1. Department of Chemistry, University of Siena, via Aldo Moro, 53100 Siena, Italy, the Max-Volmer Laboratory for Biophysical Chemistry, Technical University, Berlin, Germany, the Max-Planck-Institute for Bioinorganic Chemistry, Mülheim, Germany, and Instituto de Biotecnologia, UNAM, Apartado Postal 510-3, Cuernavaca, Morelos 62250, Mexico