1. Department of Chemistry Northwestern University Evanston IL 60208 USA
2. Present address: Max Planck Institute for Brain Research Frankfurt am Main Germany
3. U.S. Army Research Laboratory Aberdeen Proving Grounds MD 21005 USA
4. Chemical Sciences and Engineering Division Argonne National Laboratory Argonne IL 60493 USA
5. Departments of Materials Science and Engineering Biomedical Engineering International Institute for Nanotechnology Simpson Querry Institute Chemistry of Life Processes Institute Northwestern University Evanston IL 60208 USA