1. Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0506, National Institute of Environmental Health Sciences, P.O. Box 12233, Research Triangle Park, North Carolina 27709-2233, and Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030