1. P. A. Patten, N. S. Gray, P. L. Yang, and P. G. Schultz are in the Howard Hughes Medical Institute, Department of Chemistry, University of California, Berkeley, CA 94720, USA.
2. C. B. Marks, G. J. Wedemayer, and R. C. Stevens are in the Department of Chemistry, University of California, Berkeley, CA 94720, USA.
3. J. J. Boniface is in the Howard Hughes Medical Institute, Department of Microbiology and Immunology, Stanford University, Stanford, CA 94305, USA.