1. Contribution from the Max-Planck Institut für Bioanorganische Chemie, D-45470 Mülheim an
der Ruhr, Germany, Department of Biochemistry and Molecular Biology, The Pennsylvania State
University, University Park, Pennsylvania 16802, Institut für Physikalische und Theoretische
Chemie, Universität Bonn, D-53115 Bonn, Germany, and Department of Chemistry,
The Pennsylvania State University, University Park, Pennsylvania 16802