1. Institute of Medical Physics and Biophysics, University of Leipzig, Liebigstrasse 27, D-04103 Leipzig, Germany, Institute of Experimental Physics I, University of Leipzig, Linnèstrasse 5, D-04103 Leipzig, Germany, and Institute of Molecular Biology, Friedrich-Schiller-Universität Jena, Winzerlaer Strasse 10, D-07745 Jena, Germany