Binding of molecules to DNA and other semiflexible polymers

A theory is presented for the binding of small molecules such as surfactants to semiflexible polymers. The persistence length is assumed to be large compared to the monomer size but much smaller than the total chain length. Such polymers (e.g. DNA) represent an intermediate case between flexible polymers and stiff, rod-like ones, whose association with small molecules was previously studied. The chains are not flexible enough to actively participate in the self-assembly, yet their fluctuations induce long-range attractive interactions between bound molecules. In cases where the binding significantly affects the local chain stiffness, those interactions lead to a very sharp, cooperative association. This scenario is of relevance to the association of DNA with surfactants and compact proteins such as RecA. External tension exerted on the chain is found to significantly modify the binding by suppressing the fluctuation-induced interaction.Comment: 15 pages, 7 figures, RevTex, the published versio

A stochastic extension of the Uniform Theory of Diffraction accounting for geometrical uncertainty or surface and edge roughness

We present a stochastic extension of the Uniform Theory of Diffraction (UTD) which is capable to account for some uncertainty in the objects position or geometry, including roughness of surfaces or edges. Namely, we derive a solution for the electromagnetic field scattered by a perfectly conducting wedge whose faces are described as a statistical perturbation of a standard flat wedge. We give a uniform closed form expressions for the evaluation of the main statistical moments of the total electric field. The proposed statistical UTD formulation is suitable for engineering applications which involve UTD ray based codes. Some numerical examples highlight the accuracy and the effectiveness of the proposed ray description

Autoassociação de misturas dos Surfactantes Dodecanoato de Sódio (SDoD) e Decanoato de Sódio (SDeC) com o Polímero Hidrofobicamente Modificado Etil(Hidroxietil)Celulose (EHEC)

In this work, the interactions between the non-ionic polymer of ethyl(hydroxyethyl)cellulose (EHEC) and mixed anionic surfactant sodium dodecanoate (SDoD)-sodium decanoate (SDeC) in aqueous media, at pH 9.2 (20 mM borate/NaOH buffer) were investigated by electric conductivity and light transmittance measurements at 25 ºC. The parameters of the surfactant to polymer association processes such as the critical aggregation concentration and saturation of the polymer by surfactants were determined from plots of specific conductivity vs total surfactant concentration, [surfactant]tot = [SDoD] + [SDeC]. Through the results was not observed a specific link of polymer with the surfactant, implying therefore a phenomenon only cooperative association

Low-profile Dual-polarized Isoflux Antennas for Space Applications

Design, realization and measurements of a X-band antenna for data downlink (DDL) from low Earth orbit (LEO) satellites are presented. This antenna has an isoflux radiation pattern and operates in dual circular polarization with crosspolar discrimination (XPD) higher than 18dB on the operational bandwidth. Two solutions are first proposed: one realized by a metasurface with circular strips printed on a dielectric slab, and one realized by concentric metallic corrugations. Starting from a basic scattering mechanism of a self-complementary ring-type metasurface excited by a perfectly polarized source, both these antennas are designed by optimizing an equivalent anisotropic impedance surface excited by a short hat feed. The performance of the two designs are compared. The corrugated antenna is realized and measured, confirming that the performances are compliant with the requirements for next generation DDL antennas

New antennas for space applications

Metasurfaces (MTSs) are versatile, technologically advanced new materials, which have proven their capability to be successfully employed for constituting variable impedance plane and enabling precisely controlled radiation from equivalent apertures for antenna applications. Among the several requirements that can be desired in Space Communications environment the possibility of obtaining shaped beams is one of the most demanding features, in this framework MTS antennas assume a key role. In this work two new designs of shaped beam antennas are presented. These implement a low-complexity data-downlink aperture for Ka-band LEO missions and, in the second example, a printed MTS is applied for pattern shaping of a Ku-band horn for global Earth coverage