1. NE-CAT, Building 436, Argonne National Laboratory, Argonne, Illinois 60439, Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110, Division of Gastroenterology, Washington University School of Medicine, St. Louis, Missouri 63110, and Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853