IRG 2

Macromolecular Assemblies

Igal Szleifer (Leader), biomechanical engineering
Bartosz Andrzej von Poray Grzybowski, chemical and biological engineering
Monica Olvera de la Cruz, materials science & engineering
Kenneth Shull, materials science & engineering
Fraser Stoddart, chemistry
Samuel I. Stupp, chemistry
John M. Torkelson, chemical & biological engineering
Associates:
John Marko, biochemistry, molecular biology and cell biology
Guillermo Ameer, biomechanical engineering

The group studies the design and organization of functional assemblies involving polyvalent nanoparticles and macromolecules. The specific goals of the group are:

1) To understand macromolecular assembly at liquid/liquid interfaces;
2) To design and synthesize biomimetic polyelectrolyte gels;
3) To exploit electrostatics in the design of novel supramolecular and nanoscopic systems.

            The group exploits the coupling between chemical and physical interactions in the design of novel supramolecular and nanoscopic inhomogeneous systems. In particular, the idea is to take advantage of tuning electrostatic interactions through ionic concentrations and charge state controlled by chemical equilibrium shifts, such as acid-base or redox states. This type of coupling is particularly sensitive to the molecular organization and therefore to the presence of surface and interfaces.
The effort on liquid/liquid interfaces builds on our recent demonstration that functional assemblies with mechanical and transport properties relevant to biotechnology can be formed at the interface between aqueous solutions of positively and negatively charged molecules.  The hierarchical structure of these assemblies is a necessary aspect of the formation process, because it enables structures with thicknesses far greater than an individual bilayer to be formed.  Characterization of these assemblies and analysis of their dynamics at the molecular level is necessary to exploit their properties. The principles that allow robust assembly of macroions at immiscible oil/water interfaces are also investigated.  A theme of emerging importance is the role of counterion release in the interface assembly process, as well as the use of charge accumulation at interfaces to design new membranes by using a variety of ionic constituents and liquids. 
The group also designs, synthesizes and analyzes polymer assemblies that are able to reorganize their structures to respond to external stimuli. The goal is to exploit the properties of biomimetic assemblies capable of regulating their effective charge via pH changes. One of the objectives is to design and understand the physical properties of the anionic polyelectrolyte assembly with cationic molecules. The project seeks to design new materials with functions resembling those of biological gels such as chromosomes. Theoretical models and simulations of associating charged chains and proteins in complex electrolyte environments that are relevant to chromosome structure are developed to fabricate model synthethic polyelectrolyte gels that mimic these functions. 

            The overriding goal of the work on macromolecular machines is to make these systems operational in aqueous environments. The key for this extension (from presently used organic media) is to understand and control the role of counterions mediating the host-guest non-covalent interactions. Two types of systems are considered: Doubly Bistable Catenane Switches, which can perform relative circumrotational movements in aqueous solutions using orthogonal external stimuli. Systematic work is carried out on the thermodynamics and kinetics of these systems. Water-based molecular switches will be incorporated into macromolecular and nanoparticle-based scaffolds in order to explore their application in material science and biological systems. The team has collaborated on the incorporation of molecular switches into electrostatic self-assembly (ESA) schemes leading to switchable nanostructured materials. Of particular interest are assemblies of nanoparticles that can assemble and fall-apart under different red-ox conditions. These constructs can be used for targeted delivery to loci differing in electrochemical potentials.