1. Contribution from the J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, and Center for Complex Molecular Systems and Biomolecules, 182 23 Prague 8, Czech Republic, Department of Theoretical Physics, University of Paderborn, D-33098 Paderborn, Germany, and Department of Molecular Biophysics, German Research Cancer Center, D-69120 Heidelberg, Germany