1. National Institute of Standards and Technology, Biotechnology Division, 222-A353, Gaithersburg, Maryland 20899
2. Department of Physical Chemistry and the Fritz Haber Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Department of Chemistry, University of California at Irvine, Irvine, California 92717
3. Department of Chemistry, Northwestern University, 2145 Sheridan Rd, Evanston, Illinois 60208-3113