1. Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch Strasse, D-35043 Marburg, Germany, Department of Chemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, Georgia 30602-2556, and Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, J. Goethe University of Frankfurt, D-60438 Frankfurt, Germany