Species 7 Anomalous reflectivity of terminally labeled peptide species 7 in peptide/lipid complexes (Fig. 10a) showed relatively weak minima in the density corresponding to the water layer. If this is not an artifact of the rather problematic synchrotron experiment, it could be possibly attributed to increased lipid fluctuations or beginning lipid disorder upon localization of the rather bulky iodine labels at the lipid headgroup region. The electron density difference curve shows only two symmetrically distributed maxima at approximately z = ± 15.0 Å indicating a localization of the iodine labels at the lipid headgroup region tending to the membrane interior. This observation would perfectly fit with a transmembrane orientation of the b-hairpin (Fig. 11i) , although an orientation parallel to the membrane surface at the headgroup region (Fig. 11iii ) cannot be ruled out. Taking into account the observations made for the peptide/lipid complex 7/DLPC/Chol (1/47.5/ 2.5), this appears rather unlikely, since the depth of the insertion should not depend on the position of the label along the helix. The distance between the maxima of the electron density difference curve, associated with the distance of the two iodine labels is about 32 Å (see the Electronic Supplementary Material). This is in good agreement with the expectation based on a peptide length of 26 Å and the additional spacing of the Phe(4-I) side chains attached at both ends of the double helix and pointing to opposite directions (Schneggenburger et al. 2009 ).