1. Contribution from the Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J3, Canada, Department of Biology and Chemical Engineering, Box 325, Mälardalen University, 631 05 Eskilstuna, Sweden, Department of Quantum Chemistry, Box 518, Uppsala University, 751 20 Uppsala, Sweden, and Department of Biochemistry, Box 576, Uppsala University, 751 23 Uppsala, Sweden