1. CEP, UMR 8612, University of Paris XI, 5 Rue J.B. Clément, F-92296 Châtenay-Malabry, France, Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bl.21, BG-1113 Sofia, Bulgaria, Nano Analytical Laboratory, San Francisco, California 94118, and LURE, Université Paris-Sud, F-91898 Orsay, France