1. Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc, tr. 17. listopadu 12, 771 46 Olomouc, Czech Republic, Institute of Biophysics, Academy of Sciences of the Czech Republic, Kralovopolska 135, 612 65 Brno, Czech Republic, and Department of Chemistry, Single Molecule Analysis Group, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055