1. Department of Chemistry, Tulane University, New Orleans, LA 70118, USA
2. Department of Chemistry, Duke University, Durham, North Carolina 27708, USA
3. Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA
4. Department of Physics, Duke University, Durham, North Carolina 27708, USA
5. Department of Biochemistry, Duke University, Durham, North Carolina 27710, USA