1. Institute of Organic Chemistry, Chemisches Zentral-Institut, University of Tübingen, Auf der Morgenstelle 18, 72076 Tuebingen, Germany, Department of Experimental Physics 5 (Biophysics), University of Würzburg, Am Hubland, 97074 Würzburg, Germany, and Department of Bioengineering, Penn State University, 315 Hallowell Building, University Park, Pennsylvania 16802