1. Department of Organic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden, Department of Physical Chemistry, Uppsala University, Box 532, S-751 21 Uppsala, Sweden, Department of Biochemistry, Center for Chemistry and Chemical Engineering, University of Lund, Box 124, S-221 00 Lund, Sweden, and Department of Organic Chemistry, Chalmers University of Technology, 412 96 Göteborg, Sweden