1. Centro Interuniversitario di Ricerca sui Peptidi Bioattivi & Centro di Studio di Biocristallografia del C.N.R University of Naples “Federico II” Via Mezzocannone 4, I-80134 Naples, Italy Ecogen Inc., 2005 Cabot Blvd. West Langhorne, Pennsylvania 19047 The Johnson Research Foundation Department of Biochemistry and Biophysics University of Pennsylvania Philadelphia, Pennsylvania 19104-6059