1. Department of Physics and Astronomy and Institute for Condensed Matter Physics and Spectroscopy, Free University of Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam, The Netherlands, Section de Biophysique des Protéines et des Membranes, DBCM/CEA & URA 2096/CNRS CE-Saclay, F-91191 Gif-sur-Yvette, France, and Robert Hill Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2UH, United Kingdom