1. Department of Chemistry and Chemical Biology and Department of Physics, Harvard University, Cambridge, Massachusetts 02138; Department of Theoretical Physics, University of Paderborn, D-33098 Paderborn, Germany; Laboratoire de Chimie Biophysique, ISIS Universitè Loius Pasteur, 67000 Strasbourg, France; and New Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QT U.K.