1. Department of Chemistry, University of California, Berkeley, California 94720, Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, Dipartimento di Biotecnologie, Facoltà di Scienze, Universitá di Verona, Strada Le Grazie, I-37134 Verona, Italia, and Department of Biophysics, Division of Physics and Astronomy, Faculty of Sciences, Vrije University Amsterdam, NL-1081 HV Amsterdam, The Netherlands