1. Contribution from the Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands, Laboratoire de Physico-Chimie Bioinorganique, UMR 7509, Université Louis Pasteur, 1, rue Blaise Pascal, 67000 Strasbourg, France, Kluyver Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands, and Laboratoire de Cristallochimie, UMR 7513, Université Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg, France