1. School of Chemistry, University of Sydney, Sydney NSW 2006, Australia, Department of Biophysical Chemistry, Max-Planck-Institut für Biophysik, D-60596 Frankfurt am Main, Germany, Department of Biology, University of Konstanz, D-78435 Konstanz, Germany, and Department of Biophysics, University of Aarhus, Ole Worms Allé 185, DK-8000 Aarhus C, Denmark