1. Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
2. Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.
3. Department of Chemistry - BMC, Uppsala University, BMC Box 576, S-751 23 Uppsala, Sweden
4. Department of Biosciences, Durham University, Durham DH1 3LE, U.K.
5. Biophysical Sciences Institute, Durham University, Durham DH1 3LE, U.K.