1. Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200 F, 3001 Heverlee, Belgium, Department of Agriculture, Katholieke Universiteit Leuven, Kardinaal Mercierlaan, 3001 Heverlee, Belgium, and Department of Biomolecular Sciences, Wageningen Agricultural University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands