where f 0 is the number of electrons of the ion (nonresonant term) and f 0 and if 00 are the real and imaginary corrections of the scattering factor, respectively (Evans and Pettifer 2001 ). Both f 0 and f 00 , connected by the Kramers-Kronig relations, strongly vary in the vicinity of an absorption edge and can be measured (f 00 ) or obtained from databases. At an absorption edge the absolute values of f 0 and f 00 are the highest with respect to nonresonant energy regions, where f & f 0 . Thus, in a resonant diffraction experiment it is very important to determine the energy value of the absorption edge accurately to a resolution of about 1 eV. It is known that the scattering factors f 0 and f 00 of a chemical species within a sample, here iodine, depend very sensitively on its environment. Since there are many influences on these values, their accurate calculation by theoretical consideration is not feasible. Therefore, it is the most reliable strategy to measure f 00 and calculate f 0 during the experimental setup (Als-Nielsen and McMorow 2001). Experimentally, it is possible to access f 00 by absorption or fluorescence measurements. In order to Fig. 3 GID scattering geometry (left) and exemplary results from low q k scanning at different angles of a i in search of ''superstructure peaks'' (right) also required for determination of parameters concerning the scattering geometry. As the angle of incidence increases, the specular beam reflex (sb) moves along the q z axis. The primary beam (pb) is located at the origin of the q z axis, here shadowed by the sample horizon mimic the hydrophobic environment within a lipid membrane where peptides should be placed, absorption measurements were performed using quartz capillaries (700 lm diameter) filled with 0.5 M iodoform dissolved in chloroform. Instead of measuring f 00 and calculating f 0 the distinct iodine L III edge was found by varying the beam energy about ±0.025 keV around the tabulated L III edge by means of modulating the undulator gap and detecting the absorption. The iodine L III edge was found at E = 4.5578 keV. For the contrast variation in the anomalous scattering experiments, the reflectivity measurements were performed at energies of 4.5578 and 5.8000 keV.