Effects of cholesterol on the structure and collapse of DPPC monolayers.

Cholesterol induces faster collapse by compressed films of pulmonary surfactant. Because collapse prevents films from reaching the high surface pressures achieved in the alveolus, most therapeutic surfactants remove or omit cholesterol. The studies here determined the structural changes by which cholesterol causes faster collapse by films of dipalmitoyl phosphatidylcholine, used as a simple model for the functional alveolar film. Measurements of isobaric collapse, with surface pressure held constant at 52 mN/m, showed that cholesterol had little effect until the mol fraction of cholesterol, Xchol, exceeded 0.20. Structural measurements of grazing incidence X-ray diffraction at ambient laboratory temperatures and a surface pressure of 44 mN/m, just below the onset of collapse, showed that the major structural change in an ordered phase occurred at lower Xchol. A centered rectangular unit cell with tilted chains converted to an untilted hexagonal structure over the range of Xchol = 0.0-0.1. For Xchol = 0.1-0.4, the ordered structure was nearly invariant; the hexagonal unit cell persisted, and the spacing of the chains was essentially unchanged. That invariance strongly suggests that above Xchol = 0.1, cholesterol partitions into a disordered phase, which coexists with the ordered domains. The phase rule requires that for a binary film with coexisting phases, the stoichiometries of the ordered and disordered regions must remain constant. Added cholesterol must increase the area of the disordered phase at the expense of the ordered regions. X-ray scattering from dipalmitoyl phosphatidylcholine/cholesterol fit with that prediction. The data also show a progressive decrease in the size of crystalline domains. Our results suggest that cholesterol promotes adsorption not by altering the unit cell of the ordered phase but by decreasing both its total area and the size of individual crystallites.

Comparison of Ligand Architecture on Vapor Deposition Precursors: Synthesis, Characterization, and Reactivity of Volatile Cadmium Bis-Amidinate Complexes.

The lack of low-temperature (<200 °C) and easy-to-handle vapor deposition precursors for cadmium has been a limitation for cadmium chalcogenide ALD. Here, the cadmium amidinate system is presented as a scaffold for vapor deposition precursor design because the alkyl groups can be altered to change the properties of the precursor. Thus, the molecular structure affects the precursor stability at elevated temperature, onset of volatility, and reactivity. Cadmium bis-N,N-diisopropylacetamidinate (1) was synthesized and evaluated for its thermal stability, volatility, and reactivity-properties relevant to ALD precursors. Compounds 2, cadmium bis-N,N-diisopropyltertertiarybutylamidinate, and 3, cadmium bis-N,N-diisopropylbutylamidinate, are analogous to 1 and were synthesized by substituting the alkyl group on the bridging carbon during amidinate synthesis. All three compounds are volatile under reduced pressure, and thermal stability studies showed 1 and 3 to be stable at 100 °C in solution for days to weeks, while 2 decomposed at 100 °C within 24 h. Solution phase reactivity studies show 1 to be reactive with thiols at room temperature in a stoichiometric manner. No reactivity with either bis-silyl sulfides or alkyl sulfides was observed up to 110 °C over more than 3 days. Overall, the cadmium amidinate compounds presented here show potential as precursors in ALD/CVD processing, which can contribute to research critical for semiconductor processing.