1. Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, U.K., National Institute of Chemistry, Ljubjana, Hajdrihova 19, Slovenia, and Laboratoire de Chimie Biophysique, Institut le Bel, Université Louis Pasteur, 4, Rue Blaise Pascal, 67000 Strasbourg, France