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David A. Tirrell
Ross McCollum-William H. Corcoran Professor and Professor of Chemistry and Chemical Engineering
Chair, Division of Chemistry and Chemical Engineering

Research in the Tirrell group combines organic, biological, and materials chemistry to make new polymeric systems of controlled molecular and supramolecular architectures. Two kinds of systems are under active investigation: artificial proteins made by expression of artificial genes in microbial cells, and flexible polymeric nanowires and nanotubes made by a membrane templating approach. In each case, investigators are concerned not only with architectural control but also with the functional properties of the macromolecular system of interest.

Artificial proteins represent a new class of macromolecular materials that bridge the gap that has traditionally separated natural polymers from their synthetic counterparts. While synthetic polymers are interesting and enormously important, their utility derives in large part from their physical properties; chemists have yet to capture in synthetic polymers the more subtle catalytic, informational, and transduction properties of proteins and nucleic acids. The reason for this distinction may lie in the levels of architectural control to be found in each class of polymers; proteins and nucleic acids are characterized by defined lengths, sequences, and stereochemistries, while synthetic polymers are highly heterogeneous molecular mixtures. This raises interesting questions regarding the kinds of materials science that could be done if new macromolecular architectures could be created with precise control of the most important structural variables.

Microbial expressions of artificial genes provides a means of doing just that. The process begins with molecular design--the specification of a chain structure that the investigator believes will exhibit interesting (and perhaps useful) behavior. Th target structure is then encoded into an artificial gene, and the gene is expressed in an appropriate microbial host. Current targets include novel liquid crystal phases, macromolecular surface arrays, reversible hydrogels, and artificial extracellular matrices for use in tissue regeneration and repair. An important theme of all of these projects is the development of methods for efficient incorporation of new monomers (beyond the twenty "normal" amino acids) into artificial proteins in vivo.

The second program under development in the Tirrell group is directed toward fabrication of nanometer-scale wires, networks, and tubes. The approach involves patterning of fluid lipid bilayer membranes via micromanipulation, followed by photopolymerization and crosslinking of macromonomers confined by the membrane template. The method offers substantial advantages in comparison with other patterning techniques, in that it yields flexible structures that can be manipulated readily in three dimensions. Current efforts are directed toward development of new patterning chemistries and toward new methods for introduction of controlled electronic, mechanical, and transport properties.