1. Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences, and School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, U.K., Department of Biochemistry and Molecular Biology, Oregon Graduate Institute of Science and Technology, Beaverton, Oregon 97006-8921, Department of Chemistry, Montana State University, P.O. Box 173400, Bozeman, Montana 59717-3400, and Lehrstuhl für Mikrobiologie, Universität Fridericiana, D-76228 Karlsruhe, Germany