1. Institut de Biologie Structurale - Jean-Pierre Ebel, UMR5075 CNRS-CEA-UJF, 41, rue Jules Horowitz, 38027 Grenoble Cedex, France, Department of Chemistry and Biochemistry, 215 UCB, University of Colorado, Boulder, Colorado 80309-0215, and Laboratorium für Physikalische Chemie, ETH Hoenggerberg, CH-8093 Zürich, Switzerland