The Buoyant Behavior of Viral and Bacterial DNA in Alkaline CsCl

In equilibrium density gradient centrifugation, the banding polymer species is electrically neutral. The banding species for a negative polyelectrolyte with a polyanion P_(n)^(-z)n (where n is the degree of polymerization, and z the titration charge per monomer unit) in a CsCl salt gradient is CS_(zn)P_n. If the ion P_(n)^(-z)n is itself a weak acid, it may be titrated to the state P_(n)^(-(Zn+y)) by CsOH; the banding species is then Cs_(zn+y)P_n. Because of the large mass and high effective "density" of a Cs^+ ion, it is to be expected that the buoyant density in a CsCl gradient of a polymer acid will be increased by such a partial alkaline titration with CsOH. This expectation has been confirmed for polyglutamic acid (where z = 0 at low pH). The guanine and thymine monomer units of DNA are weak acids. The present communication is concerned with the increase in buoyant density of DNA in alkaline CsCl solutions. It is well known that the guanine and thymine protons are more readily titrated in denatured DNA than in native DNA. We find that the buoyant density of denatured DNA and of single strand ϕX-174 DNA gradually increases as the pH of the solution is increased beyond pH 9.8. The density of native DNA is not affected until a critical pH > 11 is reached, where the DNA abruptly denatures and increases in density. Similar increases in buoyant density have been observed independently by Baldwin and Shooter in their studies of 5BU[overbar]-substituted DNA's in alkaline solutions

Absorption of ant-provided carbon dioxide and nitrogen by a tropical epiphyte

ALTHOUGH ant-plant mutualisms have been described in many ecosystems, the magnitude of the direct benefits from such relationships are hard to quantify. In Bako National Park, Sarawak, Malaysia, stunted ‘kerangas’ forests occur on nutrient-poor sandstone hills1-3. As trees are widely spaced and have a sparse leaf area, a significant amount of light reaches the tree trunks and enables a diverse community of epiphytes to thrive there4. One of these epiphytes, Dischidia major (Vahl) Merr. (Asclepiadaceae), has evolved unusual methods for enhancing carbon and nitrogen acquisition. We show here that a mutualistic relationship exists between ants of the genus Philidris and their host, D. major. Using stable isotope analysis, we calculate that 39% of the carbon in occupied host plant leaves is derived from ant-related respiration, and that 29% of the host nitrogen is derived from debris deposited into the leaf cavities by ants. © 1995 Nature Publishing Groups. All Rights Reserved

Ultrafast Plasmonic Control of Second Harmonic Generation

Efficient frequency conversion techniques are crucial to the development of plasmonic metasurfaces for information processing and signal modulation. In principle, nanoscale electric-field confinement in nonlinear materials enables higher harmonic conversion efficiencies per unit volume than those attainable in bulk materials. Here we demonstrate efficient second-harmonic generation (SHG) in a serrated nanogap plasmonic geometry that generates steep electric field gradients on a dielectric metasurface. An ultrafast pump is used to control plasmon-induced electric fields in a thin-film material with inversion symmetry that, without plasmonic enhancement, does not exhibit an an even-order nonlinear optical response. The temporal evolution of the plasmonic near-field is characterized with ~100as resolution using a novel nonlinear interferometric technique. The ability to manipulate nonlinear signals in a metamaterial geometry as demonstrated here is indispensable both to understanding the ultrafast nonlinear response of nanoscale materials, and to producing active, optically reconfigurable plasmonic device

Reissner-Nordstrom and charged gas spheres

The main point of this paper is a suggestion about the proper treatment of the photon gas in a theory of stellar structure and other plasmas. This problem arises in the study of polytropic gas spheres, where we have already introduced some innovations. The main idea, already advanced in the contextof neutral, homogeneous, polytropic stellar models, is to base the theory firmly on a variational principle. Another essential novelty is to let mass distribution extend to infinity, the boundary between bulk and atmosphere being defined by an abrupt change in the polytropic index, triggered by the density. The logical next step in this program is to include the effect of radiation, which is a very significant complication since a full treatment would have to include an account of ionization, thus fieldsrepresenting electrons, ions, photons, gravitons and neutral atoms as well. In way of preparation, we consider models that are charged but homogeneous, involving only gravity, electromagnetism and a single scalar field that represents both the mass and the electric charge; in short, anon-neutral plasma. While this work only represents a stage in the development of a theory of stars, without direct application to physical systems, it does shed some light on the meaning of the Reissner-Nordstrom solution of the modified Einstein-Maxwell equations., with an application to a simple system.Comment: 19 pages, plain te

Fair scans of the seesaw. Consequences for predictions on LFV processes

Usual analyses based on scans of the seesaw parameter-space can be biassed since they do not cover in a fair way the complete parameter-space. More precisely, we show that in the common "R-parametrization", many acceptable R-matrices, compatible with the perturbativity of Yukawa couplings, are normally disregarded from the beginning, which produces biasses in the results. We give a straightforward procedure to scan the space of complex R-matrices in a complete way, giving a very simple rule to incorporate the perturbativity requirement as a condition for the entries of the R-matrix, something not considered before. As a relevant application of this, we show that the extended believe that BR(mu --> e, gamma) in supersymmetric seesaw models depends strongly on the value of theta_13 is an "optical effect" produced by such biassed scans, and does not hold after a careful analytical and numerical study. When the complete scan is done, BR(mu --> e, gamma) gets very insensitive to theta_13. Moreover, the values of the branching ratio are typically larger than those quoted in the literature, due to the large number of acceptable points in the parameter-space which were not considered before. Including (unflavoured) leptogenesis does not introduce any further dependence on theta_13, although decreases the typical value of BR(mu --> e, gamma).Comment: 22 pages, 5 figure