1. Max-Planck-Unit for Structural Molecular Biology, c/o DESY, Notkestrasse 85, D-22607 Hamburg, Germany, Clemens Schöpf-Institute for Organic Chemistry and Biochemistry, Technical University of Darmstadt, Petersenstrasse 22, D-64287 Darmstadt, Germany, and Institute for Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany