1. Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, 2205 Tech Drive, Evanston, Illinois 60208-3500, Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, and Department of Chemistry, University of North CarolinaWilmington, Wilmington, North Carolina 28403