1 Department of Physics, Tampere University of Technology, FI-33101 Tampere, Finland; Department of Physics, University of Helsinki, P. O. Box 64, FI-00014, Finland; MEMPHYS-Center for Biomembrane Physics, University of Southern Denmark, Odense, Denmark. Electronic address: Ilpo.Vattulainen@helsinki.fi.
2 Department of Physics, Tampere University of Technology, FI-33101 Tampere, Finland; Department of Physics, University of Helsinki, P. O. Box 64, FI-00014, Finland.