1. Contribution from the Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Medical Biochemistry and Biophysics, Karolinska Institute, S-171 77, Stockholm, Sweden, and Department of Biochemistry and Centre for Molecular Recognition, University of Cambridge, Building O, Downing Site, Cambridge CB2 1QW, U.K.