A complete form of the van der Waals dispersion interaction between an infinitely long anisotropic semiconducting/insulating thin cylinder and an anisotropic half space is derived for all separations between the cylinder and the half space. The derivation proceeds from the theory of dispersion interactions between two anisotropic infinite half spaces as formulated in Phys. Rev. A 71, 042102 (2005). The approach is valid in the retarded as well as nonretarded regimes of the interaction and is coupled with the recently evaluated ab initio dielectric response functions of various semiconducting/insulating single wall carbon nanotubes, enables the authors to evaluate the strength of the van der Waals dispersion interaction for all orientation angles and separations between a thin cylindrical nanotube and the half space. The possibility of repulsive and/or nonmonotonic dispersion interactions is examined in detail.

We derive the complete form of the van der Waals dispersion interaction between two infinitely long anisotropic semiconducting/insulating thin cylinders at all separations. The derivation is based on the general theory of dispersion interactions between anisotropic media as formulated in Munday et al. [Phys. Rev. A 71, 042102 (2005)]. This formulation is then used to calculate the dispersion interactions between a pair of single-walled carbon nanotubes at all separations and all angles. Nonretarded and retarded forms of the interactions are developed separately. The possibility of repulsive dispersion interactions and nonmonotonic dispersion interactions is discussed within the framework of the formulation.

We present a phenomenological one-parameter scaling equation of state that accurately represents osmotic pressures of neutral flexible polymers in good solvents from the dilute through the semidilute regime. The equation comprises a sum of scaled van't Hoff and des Cloizeaux terms including a fitted parameter alpha, the "crossover index", which encapsulates all chemical specificity and determines the relevant prefactors. Strikingly different values of alpha are found for the two very different systems poly(ethyleneglycol)/water (PEG) and poly(alpha-methylstyrene)/toluene (PAMS). Alpha-dependent rescaling collapses both data sets to a simple one-parameter scaling function. The fact that the anomalous system PEG/water and the canonical system PAMS/toluene can both be described by the same equation of state attests to the robustness of the polymer-scaling concepts introduced by de Gennes.

We investigate the osmotic equilibrium between a bulk polyethylene glycol (PEG) solution and DNA tightly packed in a spherical capsid. We base our analysis on the equations of thermodynamic equilibrium in terms of osmotic pressure. The equality between external osmotic pressure of PEG and osmotic pressure of tightly packed DNA gives us the DNA encapsidation curves. In this way we directly connect the wealth of existing osmotic pressure data for DNA in the bulk with the DNA encapsidation curves within small viral capsids. Specific calculations are made for a monovalent salt, Na+ -DNA and a divalent salt, Mn2+ -DNA that have quite different DNA encapsidation behaviors. The main conclusion of our work is that bending energy of DNA is of minor importance regarding the encapsidated DNA length, but has a non-negligible influence on the density distribution of DNA within the capsid.

Working at the macroscopic continuum level, we investigate effective van der Waals interactions between two layers within a multilayer assembly. By comparing the pair interactions between two layers with effective pair interactions within an assembly we assess the significant consequences of nonadditivity of van der Waals interactions. This allows us to evaluate the best numerical estimate to date for the Hamaker coefficient of van der Waals interactions in lipid-water multilamellar systems.

Critical to biological processes such as membrane fusion and secretion, ion-lipid interactions at the membrane-water interface still raise many unanswered questions. Using reconstituted phosphatidylcholine membranes, we confirm here that multilamellar vesicles swell in salt solutions, a direct indication that salt modifies the interactions between neighboring membranes. By varying sample histories, and by comparing with data from ion carrier-containing bilayers, we eliminate the possibility that swelling is an equilibration artifact. Although both attractive and repulsive forces could be modified by salt, we show experimentally that swelling is driven primarily by weakening of the van der Waals attraction. To isolate the effect of salt on van der Waals interactions, we focus on high salt concentrations at which any possible electrostatic interactions are screened. By analysis of X-ray diffraction data, we show that salt does not alter membrane structure or bending rigidity, eliminating the possibility that repulsive fluctuation forces change with salt. By measuring changes in interbilayer separation with applied osmotic stress, we have determined, using the standard paradigm for bilayer interactions, that 1 M concentrations of KBr or KCl decrease the van der Waals strength by 50%. By weakening van der Waals attractions, salt increases energy barriers to membrane contact, possibly affecting cellular communication and biological signaling.

To explain the concentration dependence of the partition coefficient of polyethylene glycol into α-hemolysin channels, weak nonideality of the polymer solution in the pore is taken into account. Polymer molecules in the pore form cigars with higher intramolecular monomer density and, therefore, higher overlap volume fraction.

Using microcalorimetry, we follow changes in the association free energy of beta-cyclodextrin (CD) with the hydrophobic part of adamantane carboxylate (AD) due to added salt or polar (net-neutral) solutes that are excluded from the molecular interacting surfaces. Changes in binding constants with solution osmotic pressure (water activity) translate into changes in the preferential hydration upon complex formation. We find that these changes correspond to a release of 15-25 solute-excluding waters upon CD/AD association. Reflecting the preferential interaction of solute with reactants versus products, we find that changes in hydration depend on the type of solute used. All solutes used here result in a large change in the enthalpy of the CD-AD binding reaction. In one class of solutes, the corresponding entropy change is much smaller, while in the other class, the entropy change almost fully compensates the solute-specific enthalpy. For many of the solutes, the number of waters released correlates well with their effect on air-water surface tensions. We corroborate these results using vapor pressure osmometry to probe individually the hydration of reactants and products of association, and we discuss the possible interactions and forces between cosolute and hydrophobic surfaces responsible for different kinds of solute exclusion.

Expected from theory and simulations, depletion of ions at fuzzy biomembrane interfaces has long eluded experiments. Here, we show how salt exclusion can be accurately measured by surprisingly simple yet accurate benchtop measurements. Multilamellar aggregates of common phospholipids sink in low salt but float in salt solutions that are much less dense than the lipid itself. By manipulating bath and lipid densities, using heavy water and varied lipid chain length, we obtain accurate exclusion curves over a wide range of KCl and KBr concentrations. While maintaining a constant width at low salt, the exclusion layer decreases in high salt, following the Debye screening length. Consistent with interfacial accumulation of polarizable ions, bromide salts are less excluded than chloride, with an attraction of approximately 2kBT per Br- ion. So far neglected in theoretical descriptions, the competition between salt exclusion and binding is critical to understanding membrane interactions and specific ionic effects.

Caused by biosynthesis defects, cholesterol deficiency can lead to developmental disorders and malformations, with possible implication of lipid membrane properties. We show that modification of sterol chemical structure alters membrane physical properties significantly. By X-ray diffraction and osmotic stress, we measure changes in the bending rigidity of bilayers containing either cholesterol or one of its metabolic precursors. Membrane elasticity differs dramatically between slightly different sterols and varies in the sequence lanosterol < 7-dehydrocholesterol < lathosterol < cholesterol. We interpret the results in terms of sterol location within lipid structures and modification of lateral stress, a structural feature relevant to interactions within biological membranes. We find that cholesterol is most efficient in enhancing membrane rigidity, a possible clue to why depletion or replacement with other sterols can affect cellular structures.

Using x-ray diffraction and NMR spectroscopy, we present structural and material properties of phosphatidylserine (PS) bilayers that may account for the well documented implications of PS headgroups in cell activity. At 30°C, the 18-carbon monounsaturated DOPS in the fluid state has a cross-sectional area of 65.3Å2 which is remarkably smaller than the area 72.5Å2 of the DOPC analog, despite the extra electrostatic repulsion expected for charged PS headgroups. Similarly, at 20°C, the 14-carbon disaturated DMPS in the gel phase has an area of 40.8Å2 vs. 48.1Å2 for DMPC. This condensation of area suggests an extra attractive interaction, perhaps hydrogen bonding, between PS headgroups. Unlike zwitterionic lipids, stacks of PS bilayers swell indefinitely as water is added. Data obtained for osmotic pressure versus interbilayer water spacing for fluid phase DOPS are well fit by electrostatic interactions calculated for the Gouy-Chapman regime. It is shown that the electrostatic interactions completely dominate the fluctuational pressure. Nevertheless, the x-ray data definitively exhibit the effects of fluctuations in fluid phase DOPS. From our measurements of fluctuations, we obtain the product of the bilayer bending modulus KC and the smectic compression modulus B. At the same interbilayer separation, the interbilayer fluctuations are smaller in DOPS than for DOPC, showing that B and/or KC are larger. Complementing the x-ray data, 31P-chemical shift anisotropy measured by NMR suggest that the DOPS headgroups are less sensitive to osmotic pressure than DOPC headgroups, which is consistent with a larger KC in DOPS. Quadrupolar splittings for D2O decay less rapidly with increasing water content for DOPS than for DOPC, indicating greater perturbation of interlamellar water and suggesting a greater interlamellar hydration force in DOPS. Our comparisons between bilayers of PS and PC lipids with the same chains and the same temperature enable us to focus on the effects of these headgroups on bilayer properties.

Driven by conditions set by smaller solutes, proteins fold and unfold. Experimentally, these conditions are stated as intensive variables--pH and other chemical potentials--as though small solutes were infinite resources that come at an externally varied free energy cost. Computationally, the finite spaces of simulation allow only fixed numbers of these solutes. By combining the analytic Gibbs adsorption isotherm with the computational Monte Carlo sampling of polymer configurations, we have been able to overcome an inherent limitation of computer simulation. The idea is to compute analytically the free energy changes wrought by solutes on each particular configuration. Then numerical computation is needed only to sample the set of configurations as efficiently as when no bathing solute is present. For illustration, the procedure is applied to an idealized two-dimensional heteropolymer to yield lessons about the effect of cosolutes on protein stability.

We formulate and evaluate the van der Waals part of the free energy due to a dielectric profile that varies continuously throughout the space between two interacting bodies. Not considering the work needed to create the inhomogeneous dielectric profile, focusing only on that part of the free energy affected by the inhomogeneity, we compare the ensuing interaction free energy with that of the original Lifshitz formulation with its step function changes at material boundaries and uniform dielectric medium. Rather than the monotonically varying attraction between like bodies given by the original formulation, the inhomogeneous continuous dielectric function leads to attractions as well as repulsions. The Lifshitz result emerges naturally in the limit of separations much larger than the thickness of the interfaces.