1. Department of Molecular and Cellular Biochemistry, Department of Chemistry, Kentucky Center for Structural Biology, and Department of Pharmaceutical Sciences, University of Kentucky, Lexington, Kentucky 40536-0084, Medical Biotechnology Center, University of Maryland Biotechnology Institute, and Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, and Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710