1. Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-2683, Microscopy and Imaging Center, Texas A&M University, College Station, Texas 77843-3255, Institut für Zytobiologie, Philipps-Universität Marburg, Robert-Koch-Strasse 6, D-35033 Marburg, Germany, and Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843