1. Helsinki Biophysics & Biomembrane Group, Institute of Biomedicine, University of Helsinki, Helsinki, Finland, Peptide and Protein Laboratory, Department of Virology, Haartman Institute, University of Helsinki, Helsinki, Finland, and CNR, IMC-Sezione Meccanismi di Reazione and Dipartimento di Chimica “La Sapienza”, P.le A. Moro 5, 00185 Rome, Italy