1. Laboratory of Applied Physics, Faculty of Electrical Engineering, University of Ljubljana, Tržaška 25, SI-1000 Ljubljana, Slovenia, Institute of Biophysics, Faculty of Medicine, University of Ljubljana, Lipičeva 2, SI-1000 Ljubljana, Slovenia, and Institute of Molecular Biology, Friedrich-Schiller-University, Winzerlaer Strasse 10, Jena 07745, Germany