1. Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York 14853, Molecular Probes, Inc., 4849 Pitchford Avenue, Eugene, Oregon 97402, Max-Planck-Institute for Biophysics, Kennedyallee 70, D60596 Frankfurt, Germany, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, and Institute of Biochemistry, University of Erlangen-Nuremberg, D-91054 Erlangen, Germany