1. Department of Biochemistry and Biophysics, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden, Department of Biochemistry and Biophysics, Göteborg University, P.O. Box 462, SE-405 30 Göteborg, Sweden, and Department of Chemistry and Bioscience, Chalmers University of Technology, P.O. Box 462, SE-405 30 Göteborg, Sweden