1. M. Lewis and G. Chang are in the Johnson Research Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104, USA.
2. N. C. Horton, M. A. Kercher, H. C. Pace, and P. Lu are in the Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA.
3. M. A. Schumacher and R. G. Brennan are in the Department of Biochemistry and Molecular Biology, Oregon Health Sciences University, Portland, OR 97201, USA.