1. CNRS, URA 2096. CEA, IBITEC, Laboratoire de Hyperfréquences, Métalloprotéines et Systèmes de Spin, F-91191 Gif-sur-Yvette, France, Dipartimento di Chimica I.F.M., Università di Torino, Torino, Italy, and Department of Microbiology, University of Manitoba, Winnipeg, MB R3T 2N2, Canada