352 research outputs found
Rigid unit modes in tetrahedral crystals
The 'rigid unit mode' (RUM) model requires unit blocks, in our case
tetrahedra of SiO_4 groups, to be rigid within first order of the displacements
of the O-ions. The wave-vectors of the lattice vibrations, which obey this
rigidity, are determined analytically. Lattices with inversion symmetry yield
generically surfaces of RUMs in reciprocal space, whereas lattices without this
symmetry yield generically lines of RUMs. Only in exceptional cases as in
beta-quartz a surface of RUMs appears, if inversion symmetry is lacking. The
occurence of planes and bending surfaces, straight and bent lines is discussed.
Explicit calculations are performed for five modifications of SiO_2 crystals.Comment: 18 pages, 6 figures, improved notatio
Fluorescence-based incision assay for human XPF-ERCC1 activity identifies important elements of DNA junction recognition
The structure-specific endonuclease activity of the human XPF–ERCC1 complex is essential for a number of DNA processing mechanisms that help to maintain genomic integrity. XPF–ERCC1 cleaves DNA structures such as stem–loops, bubbles or flaps in one strand of a duplex where there is at least one downstream single strand. Here, we define the minimal substrate requirements for cleavage of stem–loop substrates allowing us to develop a real-time fluorescence-based assay to measure endonuclease activity. Using this assay, we show that changes in the sequence of the duplex upstream of the incision site results in up to 100-fold variation in cleavage rate of a stem-loop substrate by XPF-ERCC1. XPF–ERCC1 has a preference for cleaving the phosphodiester bond positioned on the 3′-side of a T or a U, which is flanked by an upstream T or U suggesting that a T/U pocket may exist within the catalytic domain. In addition to an endonuclease domain and tandem helix–hairpin–helix domains, XPF has a divergent and inactive DEAH helicase-like domain (HLD). We show that deletion of HLD eliminates endonuclease activity and demonstrate that purified recombinant XPF–HLD shows a preference for binding stem–loop structures over single strand or duplex alone, suggesting a role for the HLD in initial structure recognition. Together our data describe features of XPF–ERCC1 and an accepted model substrate that are important for recognition and efficient incision activity
Amorphization induced by pressure: results for zeolites and general implications
We report an {\sl ab initio} study of pressure-induced amorphization (PIA) in
zeolites, which are model systems for this phenomenon. We confirm the
occurrence of low-density amorphous phases like the one reported by Greaves
{\sl et al.} [Science {\bf 308}, 1299 (2005)], which preserves the crystalline
topology and might constitute a new type of glass. The role of the zeolite
composition regarding PIA is explained. Our results support the correctness of
existing models for the basic PIA mechanim, but suggest that energetic, rather
than kinetic, factors determine the irreversibility of the transition.Comment: 4 pages with 3 figures embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
Bullying: Bullies, victims, and witnesses
This study examines bullying by focusing on bullies, victims, and witnesses. In an effort to examine long-term correlates of bullying, we asked university students about their experiences with bullying in middle school. We administered a 65 question survey to 191 college students from several university campuses. The survey was made up of the Handling Bullying Questionnaire (Bauman, Rigby & Hoppa, 2008), the Bullying Prevalence Questionnaire (Rigby & Slee, 2003), the Revised Pro-Victim Scale (Rigby, 1997), and 13 researcher created questions which dealt with the participants\u27 experiences with bullying. We hypothesized that rates of bullying would be high and that the long-term correlates of witnessing bullying would be the same as those for being a victim of bullying. Most of the participants reported witnessing acts of bullying and being victims of bullying in middle school. Very few participants reported bullying others. We found non-violent forms of bullying to be the most common. Bullies and witnesses, but not victims, were more likely to say they would intervene to stop a case of bullying. Otherwise, witnesses and victims responded similarly
Isostaticity, auxetic response, surface modes, and conformal invariance in twisted kagome lattices
Model lattices consisting of balls connected by central-force springs provide
much of our understanding of mechanical response and phonon structure of real
materials. Their stability depends critically on their coordination number .
-dimensional lattices with are at the threshold of mechanical
stability and are isostatic. Lattices with exhibit zero-frequency
"floppy" modes that provide avenues for lattice collapse. The physics of
systems as diverse as architectural structures, network glasses, randomly
packed spheres, and biopolymer networks is strongly influenced by a nearby
isostatic lattice. We explore elasticity and phonons of a special class of
two-dimensional isostatic lattices constructed by distorting the kagome
lattice. We show that the phonon structure of these lattices, characterized by
vanishing bulk moduli and thus negative Poisson ratios and auxetic elasticity,
depends sensitively on boundary conditions and on the nature of the kagome
distortions. We construct lattices that under free boundary conditions exhibit
surface floppy modes only or a combination of both surface and bulk floppy
modes; and we show that bulk floppy modes present under free boundary
conditions are also present under periodic boundary conditions but that surface
modes are not. In the the long-wavelength limit, the elastic theory of all
these lattices is a conformally invariant field theory with holographic
properties, and the surface waves are Rayleigh waves. We discuss our results in
relation to recent work on jammed systems. Our results highlight the importance
of network architecture in determining floppy-mode structure.Comment: 12 pages, 7 figure
Simulation study of pressure and temperature dependence of the negative thermal expansion in Zn(CN)(2)
12 pages, 16 figures12 pages, 16 figures12 pages, 16 figures12 pages, 16 figure
Cardiovascular and Stride Frequency Differences During Land and Aquatic Treadmill Walking
This study examined heart rate (HR) and stride frequency (SF) values of 30 college-aged males and females during dry-land (DL) and aquatic walking (AW). Aquatic walking trials were completed in an underwater treadmill with the water depth at waist level; the water temperature (31℃ ± 0.1℃) and room temperature (26.6℃ ± 0.1℃) were maintained at thermoneutral levels throughout the study. During each walking condition, HR and SF were recorded at treadmill speeds of 1 mph, 2 mph, and 3 mph. Participants were instructed to walk with their hands at their sides swinging as they would when walking on dry-land unless they felt the need to use the handrails to steady themselves. Heart rate monitors were used to record cardiovascular changes, and strides were measured from consecutive left and right toe strikes. Results of the study indicated HR was significantly higher during DL than AW at 1 mph (p \u3c .001) and 3 mph (p \u3c .001) but was not significantly different (p = .64) at 2 mph. The SF of the participants was significantly lower (p \u3c .001) during AW than DL at all speeds. When comparing sex, females had significantly higher HR at 1 mph (p = .012), 2 mph (p = .007), and 3 mph (p \u3c .001) than males for DL conditions. No differences in HR were found during the AW conditions (F = 0.66, p = 0.44, ηp2 = 0.02). No differences in SF were observed between males and females in both DL (F = 2.96, p = 0.06, ηp2 = 0.09) and AW (F = 1.03, p = 0.32, ηp2 = 0.036) conditions. As a result, AW increased HR values similar to those of DL, but without the added stress to the lower extremities due to the buoyancy of the water. Thus, AW provided an exercise medium capable of meeting the ACSM intensity guidelines for PA and allowed adults to be physically active presumably with less stress on the lower body
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Published November 1976. Facts and recommendations in this publication may no longer be valid. Please look for up-to-date information in the OSU Extension Catalog: http://extension.oregonstate.edu/catalo
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