1. Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada, Department of Biochemistry, School of Medicine, Duke University, Durham, North Carolina 27710, Department of Biophysics, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226, and ExxonMobil Biomedical Sciences Inc., Annandale, New Jersey 08801