Without relying on the contrast variation, a thickening of the bilayer by stretching of lipids in the z direction could be clearly and consistently evidenced. Thus, membrane insertion of b-helical structures is characterized by a positive mismatch situation. This hydrophobic stretching of the membrane and a good accommodation of the peptide species, which might be based on their extraordinary sidechain composition, also result in an increased membrane packing and lipid ordering of annular lipids. These findings are in contrast to theoretical studies that describe peptide helices as soft cylinders (Lagüe et al. 1998 (Lagüe et al. , 2001 . Two effects could possibly contribute to the enhanced lateral packing, the hydrophobic mismatch and/or hydrophobic, attractive lipid/peptide interactions resulting from sidechain intercalation into the lipid matrix.