1. Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
2. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
3. Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, 79104 Freiburg, Germany
4. Department of Biochemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States