1. Biophysical Organic Chemistry, Leiden Institute of Chemistry, Einsteinweg 55, 2300 RA Leiden, The Netherlands, Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan, and Max-Planck-Institut für Bioanorganische Chemie, Postfach 101365, D-45413 Mülheim an der Ruhr, Germany