1. Contribution from the Department of Physical Chemistry, Uppsala University, P.O. Box 579, SE-751 23 Uppsala, Sweden, Department of Molecular Biomimetics, Uppsala University, Villavägen 6, SE-752 36 Uppsala, Sweden, Department of Organic Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden, and Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden