1. Department of Biophysical Chemistry, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Department of Physics and Astronomy, Institute of Molecular Biological Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, and Università degli Studi di Verona, Facoltà di Scienze MM. FF. e NN., Strada Le Grazie, Ca'Vignal, I-37134 Verona, Italy