1. Department of Chemistry and
Biochemistry, Swarthmore College, 500 College
Avenue, Swarthmore, Pennsylvania 19081, United States
2. Department of Molecular Biology, Princeton University, Lewis Thomas Laboratory, Washington
Road, Princeton, New Jersey 08544-1014, United States
3. Departments of Molecular Biosciences
and of Chemistry, Northwestern University, Evanston, Illinois 60208, United States