Changes in the lateral bilayer structure upon peptide insertion are mostly discussed in theoretical studies and appear contradictory with respect to their conclusions. The intensity profiles (Fig. 4b) at a certain constant / were fitted to Lorentzian lineshapes (Eq. 2). The resulting fits at / = 8°(a) were applied to calculate the mean lipid acyl chain distance a and the acyl chain correlation length n (b) Already in early work, motivated by fluorescence data, a distinction is made between lipids not in contact with protein or peptide surface and lipids 'coupled' to the reconstituted protein, which are, thereby, disturbed in their interaction (Träuble and Overath 1973) . These so-called annular lipids were shown not to undergo lipid phase transitions. The postulated protein-derived disturbance was calculated to extend up to the third lipid neighbor of a protein inclusion (Marčelja 1976) . Addressing the structure of annular lipids in greater detail by theoretical molecular field approximations revealed an increased ordering of the lipid chains in proximity to membrane-incorporated structures within the fluid lipid phase (Marčelja 1974) . With increasing fraction of ''foreign molecules'' on the phospholipid bilayer, the membrane order parameter was reported to increase and the dependency of the order parameter on temperature to become less pronounced. We will denote this scenario as the (annular) lipid ordering scenario. In the lipid condensed phase, this effect was described to be the opposite, which we denote as the lipid disordering scenario. Other authors claim that rigid inclusions such as cholesterol might cause straightening and ordering of lipid chains, while the fluid-like surface of embedded proteins does not influence the motional freedom of the lipid chains (Lafleur et al. 1990a) . Molecular dynamics simulations of three different peptide a helices, published at the same time, likewise revealed an ordering of the lipid chains close to the peptide helices for all peptide species (Edholm and Johansson 1987) . Even if the overall effect was assessed as ''not drastic,'' the annular lipid ordering could be estimated as ''much less pronounced'' for lipids with bulky side chains. More recent theoretical considerations applying statistical mechanical integral equation theories (Lagüe et al. 1998 ) predict an expansion of the area per lipid for the annular shell upon peptide insertion into different phosphatidylcholine (PC) bilayers. In these studies, peptides are treated as soft cylinders (Lagüe et al. 2001 ). The authors postulate effective long range repulsion between lipids and peptides due to the formation of a lipid depletion layer around a protein leading to an increased cross-sectional area per lipid molecule. According to this view, which also supports the lipid disordering scenario for annular lipids, a lipid molecule in close contact with the peptide must significantly reduce its order. Effective lipid-protein repulsion would arise due to its entropic disadvantage. These theoretical results were further supported by MD simulations of a hydrated 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhyPC) bilayer containing an a-helical peptide bundle of four transmembrane domains (Husslein et al. 1998 ). Comparable to the results of Lagüe et al., Husslein and coworkers revealed an increase in the area per lipid from 7.46 nm 2 for the unperturbed bilayer to 8.50 nm 2 in the vicinity of the peptide species. Finally, for the sake of completeness, we note that a pronounced annular lipid effect is not reported by all studies. Using spin labeling techniques, Marsh and Horváth found order parameters of protein-associated lipid chains to be very similar to those in the bulk liquid-crystalline phase regions (Marsh and Horváth 1998) . Considering these opposing scenarios, it is interesting that we observed a very clear indication of the lipid ordering scenario. The decrease in the average lipid chain distance a as well as an increase in the lipid chain correlation length n for the peptides studied here can be ascribed to a reduced number of lipid gauche rotamers (Fig. 6) . For different molecular systems imposing different molecular boundary conditions, the second scenario could be valid instead. It should be taken into consideration that for the sake of simplicity, the theories supporting either one of the scenarios neglect the individual chemical properties of the incorporated molecular species that are the bases for interaction at the peptide/lipid interface.