1. Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, and Gilead Sciences Research Center at IOCB, Flemingovo nam. 2, 166 10 Praha 6, Czech Republic, Department of Theoretical Chemistry, Lund University, Chemical Center, P.O. Box 124, S-221 00 Lund, Sweden, Department of Chemistry, Stanford University, Stanford, California 94305, and Institut für Pysikalische und Theoretische Chemie, Universität Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany