1. Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
2. Institut de Chimie, UMR 7177, Laboratoire de Spectroscopie Vibrationnelle et Electrochimie des Biomolécules, Université de Strasbourg, 67070 Strasbourg, France
3. Laboratorio di Biochimica e Biofisica, Dipartimento di Biologia, Università di Bologna, 40126 Bologna, Italy
4. Laboratory of Bioinorganic Chemistry, University of Bologna, 40127 Bologna, Italy, and Center for Magnetic Resonance, University of Florence, Florence, Italy