1. Equipe mixte CEA-RP, Rhône-Poulenc, F 93308 Aubervilliers, France, Centre de Génétique moléculaire, CNRS, F 91198 Gif sur Yvette, France, Physical Chemistry 1 and 2, Center for Chemistry and Chemical Engineering, University of Lund, P.O. Box 124, S 221 00 Lund, Sweden, Institut Laue Langevin, B.P. 156, F 38042 Grenoble, France, and Laboratoire PMMH, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France