Robock_et_al._2010_Science[1] - PERSPECTIVES tent with a...

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29 JANUARY 2010 VOL 327 SCIENCE 530 PERSPECTIVES like, exponentially decaying attractive forces between neighboring fi laments ( 9 ). Most experimental data on nonspecific bundling interactions appear to be consistent with theoretical predictions of densely packed bundles resulting from counterion-induced attractive forces, but substantial discrepan- cies remain. Microtubules can form various bundling architectures, from tight hexagonal bundles to loose two-dimensional necklace- like morphologies with linear, branched, and loop morphologies ( 10 ) that are not predicted by theory. In contrast to the filament bundling described so far, which typically arises from attractive forces, long-range electrostatic repulsion appears to play the dominant role in inducing the formation of widely spaced, stable hexagonal fi lament bundles reported by Cui et al . The authors hypothesize that x-ray irradiation induces a reversible chemi- cal reaction, with deprotonation of carboxyl groups on glutamic acid residues leading to highly charged fi laments (see the fi gure). Their observation that the induced ordered phase occurs only above a certain x-ray dose rate rather than accumulated dose is consis- tent with a reversible switching process (see the figure). The large equilibrium spacing observed by the authors seems to result from repulsive fi laments in a confi ned geometry, reminiscent of a two-dimensional Wigner crystal [where minimization of potential energy at low concentration leads to a two- dimensional crystal ( 3 )]. Radiation-induced structural changes are usually detrimental. The term “radiation dam- age” is widely used to describe the resulting structural degradation. What is unusual and interesting in Cui et al .’s study is that x-ray irradiation induces rather than destroys order- ing. The system seems to be highly suscepti- ble to hexagonal order due to the built-in elec- trostatic repulsive force; above a critical con- centration, bundles form spontaneously with- out x-ray radiation. By increasing the charge of the fi laments, irradiation tips the transition point to much lower concentrations, where spontaneous bundling does not occur. Further studies are needed to clarify the detailed x-ray–induced ionization process responsible for the ordering observed by Cui et al . Nevertheless, the x-ray switch intro- duced by this study opens up entirely new directions in nanoscale assembly. We expect that future work will extend the discov- ery to other systems, such as other peptide- based geometric shapes, including sheets and spheres. Other new directions may involve using grazing-incidence x-ray irradiation for the controlled growth of ultrathin ordered phases±at±interfaces.± References
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This note was uploaded on 09/23/2011 for the course CHEM 380 taught by Professor Staff during the Spring '11 term at S.F. State.

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Robock_et_al._2010_Science[1] - PERSPECTIVES tent with a...

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