1. The Johnson Research Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania Medical School, Philadelphia, Pennsylvania 19104-6059;
2. Centro Interuniversitario di Ricerca sui Peptidi Bioattivi and Centro di Studio di Biocristallografia del C.N.R., University of Naples “Federico II”, Naples, Via Mezzocannone 4 I-80134 Italy;and