1. Department of Chemistry, The Scripps Research Institute and the Skaggs Institute for Chemical Biology, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.
2. Department of Chemistry, University of California Berkeley, Berkeley, CA 94720–1460, USA.
3. Department of Molecular Biology, The Scripps Research Institute, 10555 North Torrey Pines Road, La Jolla, CA 92037, USA.
4. Institute of Physical and Theoretical Chemistry, Humboldt-University Berlin, Bunsenstrasse 1 D-10117 Berlin, Germany.