1. Medicinal Chemistry, Discovery BioScience, Molecular Biology and Physical and Metabolic Sciences Departments, AstraZeneca R&D Charnwood, Bakewell Road, Loughborough, Leics LE11 5RH, U.K., AstraZeneca Structural Chemistry Laboratory, AstraZeneca R&D Mölndal, 431 83 Mölndal, Sweden, and Department of Molecular Biology and Skaggs Institute for Chemical Biology, Scripps Research Institute, La Jolla, San Diego, California 92037