1. Department of Biophysics, Institute of Physiology and Biophysics, University of Aarhus, DK-8000 Aarhus C, Denmark, Max-Planck-Institut für biophysikalische Chemie, Abteilung Spektroskopie, 37077 Göttingen, Germany, and The Water and Salt Research Centre, Department of Cell Biology, Institute of Anatomy, University of Aarhus, DK-8000 Aarhus C, Denmark