the chain is small compared to the Coulomb energy and the chains are always adsorbed on the colloid. We observe that the polarizability is large for short chains and small electrical fields and shows a non-monotonic behavior with the chain length at fixed charge density. The polarizability has a maximum for a chain length equal to half of the circumference of the colloid. For long chains we recover the polarizability of a classical conducting sphere. For short chains, the existence of a permanent dipole moment of the complexes leads to a van der Waal's-type long-range attraction between them. This attractive interaction vanishes for long chains (i.e., larger than the colloidal size), where the permanent dipole moment is negligible. For short distances the complexes interact with a deep short-ranged attraction which is due to energetic bridging for short chains and entropic bridging for long chains. Exceeding a critical chain length eventually leads to a pure repulsion. This shows that the stabilization of colloidal suspensions by polyelectrolyte adsorption is strongly dependent on the chain size relative to the colloidal size: for long chains the suspensions are always stable (only repulsive forces between the particles), while for mid-sized and short chains there is attraction between the complexes and a salting-out can occur.Comment: 13 pages, 14 figure

Long-Range Interaction between Heterogeneously Charged Membranes

Despite their neutrality, surfaces or membranes with equal amounts of positive and negative charge can exhibit long-range electrostatic interactions if the surface charge is heterogeneous; this can happen when the surface charges form finite-size domain structures. These domains can be formed in lipid membranes where the balance of the different ranges of strong but short-ranged hydrophobic interactions and longer-ranged electrostatic repulsion result in a finite, stable domain size. If the domain size is large enough, oppositely charged domains in two opposing surfaces or membranes can be strongly correlated by the elecrostatic interactions; these correlations give rise to an attractive interaction of the two membranes or surfaces over separations on the order of the domain size. We use numerical simulations to demonstrate the existence of strong attractions at separations of tens of nanometers. Large line tensions result in larger domains but also increase the charge density within the domain. This promotes correlations and, as a result, increases the intermembrane attraction. On the other hand, increasing the salt concentration increases both the domain size and degree of domain anticorrelation, but the interactions are ultimately reduced due to increased screening. The result is a decrease in the net attraction as salt concentration is increased

QUANTITATIVE STUDIES OF NATURALLY OCCURRING MURINE LEUKEMIA VIRUS INFECTION OF AKR MICE

Quantitative studies were made of the organ distribution of murine leukemia virus in AKR mice of various ages. Infectious virus first appeared shortly before or after birth and was continuously present in all mice thereafter. Highest infectivity titers were found in uterus and bone, with spleen slightly lower. Virus titers in normal thymus were relatively low, but increased significantly with the development of thymic lymphoma. The level of viremia decreased after the 1st month of life, but increased sharply in lymphomatous mice

The performance of beams and columns continuously braced with diaphragms

INTRODUCTION In Progress Report No.1, it was reported that the initial conception of the problem was that diaphragms act almost exclusively in shear when called upon to brace compression members, and that it had been decided to test single columns with finite widths of diaphragm attached, with width of the sheet being varied to provide a range of lateral support up to that which permitted full column strength, based on strong axis to be developed. However, the first test of this kind, Test CB-l, clearly indicated that the diaphragm acted essentially in flexure only and thus had the same effect as if it were regarded as a weak cover plate. It was already quite clear at the time of the First Progress Report that the shear contribution of the diaphragm was practically nil, based on observation of combined beam-sheet tests (refer to figure 1, First Progress Report). It was thought at that time that prevention of rotation of the ends of the diaphragm would produce shear-predominant action. While the sheet, thus restrained, provided far more support to the beam than in the previous tests and could be exactly calculated, the behavior was essentially flexural and not in shear. A series of tests were proposed in Progress Report No. 1 to confirm the initial conclusion that supporting diaphragms act in flexure and function essentially as corrugated cover plates. As mentioned, this was exactly confirmed by the first test, making further tests of this character pointless. It was recognized furthermore, that any purely flexural contribution of the sheet was so small that nothing was to be gained in design, in terms of increased column strength, by counting on it. On the other hand, by experience and intuition, it seemed that the supporting ability of the sheet should be far greater than observed. In rethinking the problem, it was concluded that the sheet could act in pure shear and provide the anticipated support only if all crosssections were prevented from roaating. The simplest situation producing this condition is that of a diaphragm attached to two identical, (in all respects, including loading), columns or beams, as in figure 27. In fact, this is a rather realistic situation in that corrugated building siding is or could be attached continuously across two or more columns that are more or less identical. Accordingly, it was decided that tests should be performed on pairs of columns, each loaded and supported separately and identically, but connected by a diaphragm, as in figure 14. To the inrestigators\u27 knowledge, tests of this kind had never been performed, and were regarded as exceedingly difficult because of the necessity of centering with absolute certainty two interconnected and interacting columns. As a simpler and less tedious way of checking the notion that the diaphragm would act primarily in shear, beam sheet tests as in figure 4 were devised. These tests, performed with a variety of diaphragms, gave substantial assurance that the idea was correct and permitted the experimental determination of the effective shear rigidities. With the information thus obtained, several double column tests were performed

The Radiative Forcing Model Intercomparison Project (RFMIP): experimental protocol for CMIP6

The phrasing of the first of three questions motivating CMIP6 – “How does the Earth system respond to forcing?” – suggests that forcing is always well-known, yet the radiative forcing to which this question refers has historically been uncertain in coordinated experiments even as understanding of how best to infer radiative forcing has evolved. The Radiative Forcing Model Intercomparison Project (RFMIP) endorsed by CMIP6 seeks to provide a foundation for answering the question through three related activities: (i) accurate characterization of the effective radiative forcing relative to a near-preindustrial baseline and careful diagnosis of the components of this forcing; (ii) assessment of the absolute accuracy of clear-sky radiative transfer parameterizations against reference models on the global scales relevant for climate modeling; and (iii) identification of robust model responses to tightly specified aerosol radiative forcing from 1850 to present. Complete characterization of effective radiative forcing can be accomplished with 180 years (Tier 1) of atmosphere-only simulation using a sea-surface temperature and sea ice concentration climatology derived from the host model's preindustrial control simulation. Assessment of parameterization error requires trivial amounts of computation but the development of small amounts of infrastructure: new, spectrally detailed diagnostic output requested as two snapshots at present-day and preindustrial conditions, and results from the model's radiation code applied to specified atmospheric conditions. The search for robust responses to aerosol changes relies on the CMIP6 specification of anthropogenic aerosol properties; models using this specification can contribute to RFMIP with no additional simulation, while those using a full aerosol model are requested to perform at least one and up to four 165-year coupled ocean–atmosphere simulations at Tier 1

A lattice model of hydrophobic interactions

Hydrogen bonding is modeled in terms of virtual exchange of protons between water molecules. A simple lattice model is analyzed, using ideas and techniques from the theory of correlated electrons in metals. Reasonable parameters reproduce observed magnitudes and temperature dependence of the hydrophobic interaction between substitutional impurities and water within this lattice.Comment: 7 pages, 3 figures. To appear in Europhysics Letter

A MAJOR GENETIC LOCUS AFFECTING RESISTANCE TO INFECTION WITH MURINE LEUKEMIA VIRUSES : II. APPARENT IDENTITY TO A MAJOR LOCUS DESCRIBED FOR RESISTANCE TO FRIEND MURINE LEUKEMIA VIRUS

The N-B locus affecting tissue culture infectivity with naturally occurring murine leukemia viruses appears to be identical to the Fv-1 locus described for sensitivity to Friend leukemia virus. Results of tissue culture studies were parallel to results of studies in vivo and indicate that the F-S virus is N-tropic and the F-B virus is NB-tropic. Inbred and partially congenic mouse strains sensitive at Fv-1 show N-type sensitivity; strains resistant at Fv-1 show B-type sensitivity. The Fv-2 locus does not appear to exert significant effect in tissue culture. Knowledge of N-B type has been useful in predicting Fv-1 sensitivity

Counterion density profiles at charged flexible membranes

Counterion distributions at charged soft membranes are studied using perturbative analytical and simulation methods in both weak coupling (mean-field or Poisson-Boltzmann) and strong coupling limits. The softer the membrane, the more smeared out the counterion density profile becomes and counterions pentrate through the mean-membrane surface location, in agreement with anomalous scattering results. Membrane-charge repulsion leads to a short-scale roughening of the membrane.Comment: 4 pages, 4 figure