G. Weidemann

Long-range tilt orientational order in phospholipid monolayers - the inner structure of dimyristoyl-phosphatidyl-ethanolamine domains

Abstract The long-range tilt orientational order in l -β,γ-dimyristoyl-α-phosphatidyl-ethanolamine (DMPE) monolayers is visualized using Brewster angle microscopy. Compact condensed phase domains in DMPE monolayers exhibit an inner structure of segments distinguished by their different reflectivity due to a different azimuthal orientation of the molecules. The azimuthal molecular orientation jumps at the boundaries of each segment. The inner structure of domains can be understood analogously to the star textural defect in thin liquid-crystal films. The orientational order in DMPE monolayers is lower than in monolayers of other amphiphiles showing similar segment structures indicated mainly by the occurrence of segment structures with varying number of segments. The segment boundaries are wound and the orientation changes continuously within each segment. The same sense of bowing in all domains reflects the chirality of the molecule.

Isotherms of phospholipid monolayers measured by a pendant drop technique

The Axisymmetric Drop Shape Analysis (ADSA) has been used to study the surface pressure/area isotherms of insoluble surfactant monolayers. The continuous measurement of surface tension as a function of surface area by increasing and decreasing the drop volume allows to investigate the phase transitions in monolayers. The isotherms of two phospholipids, dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatidyl ethanolamine (DMPE), show good agreement with those measured by using a conventional Langmuir-Blodgett film balance, except in the coexistence region. The observed disagreements are discussed in terms of differences in compression rate, curvature of the surface and effect of impurities. Evidence of possible geometric effects on monolayer domain formation and growth is given on the basis of BAM images.Due to the small total surface area, the ADSA technique provides advantages as regards homogeneity of temperature, surface pressure, surface concentration and the symmetry of area changes.

Structure features and phase behaviour of amphiphilic N-tetradecyl-β-hydroxy-propionic acid amide monolayers

Abstract Two phase transitions of N-tetradecyl-β-hydroxy-propionic acid amide monolayers at the air/water interface were studied by means of pressure/surface area ( π A ) isotherms, Brewster angle microscopy (BAM) and synchrotron X-ray grazing-incidence diffraction (GID). At T≤15 °C the π A isotherms show two plateau regions indicating two first-order phase transitions. In the first main transition from the lowdensity fluid-like phase to a condensed phase, dendritic growth of condensed phase domains with six main growth directions was observed by BAM. A second transition from a condensed phase with large tilt angle to a condensed phase with smaller tilt angle and a denser arrangement of molecules was found. The molecular area decreases jump-like by a value of ca. 0.02 nm2. The diffraction patterns (GID) of the low-density condensed phase show three peaks. In this state the crystal lattice of the monolayer is oblique. The azimuthal tilt direction of alkyl chains is close to nearest neighbours. The amphiphilic molecules change their conformation abruptly in a second transition. In the region above the second phase transition at high surface pressures, the crystal lattice remains oblique, but the molecules occupy a smaller molecular area parallel to the water surface and the tilt direction of the molecules is changed abruptly to an intermediate tilt direction. The balance of interactions between the polar head groups, dominated by hydrogen bonding between the amide and hydroxyl groups of the acid amide moieties, influences the morphological features of domains and the crystal structure of these monolayers, as well as the conformation of molecules at the air/water interface.

Influence of head group methylation on the phase behavior of lipid monolayers

Abstract Monolayers of racemic 1-(2-tetradecylhexadecanoyl)-2-O-hexadecyl-phospholipids 1(2C14-16:0)-2H-PE(CH3)n, with n=0, 1, 2 and 3, at the air/water interface are studied by film balance measurements, Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD). Although the area per three tails exceeds that per head, an influence of the head group methylation on the monolayer structure is observed. The tilt angles at lower lateral pressures and the transition pressure to a hexagonal packing of upright oriented chains increase with increasing methylation degree. The transition from the NN tilted rectangular to this hexagonal phase is connected with a pressure region where the in-plane components Qxy of the two peaks coincide while the out-of-plane components Qz differ. This indicates an undistorted hexagonal in-plane lattice even for tilted chains. The area-pressure isotherm of the 1(2C16-18:0)-2H-PE monolayer shows at 20°C a plateau region which does not directly correspond to the transition from the NN tilted to the hexagonal phase with untilted molecules. BAM measurements show that the large differences between the molecular areas, derived from isotherm and X-ray measurements below 10 mN/m, can be explained by the formation of holes in the monolayer. Possibly the tilting of the triple-chain molecules leads to an orientational ordering of the head group dipoles and therefore to an electrostatic repulsion between condensed phase domains.