1. Contribution from the Protein Engineering Network Centers of Excellence and Departments of Molecular and Medical Genetics, Biochemistry, and Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A8, Banting and Best Department of Medical Research and Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada M5G 1L6, and Institute of Organic Chemistry, University of Innsbruck, Innrain 52A, A-6060, Innsbruck, Austria