1. Department of Biophysics, Arrhenius Laboratories, Stockholm University, S-106 91 Stockholm, Sweden, Faculty of Biology and Pharmacy, Institute of Molecular Biology, Friedrich Schiller University, Jena, Winzerlaer Strasse 10, D-07745 Jena, Germany, Department of Organic Chemistry, Arrhenius Laboratories, Stockholm University, S-106 91 Stockholm, Sweden, and Laboratory of Organic and Bioorganic Chemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institute, S-171 77 Stockholm, Sweden