1. Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Centre de Recherche Paul Pascal CNRS UPR-8641, 115 avenue du dr. Schweitzer, 33600 PESSAC, France, and Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, The Netherlands