1. Department of Biochemistry and Biophysics, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden, Institut für Chemie, Max-Volmer-Laboratorium, TU Berlin, PC 14, Straβe des 17. Juni 135, D-10623 Berlin, Germany, and Department of Physics, Albanova, and Department of Biochemistry and Biophysics, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden