1. Contribution from the Biocrystallography Centre of Excellence, Department of Chemical Science, University of Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy, Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6059A, and Department of Chemistry, University of Napoli “Federico II”, Complesso Universitario Monte S. Angelo, Via Cynthia 45, I-80126 Napoli, Italy