1. Department of Chemistry University of Waterloo Waterloo, ON N2L 3G1 Canada
2. Current address: Department of Clinical Neurosciences University of Calgary Calgary, AB T2N 1N4 Canada
3. Current address: Department of Molecular Genetics University of Toronto Toronto, ON M5S 1A1 Canada
4. Current address: Department of Biology University of Waterloo Waterloo, ON N2L 3G1 Canada