1. Department of Chemistry and Biochemistry, Ludwig-Maximilians-Universität München, Butenandtstrasse 11, D-81377 München, Germany, and BioMolecular Optics and Munich Center for Integrated Protein Science CIPSM, Fakultät für Physik, Ludwig-Maximilians-Universität München, Oettingenstrasse 67, D-80538 München, Germany