1. Department of Physics and Astronomy, VU Station B 351807, Vanderbilt Institute for Integrative Biosystems Research and Education, VU Station B 351807, Department of Chemistry, VU Station B 351822, Department of Biomedical Engineering, VU Station B 351631, and Department of Molecular Physiology and Biophysics, 702 Light Hall (0615), Vanderbilt University, Nashville, Tennessee 37232