1. Contribution from the Georgetown University Medical Center, Institute for Cognitive and Computational Sciences, 3970 Reservoir Road NW, Washington, DC 20007-2197, Dipartimento Farmaco Chimico Tecnologico, Siena University, Banchi di Sotto 55, 53100 Siena, Italy, Department of Chemistry, Ohio State University, Columbus, Ohio 43210, and Division of Biochemistry, Walter Reed Army Institute of Research, Washington, DC 20307