1. Institute of Biophysics, Academy of Sciences of the Czech Republic, Kralovopolska 135, CZ-61265 Brno, Czech Republic, and Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques, Universite Rene Descartes, UMR 8601 CNRS, 45 rue des Saints-Peres, F-75006 Paris 06, France