1. Contribution from the Department of Biophysics, Arrhenius Laboratories, Stockholm University, S-10691 Stockholm, Sweden, Department of Physical Chemistry, Chalmers University of Technology, S-41296 Gothenburg, Sweden, and Center for Biomolecular Recognition, The Panum Institute, IMBG, Department of Biochemistry B, Blegdamsvej 3c, DK-2200 Copenhagen N, Denmark