1. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Department of Chemistry, Iowa State University, Ames, Iowa 50011, Department of Chemistry and Molecular Biology and Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California 92037, and NMR Facility, Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011