Cryoelectron microscopy has been used to study the reorganization of unilamellar cationic lipid vesicles upon the addition of DNA. Unilamellar DNA-coated vesicles, as well as multilamellar DNA lipid complexes, could be observed. Also, DNA induced fusion of unilamellar vesicles was found. DNA appears to adsorb to the oppositely charged lipid bilayer in a monolayer of parallel helices and can act as a molecular "glue" enforcing close apposition of neighboring vesicle membranes. In samples with relatively high DNA content, there is evidence for DNA-induced aggregation and flattening of unilamellar vesicles. In these samples, multilamellar complexes are rare and contain only a small number of lamellae. At lower DNA contents, large multilamellar CL-DNA complexes, often with >10 bilayers, are formed. The multilamellar complexes in both types of sample frequently exhibit partially open bilayer segments on their outside surfaces. DNA seems to accumulate or coil near the edges of such unusually terminated membranes. Multilamellar lipid-DNA complexes appear to form by a mechanism that involves the rupture of an approaching vesicle and subsequent adsorption of its membrane to a "template" vesicle or a lipid-DNA complex.
A fingerprint-like pattern across multilamellar, lipid-DNA complexes is attributed to DNA condensed as parallel helices between lipid bilayers. It is argued that the patterning indicates the existence of 3-D correlation forces between DNA-covered bilayers, following the DNA-driven formation of multilamellar liposomes from unilamellar vesicles.
We have investigated the molecular conformations of a lipopolymer with a polyoxazoline
headgroup at air/water interfaces as a function of lateral area per molecule with X-ray and neutron
reflectometry. The polymer 1,2-dioctadecanyl-sn-glycero-3-poly(2-methyl-2-oxazoline), PMO−(C18)2, forms
stable surface monolayers. Pressure/area isotherms around room temperature show a plateau region,
indicative of a phase transition whose origin was examined. For data evaluation, a novel approach was
used that acts on explicit quasi-molecular ensemble conformations of the polymer [Politsch et al., preceding
paper in this issue]. At lower surface pressure, the polymer density distribution exhibits a maximum
near the interface, indicative of attractive interaction between the predominantly hydrophilic polymer
chains and the hydrophobic surface. Across the plateau region of the isotherm, a change in the volume
density distribution of the alkyl chains was observed which is indicative of a partial immersion of the
lipid moieties into the aqueous subphase. In contrast, no major structural change across the phase
transition was detected in the polymer volume density profiles which comply with scaling predictions at
both sides of the phase transition if deviations due to nonidealities are neglected. We interpret these
observations as an alkyl chain ordering induced by the steric interference between the PMO: Immersion
of alkyl chains into the subphase relaxes the strain on the hydrophobic anchors which derives from a
reduction of the configurational entropy of the PMO chains due to their confinement to the interface.
Abstract-A novel x-ray transmission window based on graphenic carbon has been developed with superior performance compared to beryllium transmission windows that are currently used in the field. Graphenic carbon in combination with an integrated silicon frame allows for a window design which does not use a mechanical support grid or additional light blocking layers. Compared to beryllium, the novel x-ray transmission window exhibits an improved transmission in the low energy region (0.1 keV -3 keV) while demonstrating excellent mechanical stability, as well as light and vacuum tightness. Therefore, the newly established graphenic carbon window, can replace beryllium in x-ray transmission windows with a nontoxic and abundant material.
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