1. Department of Bioorganic Photochemistry, Leiden Institute of Chemistry, Gorlaeus Laboratories, University of Leiden, Post Office Box 9502, 2300 RA Leiden, The Netherlands, and Department of Biochemistry UMC-160, Nijmegen Center for Molecular Life Sciences, University of Nijmegen Medical School, Post Office Box 9101, 6500 HB Nijmegen, The Netherlands