1. Medical University of South Carolina, Department of Pharmaceutical Sciences, 171 Ashley Avenue, Charleston, South Carolina 29425-2303, Institut für Organische Chemie der Universität, Tammannstr. 2, D-37077 Göttingen, Germany, and Departamento de Biología Funcional e Instituto Universitario de Biotecnologia de Asturias, Universidad de Oviedo, E-33006 Oviedo, Spain