1. FOM Institute for Plasma Physics, ‘Rijnhuizen', Molecular Dynamics Group, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands, Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, and Department of Cellular and Developmental Biology and Biomolecular Structure Program, University of Colorado Health Sciences Center, Aurora, Colorado 80010-7163