1. Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bl.21, BG-1113 Sofia, Bulgaria, Equipe “Physicochimie des Systèmes Polyphasés”, UMR CNRS 8612, Université Paris-Sud, F-92296 Châtenay-Malabry, France, and College of Sciences “Leonardo da Vinci”, P.O.Box 946, BG-1000 Sofia, Bulgaria