1. Department of Physics and Materials Science Institute, University of Oregon, Eugene, Oregon 97403
2. Department of Chemistry, University of California, Berkeley, California 94720
3. Department of Molecular and Cell Biology, University of California, Berkeley, California 94720
4. Physical Bioscience and Materials Science Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Howard Hughes Medical Institute, University of California, Berkeley, California 94720