1. Contribution from the Institute of Physical Biology, Heinrich-Heine-Universität, 40225 Düsseldorf, Germany, Institute of Structural Biology, IBI-2, Forschungszentrum Jülich, 52425 Jülich, Germany, Department of Chemistry and Biochemistry and Molecular Biology Institute, University of California, Los Angeles, California 90095-1569, and Department of Chemistry, University of Arizona, Tucson, Arizona 85721