1. Nanoscience Center, Department of Chemistry, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
2. Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, United States
3. Nanoscience Center, Department of Physics, P.O. Box 35, FI-40014 University of Jyväskylä, Finland