1. National Institute for Medical Research, The Ridgeway, London NW7 1AA, U.K., Laboratory of Biological Chemistry, Medical School, University of Ioannina, Ioannina 45110, Greece, Research Group for Molecular and Structural Biophysics, Max-Planck-Institute for Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany, and Dipartimento di Chimica, University of Naples, via Cinthia 45, 80126 Napoli, Italy