Shape and Dimensional Precision in Polymeric Nanostructures

Northwestern University, Evanston IL

Investigators

Abstract

TECHNICAL SUMMARY: Precise dimension and shape control remains a great challenge in the design of nanoscale organic structures. This program studies two different strategies toward precise control over size and shape in supramolecular assemblies. One approach involves the use of molecular templates to truncate one-dimensional self-assembled structures, and a second one explores the use of mechanical forces to spontaneously terminate the growth of assemblies or determine their precise shape and size. In the first approach synthetic dumbbell-shaped molecules with a rigid and hydrophobic linear core are proposed as templates to dictate the length of assemblies formed by peptide amphiphiles. The hydrophobic segments of these molecules should be attracted to the template core and growth of the assembly limited by the hydrophilic termini of the template. In terms of function, it is also of interest to investigate the electronic properties of these assemblies, as well as their ability to encapsulate active molecules with reproducible stoichiometry. The second approach examines two problems, the use of supramolecular strain mechanics to define helical shape in one-dimensional nanostructures and also to define the shape of nanostructures shaped as platonic solids. The first problem utilizes charged peptide lipid molecules in which bulky groups strain the b-sheets formed when self-assembly occurs in organic solvents. Preliminary studies have shown that bulky endgroups attached to the periphery of b-sheets cause nanofibers to relax into helical nanostructures with pitch determined by sterics. The research will validate this model and also study the use of templates to obtain precise assemblies. In the second problem, the challenge is to achieve precise definition of three-dimensional structures. The goal in the program is to construct structures with non-spherical geometries similar to natural virus capsids. Mixtures of oppositely charged amphiphilic molecules containing various types of rigid and hydrophobic segments are proposed as a strategy to assemble nanoscale platonic solids such as icosahedral objects. In these objects supramolecular strains associated with curvature could lead to packing with planar facets. NON-TECHNICAL SUMMARY: Learning from the complex structure formations seen in nature has been one of the major goals of synthetic chemistry. Ultimately, this would not only allow us to design materials more effective in "human repair", but it would also enable us to construct artificial molecular machinery to carry out processes important for modern technology, for example in electronic and sensing devices. This is a subject requiring interdisciplinary research and links to other scientists around the world. The work cuts across physical sciences, life sciences, and engineering disciplines, and it is therefore an excellent platform for education of future scientists and for international collaborations. As a step toward this goal, the research proposed herein involves accurately controlling the shape and size of several self-assembling polymers previously discovered in this laboratory. This program explores two approaches toward this goal. One involves the use of molecules as external templates to dictate the dimension of the assemblies. The other approach programs assemblies through the molecular structure of building blocks to form nanostructures with non-spherical shapes similar to those acquired by viruses.