1. Department of Chemistry, Duke University, French Family Science Center, 124 Science Drive, Durham, North Carolina 27708-0346, United States
2. Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States
3. Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium
4. Department of Biochemistry, Duke University, Durham, North Carolina 27710, United States
5. Department of Physics, Duke University, Durham, North Carolina 27708-0346, United States