1. Department of Molecular Biology TPC-15, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Johnson & Johnson Pharmaceutical Research and Development LLC, Welsh and McKean Roads, P.O. Box 776, Spring House, Pennsylvania 19477, and Theoretical Chemistry, Department of Biotechnology, Royal Institute of Technology, ALBANOVA, S-106 91 Stockholm, Sweden