M. C. Shih

Tilted phases of fatty acid monolayers

X‐ray diffraction data from water‐supported monolayers of fatty acids with chain lengths from 19 to 22 is presented. The structures of the tilted mesophases L2’, L2, and Ov are characterized in detail. The contributions to the unit cell distortion from the tilt and the ordering of the backbone planes of the molecules are separated. It is shown that at the swiveling transition L2’–L2, not only the tilt azimuth but also the packing of the backbone planes change discontinuously. We demonstrate that the tilting transition LS–L2 is accompanied by the ordering of the backbone planes and may be discontinuous. Evidence is presented for a herringbone ordering transition within the L2 region. The distortions are related to symmetry of the phases and described by the order parameters responsible for tilt and herringbone ordering of the backbone planes of the molecules.

X‐ray diffraction study of heneicosanol monolayers on the surface of water

We have studied Langmuir monolayers of CH3(CH2)20OH (heneicosanol) using x‐ray diffraction. Over the temperature range 7–55 °C, the monolayer isotherms show only one discontinuity, which may be either a ‘‘flat section’’ or a ‘‘kink;’’ we find that these are transitions between tilted and untilted phases. There are several untilted phases, with hexagonal and distorted‐hexagonal structures, as in monolayers of the corresponding (heneicosanoic) acid. However, in contrast to the acid, when the pressure is lowered the molecules tilt only towards a next‐nearest neighbor. The dependence of the lattice spacings on pressure and temperature indicate that the monolayer does not form a ‘‘tilted hexagonal’’ phase, even at temperatures where the untilted phase is hexagonal (rotator‐II).

Lipid-mediated preferential localization of hypericin in lipid membranes

Subcellular localization of a photosensitizer is critical to its therapeutic outcome during photodynamic therapy (PDT). We delineated the distribution of hypericin, a new generation photosensitizer, in model membrane systems to identify the operating principles of its subcellular accumulation. Results from fluorescence microscopy indicated preferential incorporation of hypericin in lipid of giant unilamellar vesicles. Monolayer fluorescence measurements further identified cholesterol as the key determinant for the observed selectivity of hypericin. The emission spectra of hypericin in lipid monolayers varied in a lipid-dependent manner and Stokes shift behavior suggests that hypericin may form closely packed structure with cholesterol. Overall, our data lead to the conclusion that cholesterol is the major origin of the selectivity for hypericin in membrane systems. A hypothetical model depicting the intracellular and intravascular co-transport of hypericin and cholesterol because of their high affinity is presented.

Lipophilic C60-derivative-induced structural changes in phospholipid layers

Abstract We have studied the interactions of a lipophilic C60-derivative, synthesized recently for potential biomedical applications, with phospholipid monolayers and bilayers. The results of surface pressure–area isotherms, atomic force microscopic images, and neutron and X-ray scattering show consistently that the lipophilic C60 can intercalate into monolayers as well as vesicle bilayers of the phospholipids studied. In general, the lipophilic C60 can incorporate into the lipid membranes better in the liquid crystal phase, and modify the bending and compression modulus of the host lipid membranes significantly.

Isotherm and X-ray diffraction studies of mixed monolayers

We have studied Langmuir monolayers of saturated fatty acid-alcohol mixtures (same chain length) and acid-acid mixtures (different chain length) using isotherms and X-ray diffraction. The diffraction data confirm that the constituents are miscible, and that at zero pressure the mixed acid-alcohol monolayer areas are less than the areas of the pure components. We have determined the phase diagrams of a number of different mixtures. In any given phase, the intermolecular distances measured perpendicular to the molecules are independent of the pressure and of the constituents of the mixture. However, the location of some phase boundaries is a strong function of the composition of the mixture.

The DNA Adsorption by the Charged Cholesterol Monolayer at the Air‐liquid Interface

The adsorption of DNA by the 3‐,‐[N‐(N′,N′‐dimethyl amino ethane) carbamoyl] cholesterol (DC‐Chol) monolayer at the air‐liquid interface was studied by using the Langmuir‐Blodgett film balance. With the presence of 1 μ M DNA in the subphase, the surface pressure increases right at the beginning of the compression. The liquid expanded phase of the DC‐Chol disappears due to the adsorption of DNA. The AFM image of the prepared DC‐Chol/DNA film has tree‐branch‐like fractal structure with a height of 2 nm that correspond to the diameter of DNA.

X-Ray Diffraction Studies of Fatty Acid Monolayers on the Surface of Water

We review the current status of X-ray diffraction studies of Langmuir films. The structures of most known fatty acid monolayer phases have now been determined (and found to be different from what had historically been assumed); and various previously unexplained observations have now been given a microscopic basis.