1. Institute of Biophysics, Bulgarian Academy of Sciences, BG-1113 Sofia, Bulgaria, CNRS UMR8612 Physico-chimie-Pharmacotechnie-Biopharmacie, Châtenay-Malabry, F-92296 France, University Paris Sud, Châtenay-Malabry, F-92296 France, Institute of Materials Research, GKSS Research Center, Geesthacht, D-21502 Germany, and HASYLAB c/o DESY, Notkestrasse 85, Hamburg, D-22603 Germany