1. Lawrence Berkeley National Laboratory, Berkeley, California 94720, Department of Applied Science, University of California, Davis, California 95616, Biophysics Research Division, University of Michigan, Ann Arbor, Michigan 48109, Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02167-3860, and Department of Chemistry, Indiana University, Bloomington, Indiana 47405