1. Biophysics Department, Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands; Institute of Biochemistry, Odense University, Campusvej 55, DK-5230 Odense M, Denmark; and Biophysics Department, Free University, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands