1. Optical Spectroscopy Section, Laboratory of Molecular Biophysics, National Heart, Lung and Blood Institute, National Institutes of Health, Building 10-Magnuson CC, 5D14, 10 Center Dr, Bethesda, Maryland 20892, Laboratory of Computational Biology, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892, and Department of Chemistry & Biochemistry, Montana State University, Bozeman, Montana 59717