1. Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011, Department of Chemistry, Brock University, St. Catharines, Ontario, Canada L2S 3A1, and Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, D14195 Berlin, Germany