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The first three minutes: smooth muscle contraction, cytoskeletal events, and soft glasses

admin — Mon, 08 Mar 2010 06:02:30 +0000

Smooth muscle exhibits biophysical characteristics and physiological behaviors that are not readily explained by present paradigms of cytoskeletal and cross-bridge mechanics. There is increasing evidence that contractile activation of the smooth muscle cell involves an array of cytoskeletal processes that extend beyond cross-bridge cycling and the sliding of thick and thin filaments. We review here the evidence suggesting that the biophysical and mechanical properties of the smooth muscle cell reflect the integrated interactions of an array of highly dynamic cytoskeletal processes that both react to and transform the dynamics of cross-bridge interactions over the course of the contraction cycle. The activation of the smooth muscle cell is proposed to trigger dynamic remodeling of the actin filament lattice within cellular microdomains in response to local mechanical and pharmacological events, enabling the cell to adapt to its external environment. As the contraction progresses, the cytoskeletal lattice stabilizes, solidifies, and forms a rigid structure well suited for transmission of tension generated by the interaction of myosin and actin. The integrated molecular transitions that occur within the contractile cycle are interpreted in the context of microscale agitation mechanisms and resulting remodeling events within the intracellular microenvironment. Such an interpretation suggests that the cytoskeleton may behave as a glassy substance whose mechanical function is governed by an effective temperature.

mechanical plasticity; latch state; actin cytoskeleton; mechanotransduction; glass hypothesis

Susan J. Gunst1 and Jeffrey J. Fredberg2
1Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana 46202; and 2Harvard School of Public Health, Boston, Massachusetts 02067

The origin of the boson peak and thermal conductivity plateau in low-temperature glasses

admin — Mon, 08 Mar 2010 06:00:10 +0000

We argue that the intrinsic glassy degrees of freedom in amorphous solids giving rise to the thermal conductivity plateau and the “boson peak” in the heat capacity at moderately low temperatures are directly connected to those motions giving rise to the two-level-like excitations seen at still lower temperatures. These degrees of freedom can be thought of as strongly anharmonic transitions between the local minima of the glassy energy landscape that are accompanied by ripplon-like domain wall motions of the glassy mosaic structure predicted to occur at T g by the random first-order transition theory. The energy spectrum of the vibrations of the mosaic depends on the glass transition temperature, the Debye frequency, and the molecular length scale. The resulting spectrum reproduces the experimental low-temperature boson peak. The “nonuniversality” of the thermal conductivity plateau depends on k B T g/ℏωD and arises from calculable interactions with the phonons.

Vassiliy Lubchenko and Peter G. Wolynes†
+ Author Affiliations

Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093
Contributed by Peter G. Wolynes

Physics and chemistry of silicate glasses and melts

admin — Mon, 08 Mar 2010 05:41:45 +0000

Property and structure data from binary and ternary silicate and aluminosilicate melts and glasses often reveal simple and systematic relationships to composition and temperature. Properties whose dominant structural control is the abundance of fully polymerized Q4-species, exhibit smooth variations with silica content. Such properties include activation energies associated with transport. Properties that depend on the type and abundance of coexisting structural species in depolymerized melts, either show smooth functional relationships to melt polymerization, NBO/T, or exhibit pronounced minimum or maximum values at intermediate NBO/T-values. The former properties include those, which are related to the availability of configurational states. The latter group includes those whose behavior is controlled by the energetics of mixing of the individual structural species. An example is enthalpy of mixing. Crystal-liquid element partition coefficients also exhibit behavior governed by thermodynamics of mixing of the structural species of the melt.

Aluminum in most silicate melts is in tetrahedral coordination at ambient pressure and substitutes for silicon. The Al3+ <=> Si4+ substitution lowers (Si,Al)-O-(Si,Al) bond energy. In principle, the value of properties that depends on bridging oxygen bond strength, decreases, therefore, with increasing Al/(Al+Si). Properties that depend on the proportion of polymerized and depolymerized Qn-species in the melt can show minima or maxima in relationships to Al/(Al+Si) because the abundance of the Qn-species in depolymerized silicate melts depends on the bulk melt Al/(Al+Si). Transport properties fall in this category. It is possible that Al-distribution between Qn-species in depolymerized aluminosilicate melts depends on temperature. It is likely, therefore, at least for high-temperature viscosity, its temperature-dependence is non-Ahrrenian. Other melt transport properties that are functionally related to viscous flow (e.g., diffusivity, electrical, and thermal conductivity) may also show non-Ahrrenian temperature-dependence at high temperature.

Bjorn MYSEN*

Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., NW, Washington DC 20015, USA

* E-mail: mysen@gl.ciw.edu

Key-words: silicate melt, structure, spectroscopy, rheology, thermodynamics.

Determination of molar absorptivity of IR fundamental OH-stretching vibration in rhyolitic glasses

admin — Mon, 08 Mar 2010 05:41:02 +0000

Molar absorptivity of the infrared (IR) fundamental OH-stretching vibration band at 3550 cm–1 was determined for rhyolitic glasses. Five obsidian samples, unheated and heated at 500–700 °C using an internally heated pressure vessel, were used to evaluate the dependence of the molar absorptivity and final quenched H2O speciation on H2O contents and temperature. Water contents of the obsidians were measured by Karl-Fischer titration first, then the amount of unextracted H2O was calibrated by IR spectroscopy and a conventional vacuum extraction method. Total H2O contents of the obsidians were determined to be 0.24–1.25 wt%. IR spectra of the unheated and heated obsidian samples were obtained using an FT-IR microspectrometer. We determined the molar absorptivity for the 3550 cm–1 band to be 75 ± 4 L/mol/cm without significant dependence on the H2O contents and heating temperature. This value can be used to determine precise H2O contents up to 1.25 wt% in rhyolitic volcanic glasses.

Satoshi Okumura1,*, Michihiko Nakamura2 and Satoru Nakashima1
1 Interactive Research Center of Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguroku, Tokyo 152-8551, Japan
2 Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Aobaku, Sendai 980-8578, Japan

Correspondence: * E-mail: sokumura@geo.titech.ac.jp

Evidence for rhenium enrichment in the mantle wedge from submarine arc–like volcanic glasses (Papua New Guinea)

admin — Mon, 08 Mar 2010 05:40:09 +0000

AbstractThe low Re abundance in arc-type volcanic rocks characterized by high 187Os/188Os ratios is an unsolved puzzle of the 187Re-187Os isotope system, leaving a significant gap in our understanding of the evolution of the upper mantle–continental crust system. Here we report new observations of high Re concentrations in fresh, submarine-erupted—i.e., relatively undegassed—island arc–like volcanic glasses dredged from the eastern Manus Basin, offshore Papua New Guinea. These observations, together with previously published reports of high Re concentrations in arc-type melt inclusions, indicate that undegassed arc-type volcanic rocks and the mantle wedge are enriched in Re. Consequently, the Re concentration in the continental crust is likely to be as high as ∼2 ppb, much higher than previously estimated. The low Re concentrations in subaerial arc-type volcanic rocks are probably due to Re loss during magma degassing.

Weidong Sun*1, Richard J. Arculus*2, Vickie C. Bennett*3, Stephen M. Eggins*3 and Raymond A. Binns*4
+ Author Affiliations

1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2Department of Geology, Australian National University, Canberra, ACT 0200, Australia
3Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
4Commonwealth Scientific and Industrial Research Organisation, Exploration and Mining, North Ryde, NSW 1670, Australia

Titanium coordination in silicate glasses investigated using O K-edge X-ray absorption spectroscopy

admin — Mon, 08 Mar 2010 05:38:54 +0000

A series of titanium silicate glasses along the composition joins TiO2–SiO2, TiO2–Na2SiO3, TiO2–K2SiO3 and TiO2–CaSiO3, has been examined using oxygen K-edge X-ray absorption near edge structure spectroscopy (XANES) confined to the near-surface region. Sharp pre-edge features in the spectra can be used to determine the Ti coordination in the glasses. The presence of [4]Ti is indicated by a pre-edge peak at ~534 eV while [5]Ti is indicated by a peak at ~533 eV. Titanium exists in all these glasses as [4]Ti and [5]Ti with no [6]Ti being present. For alkali-containing glasses the [5]Ti site becomes more prevalent with increasing TiO2. TiO2-K2SiO3 compositions contain a greater proportion of [4]Ti than comparable Na2O compositions. This is consistent with earlier Ti L-edge findings. The TEY spectra for the TiO2-CaSiO3 compositions indicate the presence of significant amounts of [5]Ti at high TiO2 contents; however, comparison of TEY (near surface) and FY (bulk sample) XANES shows that the [5]Ti is confined to the surface of the glass sample with the bulk of the glass containing [4]Ti.

KEYWORDS: Ti, XANES, silicate glasses

G. S. Henderson1,*, Xiaoyang Liu2 and M. E. Fleet2
1 Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada
2 Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada

* E-mail: henders@geology.utoronto.ca