1. Department of Physics, Center for the Physics of Living Cells, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801, United States
2. Howard Hughes Medical Institute, Urbana, Illinois 61801, United States
3. Munich Center for Advanced Photonics and Center for Integrated Protein Sciences, Department of Chemistry and Biochemistry, Gene Center, Ludwig-Maximilians-University Munich, Feodor-Lynen-Strasse 25, D-81377 Munich, Germany