1. Contribution from the University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, United Kingdom, Centre for Protein Engineering, Hills Road, Cambridge CB2 2HQ, United Kingdom, Departments of Chemistry and Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and Laboratory of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Universitätstrasse 16, CH-8092 Zürich, Switzerland