1. J. M. Rivera, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA, and Skaggs Institute for Chemical Biology and Department of Chemistry, Scripps Research Institute, La Jolla, CA 92037, USA.
2. T. Martı́n and J. Rebek Jr., Skaggs Institute for Chemical Biology and Department of Chemistry, Scripps Research Institute, La Jolla, CA 92037, USA.