1. Contribution from the Center in Molecular Toxicology and Vanderbilt Institute of Chemical Biology, Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, D-28759 Bremen, Germany, Department of Chemistry and Centre for Biotechnology, Brock University, St. Catharines, Ontario L2S 3A1, Canada, and Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada