1. Biomolecular Measurement Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States
2. Institute for Bioscience & Biotechnology Research, University of Maryland, 9600 Gudelsky Way, Rockville, Maryland 20850, United States
3. Department of Chemistry, Oakland University, 146 Library Drive, Rochester, Michigan 48309, United States
4. National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, Michigan 48824, United States