1. Dipartimento di Scienze Farmaceutiche, Università di Pisa, via Bonanno 6, 56126 Pisa, Italy, Sbarro Institute for Cancer Research and Molecular Medicine, Center for Biotechnology, College of Science and Technology, Temple University, Philadelphia, Pennsylvania 19122, and Centro de Química Estrutural, Instituto Superior Técnico, Rua Rovisco Pais 1, 1049-001 Lisboa, Portugal