1. Departments of Chemistry and Biology, Indiana University, Bloomington, Indiana 47405, Institute of Crystallography, Russian Academy of Sciences, 59 Leninsky pr., 117333 Moscow, Russia, European Molecular Biology Laboratory, Outstation, Notkestrasse 85, D-22603 Hamburg, Germany, Department of Chemistry, Dalhousie University, Halifax, NS, B3H 4J3 Canada