1. Departments of Structural Biology, Protein Biochemistry, Anti-Infectives, and Medicinal Chemistry, SmithKline Beecham Pharmaceuticals, 709 Swedeland Road, King of Prussia, Pennsylvania 19406, and Laboratoire d'Enzymologie & Centre d'Ingénierie des Protéines, Institut de Chimie, B6 Sart Tilman, Université de Liège, B-4000 Liège, Belgium