1. INFM, UdR Roma I, University “La Sapienza”, Rome, Italy, Section de Biophysique des Protéine et de Membranes, DBCM, CEA/Saclay 91191 Gif-sur-Yvette, Cedex, France, INFM and Department of Physics, University “La Sapienza”, P. le A. Moro 5, 00185 Rome, Italy, and INFM and Department of Physics, University of L'Aquila, Via Vetoio, Coppito, 67010 L'Aquila, Italy