1. School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K.
2. Department of Medical Microbiology, Centre for Infectious Diseases, University of Edinburgh, 49 Little France Crescent, Edinburgh, EH16 4SB, U.K.
3. Current address: School of Biosciences, University of Exeter, Stocker Road, Exeter EX4 4QD, U.K