1. Contribution from the Computational Molecular Biophysics, IWR, Heidelberg University, Im Neuenheimer Feld 368, Heidelberg, Germany; Molecular Biophysics Department, German Cancer Research Center, Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany; and Theoretische Physik, University of Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany