1. Contribution from the Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bl.21, BG-1113 Sofia, Bulgaria, Physico-chimie, Pharmacotechnie, Biopharmacie, UMR8612 CNRS, University of Paris XI, 5 Rue J.B. Clément, F-92290 Châtenay-Malabry, France, NanoAnalytical Laboratory, San Francisco, California 94118, and Laboratoire de Physique des Solides, UMR8502 CNRS, Université de Paris Sud, Bât. 510, F-91405 Orsay, France