Michael P Ledbetter1, Floyd E Romesberg2. 1. Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA. Electronic address: michaell@scripps.edu. 2. Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA. Electronic address: floyd@scripps.edu.