1. Department of Chemical Engineering and Helen Wills Neuroscience Institute, University of California, Berkeley, California 94720
2. Division of Molecular and Cellular Biology, Beckman Research Institute of City of Hope, Duarte, California 91010
3. Department of Bioengineering, University of California, Berkeley, California 94720
4. Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720