1. Contribution from the Departments of Biochemistry & Molecular Biology, Indiana University School of Medicine, and Chemistry & Chemical Biology, Purdue School of Science, Indiana University-Purdue University Indianapolis (IUPUI), Indianapolis, Indiana 46202, Department of Chemistry, Georgia State University, P.O. Box 4098, Atlanta, Georgia 30302-4098, and the Department of Pathology, University of North Carolina, Chapel Hill, North Carolina 27599