1. Contribution from the Centre of Excellence in Biocrystallography, Department of Chemical Science, University of Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy, Department of Physics and Astronomy, and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia Pennsylvania 19104, and Department of Chemistry, University of Napoli “Federico II”, Complesso Universitario Monte S. Angelo, Via Cynthia 45, I-80126 Napoli, Italy