1. Department of Chemistry, University of Georgia, Athens, Georgia 30602, Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602, Department of Molecular Biosciences, University of Kansas, Lawrence, Kansas 66045, and Cell Biology and Biochemistry Group, Biological Sciences Division, Fundamental Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352