Most of the conjugated polymer employed as fluorescent biosensors present low solubility and emission in aqueous
environment. In order to solve this feature, we have reconstituted, in buffer phosphate, a neutral conjugated poly[9,9-
bis(6'-bromohexil)-2,7-fluorene-co-alt-1,4-phenylene], as PFPBr2 (insoluble in water), in the presence of an artificial
zwitterionic phospholipids bilayers, as 1,2-dimyristoyl-sn-glycero-3-phospho-choline (DMPC). Quantum yield of
PFPBr2-DMPC was around 20% in phosphate buffer, it was identical value calculated from ammonium polyelectrolytes
(PFPNMe3+). In addition, the maximum of bluish emission for buffer solution of PFPBr2-DMPC was at 420nm, a red-shift emission with regard to chloroform solution (at 410 nm). The structural study at different concentrations of PFPBr2
and DMPC was carried out using different approaches: steady state fluorescence spectroscopy, confocal fluorescence
microscopy and calorimetry. A positive interaction takes place involving neutral conjugated polymer and zwitterionic
phospholipids bilayer. Novels complexes or associations of poly(fluorene-phenylene) (PFPBr2) and zwitterionic
phospholipids (DMPC) have been suggested and visualized by epifluorescence. Phase transitions of the liposomes have
been also detected by differential scanning calorimetry.
Immobilization of ion channels requires of a methodology able to retain the physical properties of the lipid bilayer where their activity is performed. However, most of lipid membrane immobilization methods have been observed to alter the structural properties of the bilayers. Use of sol-gel routes seems to be an interesting alternative, although unstable liposomes were obtained when conventional sol-gel methodology was employed for immobilizing. Recently, we have suggested that use of alcohol-free sol-gel routes combined with negatively charged lipids could minimize effects exerted by host matrix on liposome structure, increasing its stability. Here we confirm this assumption by analysing the physical properties of a series of zwitterionic and anionic liposomes entrapped in a sol-gel matrix and we develop a methodology able to retain the physical properties of the lipid bilayer. This methodology has been successfully used to immobilize the transmembrane ion channel peptide gramicidin. Gramicidin was reconstituted in anionic liposomes and its immobilization was confirmed from changes observed in the photophysical properties of the tryptophan residues. Ion channel activity was determined using the fluorescent dye pyrene-1,3,6,8-tetrasulphonic acid (PTSA) and long term stability of the immobilized system was checked from steady-state fluorescence anisotropy measurements.
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