1. Department of Chemistry, Linköping University, S-581 83 Linköping, Sweden, Arrhenius Laboratory, Department of Organic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden, Department of Biochemistry, Uppsala University, BMC, S-751 23 Uppsala, Sweden, Department of Molecular Biology, Uppsala University, BMC, Box 590, S-751 24 Uppsala, Sweden, and Department of Organic Pharmaceutical Chemistry, Uppsala University, BMC, S-751 23 Uppsala, Sweden