1. Contribution from the Department of Biochemistry, Molecular Biology, and Biophysics, and Center for Metals in Biocatalysis, University of Minnesota, 6-155 Jackson Hall, 321 Church Street SE, Minneapolis, Minnesota 55455, Department of Chemistry, Bates College, 5 Andrews Road, Lewiston, Maine 04240, and Department of Chemistry, Princeton University, Princeton, New Jersey 08544