Dimensions
  • Issue 2  |
  • January 2010

Computer-Aided Design of Self-Assembled Cage Tailor-Made for Sulfate

As recently reported in Angewandte Chemie and highlighted in the Chemistry World magazine (http://www.rsc.org/chemistryworld/News/2009/April/27040901.asp) computer-aided design can effectively guide the construction of self-assembled nanoscale containers with predetermined functionality. Specifically, with the help of molecular modeling, a cage receptor functionalized for optimal encapsulation and efficient sequestration of sulfate from water was efficiently crafted. Sulfate sequestration from aqueous solutions is extremely challenging due to the strong interactions of this anion with water molecules. The designer cage receptor developed in this study proved to be the strongest synthetic sulfate binder known to date, and on a par with sulfate-binding protein. The exceptional sulfate binding observed was made possible by precise positioning of functional groups inside the cage cavity, guided by computer design, which resulted in an ideal environment for this anion.

Superheavies

Previous molecular modeling and structural studies established that sulfate can ideally accept 12 hydrogen bonds from six urea groups arranged tetrahedrally around the anion. Reasoning that a self-assembled cage could provide this arrangement if the urea groups acted as edges, and Ni(bpy)3 (bpy = 2,2’-bipyridine) as vertices of a tetrahedron, the challenge was  to identify molecular linkages that would hold the assembly together. This is where HostDesigner, a homegrown software package, proved invaluable. Using a virtual library of molecular fragments, the software generated within minutes more than 270,000 possible structures, which were screened down to a handful of candidates that were able to form stable cages. One of the cages was selected for synthesis, and X-ray structural analysis subsequently confirmed the encapsulation of sulfate, as predicted by the model (see figure: yellow–predicted; blue–experimental).

This basic research has potential implications to environmental decontamination and nuclear waste cleanup, touching on the specific need for technologies to remove sulfate from nuclear wastes prior to vitrification into borosilicate glass. In a broader and more fundamental context, these results significantly advance the areas of nanoscience and self-assembly by demonstrating that (i) nanoscale architecture and function can be predetermined with an unprecedented level of control, and (ii) computer-aided design can be used as a powerful tool in the pre-selection of molecular candidates for the assembly of nanostructures, thereby replacing years of tedious trial-and-error experimentation.

Radu Custelcean, Jerome Bosano, Peter V. Bonnesen, Vilmos Kertesz, Benjamin P. Hay, Angew. Chem. Int. Ed. 2009, 48, 4025-4029.

This research was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.

Using microscopic density functional theory, nuclear theorists from the University of Tennessee/Oak Ridge National Laboratory carried out calculations [1,2] showing that fission barriers of excited superheavy nuclei vary rapidly with particle number, pointing to the importance of shell effects even at large excitation energies. The results are consistent with recent experiments where superheavy elements were created by bombarding an actinide target with 48-calcium; yet even at high excitation energies, sizable fission barriers remained. Not only does this reveal clues about the conditions for creating new elements, it also provides a wider context for understanding other types of fission, such as that used in reactors to provide energy.

[1]   “Fission barriers of compound superheavy nuclei,”J.C.Pei, W.Nazarewicz, J.A. Sheikh,  and A.K. Kerman, Phys. Rev. Lett. 102, 192501 (2009).

[2]   “Systematic study of fission barriers of excited superheavy nuclei,” J.A. Sheikh, W. Nazarewicz, and J.C. Pei, Phys. Rev. C 80, 011302(R) (2009).

This work was supported in part by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through Grant DE-FG03-03NA00083 and by the US Department of Energy, Nuclear Physics, under Contract Nos.  DE-FG02-96ER40963 (University of Tennessee), DE-AC05-00OR22725 with UT-Battelle, LLC (Oak Ridge National Laboratory), and DE-FC02-07ER41457 (UNEDF SciDAC Collaboration).