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