1. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Department of Chemistry and Biochemistry, Ithaca College, Ithaca, New York 14850, USA
3. Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
4. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
5. Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA