1 Michael Smith Laboratories and Department of Chemistry, University of British Columbia, 2185 East Mall, Vancouver, BC V6T 1Z4, Canada. Electronic address: brumer@msl.ubc.ca.
2 Institute for Cell and Molecular Biosciences, Newcastle University, The Medical School, Framlington Place, Newcastle upon Tyne NE2 4HH, UK. Electronic address: harry.gilbert@ncl.ac.uk.