1. Department of Molecular Biology, University of Aarhus, Gustav Wieds Vej 10 C, DK-8000 Aarhus C, Denmark, Laboratoire de Biophysique, Ecole Polytechnique, F-91128 Palaiseau Cedex, France, Department of Chemistry, Leiden University, Post Office Box 9502, NL-2300 RA Leiden, The Netherlands, and Akita National College of Technology, 1-1, Iijima Bunkyo machi, Akita, Japan 011-8511