1. Dept. of Structural Biology; Weizmann Institute of Science; Rehovot 76100 Israel
2. Dept. of Physics; University of Wisconsin-Madison; 1150 University Avenue Madison WI 53706 USA
3. Dept. of Chemistry; University of Wisconsin-Madison; 1101 University Avenue Madison WI 53706 USA
4. Dept. of Biomaterials; Max Planck Institute of Colloids and Interfaces; 14424 Potsdam Germany