The X-ray reflectivity curves (Fig. 9 ) and the corresponding electron density profiles (Fig. 10a, b) are exemplarily shown for compound 7 and for its unlabeled analog 2 (in-house), respectively. More data are shown in the Electronic Supplementary Material. The derived electron density difference curves for all studied constructs 5, 7, and 8 and unlabeled compounds 2 and 9 are depicted in Fig. 10b, c. a b water-layer water-layer water-layer Si-substrate Fig. 7 Schematic representation of the reflectivity scattering geometry for multilamellar P/L complexes (a) with highlighted iodine positions (red arrows) and deduced electron density profiles (right, b) . The comparison of electron density profiles originating from samples containing labeled and unlabeled peptide species or from scattering at different photon energies yields the difference curve (blue)