1. Contribution from the Research Institute of Molecular Pharmacology FMP, Robert-Rössle-Strasse 10, 13125 Berlin, Germany, Department of Biophysical Chemistry, Biocenter of the University of Basel, Klingelbergstrasse 50/70, 4056 Basel, Switzerland, and Institute of Physical Chemistry, Martin Luther University Halle−Wittenberg, Mühlpforte 1, 06108 Halle, Germany