1. Department of Chemistry, University of York, Heslington, York YO1 5DD, England, European Molecular Biology Laboratory (EMBL), DESY, Notkestrasse 85, 22603 Hamburg, Germany, and Centre de Recherche en Microbiologie Appliquée, Institut Armand-Frappier, Université du Québec, Laval-des-Rapides, C. P. 100, Québec, Canada