1. Department of Chemical Engineering, Massachusetts Institute of Technology, Building 66, 25 Ames Street, Cambridge, Massachusetts 02139, Department of Physics, University of Illinois-Urbana/Champaign, 600 S. Mathews Avenue, Urbana, Illinois 61801, and Division of Biophysics of Macromolecules, German Cancer Research Center, Im Neuenheimer Feld 580, D-69120, Heidelberg, Germany