The use of self-assembly processes to date has focused on developing static structures. But dynamic self-assembling systems widely prevalent in nature – those that develop local spatial and temporal organizations to perform time-dependent tasks – offer a tremendous opportunity to devise entirely new classes of complex functional materials with dynamic properties and spatial patterns. Many of nature’s high performance materials are complex heterogeneous mixtures of widely disparate molecular building blocks, organized hierarchically through the nanometer scale to the micrometer scale and above and temporally at vastly different time-scales. This spatio-temporal mode of organization in biological systems, which we term dynamic self-assembly, produces a rich variety of biological functions. We are fascinated by two such classes of spatial and temporal self-assemblies in biomolecules.
First, a typical biological membrane, a 7-9 nm thick bimolecular layer, is a multi-component complex fluid composed of a phospholipid matrix phase with a number of secondary functional components including glycolipids, membrane proteins, and ion channels. Complex dynamics reflecting coupling between membrane chemical composition, molecular structure, and physical interactions lead to an impressive set of time-dependent functions, including active and passive ion transport, molecular recognition, and energy transduction. To facilitate such functions, membrane architectures organize into functional sub-structures. Lipid rafts, in this regard, represent an extraordinarily powerful example of dynamic self-assembly wherein structure-function relations exhibit a strong depedence on dynamics. They are structurally, chemically, and functionally distinct lipid and proteo-lipidic domains which emerge spontaneously within the cell membrane media and provide molecular environment suitable for a host of signaling functions (e.g., molecular recognition, energy transduction, and transport). Second, in the same vein, biomineralization exemplifies an extraordinarily powerful biosynthetic tool for hierarchical self-assembly: it is a process in which nucleation, growth, and the final morphologies of crystallizing inorganic solids are constrained and directed by the organic phase templates. Examples include bone, cartilage, and shells. Here, the nanoscale (<10 nm) supramolecular assemblies of lipids and proteins template the crystallization of calcium carbonate as lamellar aragonites at a much larger scale (<0.5 μm) in single nanolaminated macroscopic structures. These natural and hierarchically templated mode of materials fabrication challenges the view that materials synthesis is confined to traditional thermodynamic “condensed matter” phases. Rather, they offer a rich, new approach of “designed synthesis”. Implicit in this approach is a shift in emphasis from thermodynamic to kinetic regimes in which equilibrium structures (global minima) based on single length scales (e.g., unit cell dimensions in single crystals) are replaced by higher-order organizational states (local minima) of consolidated matter. Exploring this theme of hierarchical self-assembly in natural systems for organizing nanoscale building blocks constitutes the conceptual framework of our research program.
We believe that understanding and using these principles will lead to new, multi-length scale organic-inorganic hybrid materials that perform advanced functions for nanotechnology (e.g., photonic, electronic, and fluidic nanostructures), separations (e.g., nanofiltering, waste encapsulation, and bioremediation), and biotechnology (e.g., biomedical implants with porosity, drug carriers, and viral and DNA delivery vectors), and also allow control of membrane protein assembly (which will help in establishing structural foundations of biochemical recognition events, e.g., host-pathogen interactions).
Within this broad intellectual framework, our current work is focused on several parallel fronts.