1. Department of Chemistry and Vanderbilt Institute of Chemical Biology, Vanderbilt University, 4226 Stevenson Center, Nashville, Tennessee 37235, Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 N. Torrey Pines Rd., La Jolla, California 92037, and Department of Chemistry and Biochemistry and Center for Bioinspired Nanomaterials, Montana State University, 103 Chemistry and Biochemistry Building, Bozeman, Montana 59717