1. Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0520, United States
2. Department of Chemistry, King’s College London, London, SE1 1UL, United Kingdom
3. Department of Chemistry, University of Southern California, 3620 S. McClintock Avenue, Los Angeles, California 90089-1062, United States