Locating intercalants within lipid bilayers using fluorescence quenching by bromophospholipids and iodophospholipids

In previous work, we have been able to determine the depth of intercalated molecules within the lipid bilayer using the solvent polarity sensitivity of three spectroscopic techniques: the ¹³C NMR chemical shift (δ); the fluorescence emission wavelength (λ_em), and the ESR β-H splitting constants (a_β-H). In the present paper, we use the quenching by a heavy atom (Br or I), situated at a known location along a phospholipid chain, as a probe of the location of a fluorescent moiety. We have synthesized various phospholipids with bromine (or iodine) atoms substituted at various locations along the lipid chain. The latter halolipids were intercalated in turn with various fluorophores into DMPC liposomes, biomembranes and erythrocyte ghosts. The most effective fluorescence quenching occurs when the heavy atom location corresponds to that of the fluorophore. The results show that generally speaking the fluorophore intercalates the same depth independent of which lipid bilayer is used. KBr (or KI) is the most effective quencher when the fluorophore resides in or at the aqueous phase. Presumably because of iodine's larger radius and spin coupling constant, the iodine analogs are far less discriminating in the depth range it quenches.

Tunable Porosity of Cross-Linked-Polyhedral Oligomeric Silsesquioxane Supports for Palladium-Catalyzed Aerobic Alcohol Oxidation in Water

Polyhedral oligomeric silsesquioxane (POSS)-based materials, poly-POSS-T_n [n = 8 (1), 10 (2), 12 (3), and mix (4)], were prepared in high yields via free radical polymerization of corresponding pure forms of methacrylate-functionalized POSS monomers, MMA-POSS-T_n (n = 8, 10, 12), and the mixture form, MMA-POSS-T_mix. Powder X-ray diffraction (XRD) spectra and BET analysis indicate that 1-4 are amorphous materials with high surface areas (683-839 m² g^-1). The surface areas and total pore volumes follow the trend: poly-POSS-T₁₂ > poly-POSS-T₁₀ > poly-POSS-T_mix > poly-POSS-T₈. In addition, on the basis of Barrett-Joyner-Halenda (BJH) analysis, poly-POSS-T₁₂ contains the highest amount of mesopores. The Pd nanoparticles immobilized on poly-POSS-T_n [n = 8 (5), 10 (6), 12 (7), and mix (8)] are well dispersed with 4-6 wt % Pd content and similar average particle sizes of 6.2-6.5 nm, according to transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDX) and microwave plasma-atomic emission spectroscopy (MP-AES). At 90 °C, the stabilized Pd nanoparticles in 5-8 catalyzed aerobic oxidation of benzyl alcohol to benzaldehyde in 72-100% yields at 6 h using a mixture of a H₂O/Pluronic (P123) solution. The PdNp@poly-POSS-T₈ catalyst (5) exhibited the lowest catalytic activity, as a result of its lowest surface areas, total pore volumes, and amounts of mesopores. With the catalyst 8, various benzyl alcohol derivatives were converted to the corresponding aldehydes in good to excellent yields. However, with alcoholic substrates featuring electron-withdrawing substituents, high conversions were achieved with 1 equiv of K₂CO₃ additive and longer reaction times.

Synthesis and isolation of non-chromophore cage-rearranged silsesquioxanes from base-catalyzed reactions

Carbonate (CO₃²⁻) anion as a stronger base but poorer in nucleophilicity gives rise exclusively to a cage expansion of unsubstituted products.

Synthesis of aromatic functionalized cage-rearranged silsesquioxanes (T8, T10, and T12) via nucleophilic substitution reactions

Organic-inorganic hybrid nano-building blocks of aromatic nitro-, aldehyde-, and bromo-functionalized polyhedral oligomeric silsesquioxanes were easily prepared through nucleophilic substitutions, starting from the reactions between octakis(3-chloropropyl)octasilsesquioxane and phenoxide derivatives. These phenoxide anions not only supply the substitution functions to a silsesquioxane cage, but can also induce a cage-rearrangement leading to the formation of octa-, deca-, and dodecahedral silsesquioxane cages.

Synthesis and isolation of methacrylate- and acrylate-functionalized polyhedral oligomeric silsesquioxanes (T8, T10, and T12) and characterization of the relationship between their chemical structures and physical properties

Novel organic-inorganic hybrid nanobuilding blocks of methacrylate- and acrylate-functionalized polyhedral oligomeric silsesquioxanes were easily prepared via nucleophilic substitution on octakis(3-chloropropyl)octasilsesquioxane, using sodium methacrylate and sodium acrylate, respectively. From a practical standpoint, these cage-rearranged silsesquioxanes (T8, T10, and T12) could be readily isolated in their pure form with conventional silica gel column chromatography. Octakis(3-propyl methacrylate)octasilsesquioxane (T8) is a colorless, crystalline solid with a melting point of 66.7-67.2 °C, while other cage products are colorless viscous liquids at room temperature. Moreover, we report that the chemical structure/physical property relationship of silsesquioxane cages not only is dependent on the symmetry of the inorganic silsesquioxane core at a given temperature but also is dictated by the organic substituent mobility. Structures of the products were confirmed by (1)H, (13)C, and (29)Si NMR spectroscopy and high resolution electrospray ionization mass spectrometry analysis.