We present a temperature dependent x-ray reflectivity study of highly oriented, fully hydrated multilamellar phospholipid membranes. Both the specular and diffuse (nonspecular) x-ray reflectivity were measured for dimyristoyl-sn-glycero-phosphocholine (DMPC) and oleoyl-palmitoyl-sn-glycero-phosphocholine (POPC) on silicon substrates in excess water. In this configuration the repeat distance as well as the fluctuation spectra can be determined as a function of temperature. Both model systems studied exhibit a discontinuous unbinding transition from a substrate bound, multilamellar state to a state of freely dispersed bilayers in water. In the unbound phase a single membrane remains on the substrate.

Although the antimicrobial, fungal peptide alamethicin has been extensively studied, the conformation of the peptide and the interaction with lipid bilayers as well as the mechanism of channel gating are still not completely clear. As opposed to studies of the crystalline state, the polypeptide structures in the environment of fluid bilayers are difficult to probe. We have investigated the conformation of alamethicin in highly aligned stacks of model lipid membranes by synchrotron-based x-ray scattering. The (wide-angle) scattering distribution has been measured by reciprocal space mappings. A pronounced scattering signal is observed in samples of high molar peptide/lipid ratio which is distinctly different from the scattering distribution of an ideal helix in the transmembrane state. Beyond simple models of ideal helices, the data is analyzed in terms of models based on atomic coordinates from the Brookhaven Protein Data Bank, as well as from published molecular dynamics simulations. The results can be explained by assuming a wide distribution of helix tilt angles with respect to the membrane normal and a partial insertion of the N-terminus into the membrane.

We present a quantitative study of specular and diffuse (non-specular) x-ray and neutron reflectivity from highly aligned phospholipid membranes deposited on solid substrates. The height-height correlation function could be obtained from the diffuse scattering without further model assumptions. The results differ significantly from the linear theory of smectic elasticity. We argue that the diffuse scattering is dominated by static liquid-crystalline defects, rather than thermal fluctuations.

We report a neutron scattering study of multilamellar membranes supported on solid substrates. In contrast to previous work, the high degree of orientational alignment allows for a clear distinction between specular and nonspecular reflectivity contributions. In particular, we demonstrate that by using the specific advantages of neutron optics the nonspecular scattering can be mapped over a wide range of reciprocal space. Several orders of magnitude in scattering signal and parallel momentum transfer can easily be recorded in multilamellar stacks of lipid membranes. This opens up the possibility to study fluctuations, and more generally lateral structure parameters of membrane on length scales between a few Å up to several µm. The first results obtained for a system of partially hydrated 1,2-dimyristoyl-snglycero-3-phosphatidylcholine (DMPC) indicate strong deviations from the predictions of the standard Caillé model.Neutron reflectivity offers unique possibilities to study the structure of thin organic layers like polymers or biological macromolecules on solid substrates or at the air/water interface and is widely used for this purpose complementing X-ray techniques by means of contrast variation. As is well known, models of the laterally averaged scattering length density profile with molecular resolution can be fitted and in some cases derived from the neutron reflectivity measured over a range of grazing incidence angles α i . More recently, it has been realized that important information on the lateral interface structure is contained in the nonspecular scattering measured at angles of exit α f = α i [1,2]. This technique has evolved to a powerful technique widely used in the X-ray counterpart. In the neutron case, as is often argued, a lack of intensity severely limits the practicability of neutron diffuse studies, which are therefore rather scarce.

We present a novel method to measure and to analyze lipid bilayer fluctuations based on time-of-flight nonspecular neutron reflectivity (TOF-NSNR). To validate this approach, we compare the results obtained on the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) to NSNR data collected in the conventional monochromatic mode. For the DMPC model system in its fluid L α phase, we determine the fluctuation spectrum of an averaged bilayer in an oriented multilamellar stack and compare the result to the predictions of the linearized smectic free energy functional. In particular, we show how the smectic penetration depth Λ can be determined from the measurements. Furthermore, significant changes in the fluctuation spectrum of DMPC are observed upon the interaction with the antimicrobial peptide magainin 2 and cannot be explained by the smectic model.