1. Department of Chemistry, Indiana University, Bloomington, Indiana 47405, Department of Chemistry, University of Massachusetts-Amherst, Amherst, Massachusetts 01002, Indiana Molecular Biology Institute, Indiana University, Bloomington, Indiana 47405, Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, Department of Biochemistry & Biophysics, Texas A&M University, College Station, Texas 77843