1. Scheikundig Laboratorium, Vrije Universiteit, De Boelelaan 1083, 1083 HV Amsterdam, The Netherlands, Fax: (internat.) + 31‐20‐4447488
2. Present address: Akzo Nobel, N.V. Organon, Molenstraat 110, P. O. Box 20, 5340 BH Oss, The Netherlands
3. Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands