1. NASA Ames Research Center, Exobiology Branch, Mail Stop 239-4, Moffett Field, California, 94035-1000, Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, 20742-4454, and Theoretical and Computational Biophysics Group, Beckman Institute for Advanced Science and Engineering, University of Illinois at Urbana−Champaign, 405 North Mathews, Urbana, Illinois 61801