[1] Univ Penn, Dept Biochem & Biophys, Johnson Res Fdn, Philadelphia, PA 19104 USA
[2] Univ Naples Federico II, CNR, Ctr Studio Biocristallog, I-80134 Naples, Italy
[3] Univ Naples Federico II, CNR, Ctr Interdipartimentale Ric Peptidi Bioacttivi, I-80134 Naples, Italy