1. Contribution from the Department of Physics and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, Lehrstuhl fuer Biophysik − E22, Technische Universitaet Muenchen, Germany, Los Alamos National Laboratory, Chemistry Division, Los Alamos, New Mexico 87545, and The Nanotechnology Lab, Kraft Foods R&D, 801 Waukegan Road, Glenview, Illinois 60025