1. Contribution from the Department of Biochemistry and Organic Chemistry, Uppsala University, Box 576, SE-75123 Uppsala, Sweden, Department of Quantum Chemistry, Uppsala University, Box 518, SE-751 20 Uppsala, Sweden, Uppsala Imanet, GE Healthcare, Box 967, SE-751 09 Uppsala, Sweden, and ISOF, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, I-40129 Bologna, Italy