1. Huygens Laboratory, Biophysics Department, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands, Service de Biophysique des Fonctions Membranaires, DBJC/CEA & URA 2096/CNRS, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France, and Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region 142290, Russian Federation