1. Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Marie-Curie-Str. 11, D60439 Frankfurt am Main, Germany, Institute of Molecular Pharmacology, Robert-Rössle-Str. 10, D13125 Berlin, Germany, and Institute of Chemistry, Humboldt University, Brook-Taylor-Str. 2, D12489 Berlin, Germany