Putative transmembrane helixes of membrane proteins in general and channel proteins in particular often contain proline residues which may induce a bend into an otherwise regular helical structure. Here, it is shown by fluorescence-energy-transfer measurements and mol.-dynamics calcns. that, in the case of synthetic bilayer-spanning helical polypeptides, a proline-induced bend in a helix acts as a flexible element mediating rigid body motions of the helical segments. Most important, such structural fluctuations in the transmembrane helixes seem to play a functional role in the formation of ionic channels in planar lipid bilayers and biol. membranes. [on SciFinder (R)]