1. Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
2. Department of Structural Biology, University of Pittsburgh School of Medicine, 3501 Fifth Ave., Pittsburgh, Pennsylvania 15261, United States
3. Division of Biophysical Chemistry, Center for Molecular Protein Science, Department of Chemistry, Lund University, P.O. Box 124, Lund SE-22100, Sweden