1. Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany, Division of Physics and Astronomy, Faculty of Sciences and Institute of Molecular Biological Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, and A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow 119899, Russia