1. Institute de Biologie Physico-chimique, 13, rue Pierre et Marie Curie, 75005 Paris, France, Johnson Research Foundation, Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, and Institut für Biophysik, Johann Wolfgang von Goethe Universität, Theordor-Stem-Kai 7, 60540 Frankfurt/Main, Germany