1. Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, Biophysics Department, Huygens Laboratories, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands, and Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region 142290, Russian Federation