1. Laboratoire de Biophysique Moléculaire, Institut de Biologie Structurale, Jean Pierre EBEL, 41 rue Jules Horowitz, F-38027 Grenoble; CEA; CNRS; Université Joseph Fourier, Dipartimento di Fisica, Universita' di Perugia, Via Pascoli, 06123 Perugia, Italy, Institut Laue-Langevin, 6 rue Jules Horowitz, B.P. 156, 38042 Grenoble Cedex 9, France, and Department of Chemistry, Center for Biomembrane Systems and Institute for Surface and Interface Science, University of California, Irvine, California