1. Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium, and Laboratoire de Chimie Biologique Structurale, FUNDP - Fac. des Sciences, 61 Rue de Bruxelles, B-5000 Namur, Belgium