1. Departments of Biochemistry and Chemistry, Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, Center for Structural Biochemistry, Department of Biosciences at Novum, Karolinska Institutet, SE-141 57 Huddinge, Sweden, Institute of Environmental Medicine, Karolinska Institutet, S-171 77 Stockholm, Sweden