1. Max-Planck-Institut für biophysikalische Chemie, Abt. Spektroskopie, 37070 Göttingen, Germany, Institute of Biophysics, Biological Research Centre, 6701 Szeged, Hungary, School of Chemistry and School of Biochemistry and Molecular Biology, University of Leeds, Leeds LS2 9JT, U.K., and Department of Biological and Biomedical Sciences, Glasgow Caledonian University, Glasgow G4 0BA, U.K.