Point defect calculations in tungsten

Point defect calculations for tungste

A molecular-field approximation for quantum crystals

Ground-state properties of quantum crystals have received considerable attention from both theorists and experimentalists. The theoretical results have varied widely with the Monte Carlo calculations being the most successful. The molecular field approximation yields ground-state properties which agree closely with the Monte Carlo results. This approach evaluates the dynamical behavior of each pair of molecules in the molecular field of the other N-2 molecules. In addition to predicting ground-state properties that agree well with experiment, this approach yields data on the relative importance of interactions of different nearest neighbor pairs

Pause Point Spectra in DNA Constant-Force Unzipping

Under constant applied force, the separation of double-stranded DNA into two single strands is known to proceed through a series of pauses and jumps. Given experimental traces of constant-force unzipping, we present a method whereby the locations of pause points can be extracted in the form of a pause point spectrum. A simple theoretical model of DNA constant-force unzipping is demonstrated to produce good agreement with the experimental pause point spectrum of lambda phage DNA. The locations of peaks in the experimental and theoretical pause point spectra are found to be nearly coincident below 6000 bp. The model only requires the sequence, temperature and a set of empirical base pair binding and stacking energy parameters, and the good agreement with experiment suggests that pause points are primarily determined by the DNA sequence. The model is also used to predict pause point spectra for the BacterioPhage PhiX174 genome. The algorithm for extracting the pause point spectrum might also be useful for studying related systems which exhibit pausing behavior such as molecular motors.Comment: 15 pages, 12 figure

Measurement of beam divergence of 30-centimeter dished grids

The beam divergence of a 30-centimeter diameter thruster with dished grids was calculated from current densities measured with a probe rake containing seventeen planar molybdenum probes. The measured data were analyzed as a function of a number of parameters. The most sensitive parameters were the amount of compensation of the accelerator grid and the ratio of net to total accelerating voltage. The thrust losses were reduced by over 5 percent with the use of compensated grids alone, and by variation of other parameters the overall thrust losses due to beam divergence were reduced to less than 2 percent

Measurement of the salt-dependent stabilization of partially open DNA by Escherichia coli SSB protein

The rezipping force of two complementary DNA strands under tension has been measured in the presence of Escherichia coli single-stranded-binding proteins under salt conditions ranging from 10– to 400 mM NaCl. The effectiveness of the binding protein in preventing rezipping is strongly dependent on salt concentration and compared with the salt dependence in the absence of the protein. At concentrations less than 50 mM NaCl, the protein prevents complete rezipping of λ-phage on the 2-s timescale of the experiment, when the ssDNA is under tensions as low as 3.5 ± 1 pN. For salt concentrations greater than 200 mM NaCl, the protein inhibits rezipping but cannot block rezipping when the tension is reduced below 6 ± 1.8 pN. This change in effectiveness as a function of salt concentration may correspond to salt-dependent changes in binding modes that were previously observed in bulk assays

Effect of genome sequence on the force-induced unzipping of a DNA molecule

We considered a dsDNA polymer in which distribution of bases are random at the base pair level but ordered at a length of 18 base pairs and calculated its force elongation behaviour in the constant extension ensemble. The unzipping force $F(y)$ vs. extension $y$ is found to have a series of maxima and minima. By changing base pairs at selected places in the molecule we calculated the change in $F(y)$ curve and found that the change in the value of force is of the order of few pN and the range of the effect depending on the temperature, can spread over several base pairs. We have also discussed briefly how to calculate in the constant force ensemble a pause or a jump in the extension-time curve from the knowledge of $F(y)$

Massively parallel single-molecule manipulation using centrifugal force

Precise manipulation of single molecules has already led to remarkable insights in physics, chemistry, biology and medicine. However, widespread adoption of single-molecule techniques has been impeded by equipment cost and the laborious nature of making measurements one molecule at a time. We have solved these issues with a new approach: massively parallel single-molecule force measurements using centrifugal force. This approach is realized in a novel instrument that we call the Centrifuge Force Microscope (CFM), in which objects in an orbiting sample are subjected to a calibration-free, macroscopically uniform force-field while their micro-to-nanoscopic motions are observed. We demonstrate high-throughput single-molecule force spectroscopy with this technique by performing thousands of rupture experiments in parallel, characterizing force-dependent unbinding kinetics of an antibody-antigen pair in minutes rather than days. Additionally, we verify the force accuracy of the instrument by measuring the well-established DNA overstretching transition at 66 $\pm$ 3 pN. With significant benefits in efficiency, cost, simplicity, and versatility, "single-molecule centrifugation" has the potential to revolutionize single-molecule experimentation, and open access to a wider range of researchers and experimental systems.Comment: 5 pages, 3 figure

ASPIRES at Georgia Southern University

Advisement and Scholarship Promoting Inquiry-based Research Experiences in STEM (ASPIRES) at Georgia Southern University is a comprehensive, strategic project that will increase the number of students graduating with bachelor\u27s degrees in STEM fields. Five action items address two interrelated goals: (1) to increase retention of STEM students and (2) to improve academic success in science and math courses. The project ended in 2012