Geometrical and spectral study of beta-skeleton graphs

We perform an extensive numerical analysis of beta-skeleton graphs, a particular type of proximity graphs. In beta-skeleton graph (BSG) two vertices are connected if a proximity rule, that depends of the parameter beta is an element of (0, infinity), is satisfied. Moreover, for beta > 1 there exist two different proximity rules, leading to lune-based and circle-based BSGs. First, by computing the average degree of large ensembles of BSGs we detect differences, which increase with the increase of beta, between lune-based and circle-based BSGs. Then, within a random matrix theory (RMT) approach, we explore spectral and eigenvector properties of random BSGs by the use of the nearest-neighbor energy-level spacing distribution and the entropic eigenvector localization length, respectively. The RMT analysis allows us to conclude that a localization transition occurs at beta = 1

Effect of Bilayer Thickness on Membrane Bending Rigidity

The bending rigidity $k_c$ of bilayer vesicles self-assembled from amphiphilic diblock copolymers has been measured using single and dual-micropipet techniques. These copolymers are nearly a factor of 5 greater in hydrophobic membrane thickness $d$ than their lipid counterparts, and an order of magnitude larger in molecular weight $\bar{M}_n$. The macromolecular structure of these amphiphiles lends insight into and extends relationships for traditional surfactant behavior. We find the scaling of $k_c$ with thickness to be nearly quadratic, in agreement with existing theories for bilayer membranes. The results here are key to understanding and designing soft interfaces such as biomembrane mimetics

An analysis of the role of public relations in the development of the industrial promotion program sponsored by the Commonwealth of Puerto Rico

Thesis (M.S.)--Boston Universit

Exact Mapping of the 2+1 Dirac Oscillator onto the Jaynes-Cummings Model: Ion-Trap Experimental Proposal

We study the dynamics of the 2+1 Dirac oscillator exactly and find spin oscillations due to a {\it Zitterbewegung} of purely relativistic origin. We find an exact mapping of this quantum-relativistic system onto a Jaynes-Cummings model, describing the interaction of a two-level atom with a quantized single-mode field. This equivalence allows us to map a series of quantum optical phenomena onto the relativistic oscillator, and viceversa. We make a realistic experimental proposal, at reach with current technology, for studying the equivalence of both models using a single trapped ion.Comment: Revtex4, submitted for publicatio

Simulation of Jahn-Teller-Dicke Magnetic Structural Phase Transition with Trapped Ions

We study theoretically the collective E$\otimes$e Jahn-Teller-Dicke distortion in a system of trapped ions. We focus in the limit of infinite range interactions in which an ensemble of effective spins interacts with two collective vibrational modes with U(1) symmetric couplings. Our model is exactly solvable in the thermodynamical limit and it is amenable to be solved by exact numerical diagonalization for a moderate number of ions. We show that trapped ions are ideally suited to study the emergence of spontaneous symmetry breaking of a continuous symmetry and magnetic structural phase transition in a mesoscopic system.Comment: 19 pages, 7 figure

Molecular Weight Dependence of Polymersome Membrane Elasticity and Stability

Vesicles prepared in water from a series of diblock copolymers and termed "polymersomes" are physically characterized. With increasing molecular weight $\bar{M}_n$, the hydrophobic core thickness $d$ for the self-assembled bilayers of polyethyleneoxide - polybutadiene (PEO-PBD) increases up to 20 $nm$ - considerably greater than any previously studied lipid system. The mechanical responses of these membranes, specifically, the area elastic modulus $K_a$ and maximal areal strain $\alpha_c$ are measured by micromanipulation. As expected for interface-dominated elasticity, $K_a$ ($\simeq$ 100 $pN/nm$) is found to be independent of $\bar{M}_n$. Related mean-field ideas also predict a limiting value for $\alpha_c$ which is universal and about 10-fold above that typical of lipids. Experiments indeed show $\alpha_c$ generally increases with $\bar{M}_n$, coming close to the theoretical limit before stress relaxation is opposed by what might be chain entanglements at the highest $\bar{M}_n$. The results highlight the interfacial limits of self-assemblies at the nano-scale.Comment: 16 pages, 5 figures, and 1 tabl

No Holes Barred: Invasion of the Intestinal Mucosa by Mycobacterium avium subsp. paratuberculosis

The infection biology of Mycobacterium avium subsp. paratuberculosis has recently crystallized, with added details surrounding intestinal invasion. The involvement of pathogen-derived effector proteins such as the major membrane protein, oxidoreductase, and fibronectin attachment proteins have been uncovered. Mutations constructed in this pathogen have also shed light on genes needed for invasion. The host cell types that are susceptible to invasion have been defined, along with their transcriptional response. Recent details have given a new appreciation for the dynamic interplay between the host and bacterium that occurs at the outset of infection. An initial look at the global expression pathways of the host has shown a circumvention of the cell communication pathway by M. avium subsp. paratuberculosis, which loosens the integrity of the tight junctions. We now know that M. avium subsp. paratuberculosis activates the epithelial layer and also actively recruits macrophages to the site of infection. These notable findings are summarized along with added mechanistic details of the early infection model. We conclude by pro- posing critical next steps to further elucidate the process of M. avium subsp. paratuberculosis invasion

Excitation Enhancement of a Quantum Dot Coupled to a Plasmonic Antenna

Plasmonic antennas are key elements to control the luminescence of quantum emitters. However, the antenna's influence is often hidden by quenching losses. Here, the luminescence of a quantum dot coupled to a gold dimer antenna is investigated. Detailed analysis of the multiply excited states quantifies the antenna's influence on the excitation intensity and the luminescence quantum yield separately

Mycobacterium avium resists exposure to the acidic conditions of the stomach

Organisms of the Mycobacterium avium complex are common pathogens in immunosuppressed patients such as individuals with AIDS. There is evidence that in AIDS patients, the main route for M. avium infection is the gastrointestinal tract. The stomach is a formidable barrier to pathogens and the ability to resist exposure to pH lower than 3 has been shown to be a virulence determinant of enteric pathogens. Incubation of three clinical isolates of M. avium under acidic pH revealed resistance of M. avium grown both to the exponential and stationary phase at pH 2.2 for 2 h. Inhibition of protein synthesis had no effect on the acid tolerance. When the duration of the incubation at pH 2.2 was extended to 24 h, bacteria grown to the stationary phase had a significantly greater tolerance to acid than exponential phase bacteria. M. avium incubated with acid in the presence of water was significantly more resistant to pH 2.2 than M. avium in the presence of buffer. Pre-adaptation in water prior to exposure to acidic conditions was also associated with increased resistance to pH 2.2. Isoosmolarity of Hank's balanced salt solution appears to be responsible for the impaired resistance to acid between 2 and 24 h of incubation. These findings indicate that M. avium is naturally tolerant to pH<3 and that pre-adaptation under conditions similar to the conditions where M. avium is found in the environment results in increased acid resistanc