1. Biophysics Group, Institut für Molekulare Medizin und Zellforschung, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 9, D-79104 Freiburg, Germany, Laboratory of Molecular Biology and Biochemistry, The Rockefeller University, New York, New York 10021, and Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel