As we are interested in a general approach including a side-directed iodine-labeling for the reflectivity studies (Arbely et al. 2004) , the present work focuses on data recording and analysis, as can be applied for any peptide/ lipid sample. Fig. 1 Schematic representation of the reciprocal space for multilamellar membrane stacks (i) as a function of the parallel (q k ) and normal (q z ) components of the momentum transfer (q). The scheme shows the reflection geometry known as grazing incidence X-ray scattering (GISAXS) at small q k or grazing incidence diffraction (GID) at high q k . The recorded patterns include the diffuse Bragg sheets (ii) in the vicinity of the (specular) q z axis. In this study the q z components (iii) were measured by line scanning under specular conditions (iv). At low q z , the lipid acyl chain correlation maximum (v) is observed. Lorentzian fits yield the lateral lipid chain distance. The width of the acyl chain peak along q k gives information about the lipid ordering (correlation length); the angular width of the peak corresponds to the acyl chain tilt. In addition, superstructures (vi) and peptide geometries (helix maximum, vii) can be observed in some cases Eur Biophys J (2011) 40:417-436 419 Design of the b-type peptide helix motif