1. Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States
2. Department of Bioengineering, Knight Campus, University of Oregon, Eugene, Oregon 97403, United States
3. Department of Chemistry, Center for Structural Biology, Vanderbilt University, Nashville, Tennessee 37235, United States
4. Department of Biochemistry, Institute for Protein Design, University of Washington, Seattle, Washington 98195, United States