1. Max-Planck-Institut für Biophysikalische Chemie, Abteilung Molekulare Biologie, Am Fassberg 11, D-37077 Göttingen, Germany, Department of Physics and Laboratory for Fluorescence Dynamics, Loomis Laboratory of Physics, 1110 West Green Street, Urbana, Illinois 61801-3080, and Department of Biophysics and Cell Biology, Medical University of Debrecen, H-4012 Debrecen, Hungary