1. Department of Chemistry, The Scripps Research Institute and the Skaggs Institute for Chemical Biology, 10550 N. Torrey Pines Road, La Jolla, California 92037, Department of Chemistry, University of CaliforniaBerkeley, Berkeley, California 94720-1460, and Department of Molecular Biology, The Scripps Research Institute, 10555 N. Torrey Pines Road, La Jolla, California 92037