1. Department of Biophysics, Arrhenius Laboratories, Stockholm University, S-106 91 Stockholm, Sweden, Institute of Environmental Medicine, Division of Biochemical Toxicology, Karolinska Institute, S-171 77 Stockholm, Sweden, Department of Toxicology, University of Mainz, D-551 31 Mainz, Germany, and Department of Physical Chemistry, Chalmers University of Technology, S-412 96 Göteborg, Sweden