Within the centrosymmetric electron density profiles deduced from reflectivity scans (Fig. 7b) , maxima correspond to the lipid headgroup regions, while the global minimum reflects the acyl-acyl contacts of opposing hydrocarbon chains. The adjacent water layers are represented by the side minima. For anomalous scattering, the electron density profiles obtained at the L III absorption edge (E = 4.5578 keV) were subtracted from the electron density profiles at E = 5.8000 keV to estimate the position of the iodine label with respect to the z direction, indicated by rises in the electron density difference curve (Fig. 7b , blue curve). In in-house experiments such curves result from subtraction of the electron density q(z) of the samples containing iodinated peptides from the sample with unlabeled peptide species.