1. Institute for Neuroscience and Biophysics 1, Forschungszentrum Jülich, D 52425 Jülich, Germany, J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Praha 8, Czech Republic, Aachen University, D-52074 Aachen, Germany, and Institute of Physical and Theoretical Chemistry, Eberhard Karls University Tübingen, D-72076 Tübingen, Germany