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1.
Charles W. Wolgemuth Raymond E. Goldstein Thomas R. Powers 《Physica D: Nonlinear Phenomena》2004,190(3-4):266-289
Certain bacteria form filamentous colonies when the cells fail to separate after dividing. In Bacillus subtilis, Bacillus thermus, and Cyanobacteria, the filaments can wrap into complex supercoiled structures as the cells grow. The structures may be solenoids or plectonemes, with or without branches in the latter case. Any microscopic theory of these morphological instabilities must address the nature of pattern selection in the presence of growth, for growth renders the problem nonautonomous and the bifurcations dynamic. To gain insight into these phenomena, we formulate a general theory for growing elastic filaments with bending and twisting resistance in a viscous medium, and study an illustrative model problem: a growing filament with preferred twist, closed into a loop. Growth depletes the twist, inducing a twist strain. The closure of the loop prevents the filament from unwinding back to the preferred twist; instead, twist relaxation is accomplished by the formation of supercoils. Growth also produces viscous stresses on the filament which even in the absence of twist produce buckling instabilities. Our linear stability analysis and numerical studies reveal two dynamic regimes. For small intrinsic twist the instability is akin to Euler buckling, leading to solenoidal structures, while for large twist it is like the classic writhing of a twisted filament, producing plectonemic windings. This model may apply to situations in which supercoils form only, or more readily, when axial rotation of filaments is blocked. Applications to specific biological systems are proposed. 相似文献
2.
In quasi-steady operation, convection currents in a Bridgmandevice, used for producing a semi-conductor crystal, createinhomogeneities that may make the crystal unusable. It has oftenbeen suggested that additional forces due to rotation or magnetismmight be efficacious in reducing the segregation of the elementsof the alloy. It has been found that, over a wide range of rotationrates, there is no improvement in performance due to rotationabout the vertical axis. However, numerical results that havebeen obtained previously (Lee & Pearlstein, J. Crys. Growth240, 2002) indicate that, when effects of centrifugal buoyancyare introduced, a substantial reduction in segregation is achieved.In the work reported here, by contrast, in which we extend previouslarge-Rayleigh-number asymptotic analysis to include centrifugalbuoyancy, we find no improvement in radial segregation, butrather increasing segregation with increasing rotation rate. 相似文献
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R. Sridhar P. C. Beaumont E. L. Powers 《Journal of Radioanalytical and Nuclear Chemistry》1986,101(2):227-237
The technique of spin trapping with nitrone spin traps nas gained wide acceptance as a method for estimating·OH yields in ESR studies. In our study, fast optical kinetic techniques applied to a series of these traps (PBN, 2-PyBN, 3-PyBN, 4-PyBN, 3-PyOBN and 4-PyOBN) reveal relaxation spectra that indicate two absorption maxima with different time constants, with all except 4-PyOBN showing second order behavior. These two spectral regions show different kinetics. Thus, two reaction sites are indicated, only one of which is necessarily a measure of initial · OH when ESR methods are used. One other trap (DMPO) after · OH reaction decays in one mode suggesting that its final product might be useful as a measure of initial · OH. Also, our ESR evidence shows that OH detection can be improved significantly by spin trapping -hydroxyalkyl radicals formed by · OH attack on alcohols. 相似文献
7.
Deechongkit S Powers ET You SL Kelly JW 《Journal of the American Chemical Society》2005,127(23):8562-8570
Low molecular weight peptidomimetics with simple amphiphilic sequences can help to elucidate the structures of cross beta-sheet assemblies, such as amyloid fibrils. The peptidomimetics described herein comprise a dibenzofuran template, two peptide strands made up of alternating hydrophilic and hydrophobic residues, and carboxyl termini, each of which can be varied to probe the structural requirements for beta-sheet self-assembly processes. The dibenzofuran template positions the strands approximately 10 A apart, allowing corresponding hydrophobic side chains in the strands to pack into a collapsed U-shaped structure. This conformation is stabilized by hydrophobic interactions, not intramolecular hydrogen bonds. Intermolecular stacking of the collapsed peptidomimetics, enabled by intermolecular hydrogen bonding and hydrophobic interactions, affords 25-27 A wide protofilaments having a cross beta-sheet structure. Association of protofilaments, mediated by the dibenzofuran substructures and driven by the hydrophobic effect, affords 50-60 A wide filaments. These widths can be controlled by changing the length of the peptide strands. Further assembly of the filaments into fibrils or ribbons can be controlled by modification of the template, C-terminus, and buffer ion composition. 相似文献
8.
[reaction: see text] The gas-phase thermal isomerization of cis,anti,cis-tricyclo[6.3.0.0(2,7)]undec-3-ene (1) to endo-tricyclo[5.2.2.0(2,6)]undec-8-ene (2) at 315 degrees C occurs cleanly through a symmetry-forbidden [1,3] suprafacial,retention (sr) pathway. 相似文献
9.
Four-component equilibria in substituted 1,3-dioxanes were applied to the determination of conformational energies not accessible by conventional equilibration, with the following conclusions: 1. The difference in free energy between the chair and twist forms of 2,2,trans - 4,6 - tetramethyl - 1,3 -dioxane is 7·4 kcal/mol. 2. Equatorial Me substituents at C-4,6 exert a palpable buttressing effect on the corresponding axial substituents. 3. Equatorial substituents at C-2 exert a similar buttressing effect on the geminal axial substituent. 4. The effect of equatorial t-Bu substitution or gem-dimethyl substitution at C-5 on conformational energy seems to be of minor importance. The more complex effects of equatorial 4-t-Bu substitution are discussed. 相似文献
10.
Petrassi HM Johnson SM Purkey HE Chiang KP Walkup T Jiang X Powers ET Kelly JW 《Journal of the American Chemical Society》2005,127(18):6662-6671
Transthyretin (TTR) amyloidogenesis requires rate-limiting tetramer dissociation and partial monomer denaturation to produce a misassembly competent species. This process has been followed by turbidity to identify transthyretin amyloidogenesis inhibitors including dibenzofuran-4,6-dicarboxylic acid (1). An X-ray cocrystal structure of TTR.1(2) reveals that it only utilizes the outer portion of the two thyroxine binding pockets to bind to and inhibit TTR amyloidogenesis. Herein, structure-based design was employed to append aryl substituents at C1 of the dibenzofuran ring to complement the unused inner portion of the thyroxine binding pockets. Twenty-eight amyloidogenesis inhibitors of increased potency and dramatically increased plasma TTR binding selectivity resulted. These function by imposing kinetic stabilization on the native tetrameric structure of TTR, creating a barrier that is insurmountable under physiological conditions. Since kinetic stabilization of the TTR native state by interallelic trans suppression is known to ameliorate disease, there is reason to be optimistic that the dibenzofuran-based inhibitors will do the same. Preventing the onset of amyloidogenesis is the most conservative strategy to intervene clinically, as it remains unclear which of the TTR misassembly intermediates results in toxicity. The exceptional binding selectivity enables these inhibitors to occupy the thyroxine binding site(s) in a complex biological fluid such as blood plasma, required for inhibition of amyloidogenesis in humans. It is now established that the dibenzofuran-based amyloidogenesis inhibitors have high selectivity, affinity, and efficacy and are thus excellent candidates for further pharmacologic evaluation. 相似文献