Membranes lecithin

The survey of over 50 artificial lipid membrane models (pION) in this chapter reveals a new and very promising in vitro GIT model, based on the use of levels of lecithin membrane components higher than those previously reported, the use of negatively charged phospholipid membrane components, pH gradients, and artificial sink conditions. Also, a novel direction is suggested in the search for an ideal in vitro BBB model, based on the salient differences between the properties of the GIT and the BBB. 

Figure 7.32 Soy lecithin membrane retentions at various concentrations in dodecane, with and without sink (a) bases (b) acids (c) neutrals.

For acids, the membrane retention actually increases in the case of egg lecithin, compared to soy lecithin. This may be due to decreased repulsions between the negatively charged sample and negatively charged phospholipid, allowing H-bond-ing and hydrophobic forces to more fully realize in the less negatively charged egg lecithin membranes. The neutral molecules display about the same transport properties in soy and egg lecithin, in line with the absence of direct electrostatic effects. These differences between egg and soy lecithins make soy lecithin the preferred basis for further model development. 

The above iso-pH measurements are based on the 2% DOPC/dodecane system (model 1.0 over pH 3-10 range). Another membrane model was also explored by us. Table 7.16 lists iso-pH effective permeability measurements using the soy lecithin (20% wt/vol in dodecane) membrane PAMPA (models 17.1, 24.1, and 25.1) The negative membrane charge, the multicomponent phospholipid mixture, and the acceptor sink condition (Table 7.1) result in different intrinsic permeabilities for the probe molecules. Figure 7.40 shows the relationship between the 2% DOPC and the 20% soy iso-pH PAMPA systems for ketoprofen. Since the intrinsic permeability of ketoprofen in the soy lecithin membrane is about 20 times greater than in DOPC membrane, the flat diffusion-limited transport region of the log Pe... 

An interesting variation of this system involves embedding surface active zinc and manganese(III) porphyrins in lecithin membranes. Provided that the porphyrin is negatively charged, and with zwitterionic PSV in the bulk aqueous phase, the Mn111 can be photochemically oxidized to Mn,v. No electron transfer from PSV" to MuIV occurs, perhaps because the MnIV is deeply embedded in the membrane.310... 

As follows from the data from Sect. 2, the primary photochemical stage in the majority of the membrane systems studied is the redox quenching of the excited photosensitizer by an electron acceptor or donor leading to electron transfer across the membrane // water interface. For electron transfer to occur from the membrane-embedded photosensitizer to the water soluble acceptor, it is necessary for the former to be located sufficiently close to the membrane surface, though the direct contact of the photosensitizer with the aqueous phase is not obligatory. For example, Tsuchida et al. [147] have shown that electron transfer to MV2 + from photoexcited Zn-porphyrin inserted into the lecithin membrane, is observed only until the distance from the porphyrin ring to the membrane surface does not exceed about 12 A. 

Furthermore, DSC can be useful for modeling the toxic effects of xenobiotics such as phenols and especially chlorinated phenols. A detailed study was performed by Smejtek et al. [44]. Among other observations, it was found that pentachlorophenol induces changes in the -potential and the gel to fluid transition temperature in model lecithin membranes. The ionized form was more potent in abolishing pretransition. The unionized species induced an increase in melting transition width (Figure 3.8). 

An interesting variation of this system involves embedding surface active zinc and manganese(III) porphyrins in lecithin membranes. Provided that the porphyrin is negatively... 

Zeaxanthin (C ) has been incorporated in DMPC and egg lecithin vesicles. This a,(D-bipolar carotenoid reinforces the DMPC vesicle with respect to mechanical stability and water permeability but has no effect on fluid egg lecithin membranes (Lazrak et al.,1987). Electron-poor derivatives with electron-withdrawing carboxyl or pyridinium end groups should reversibly take up electrons in a type of reversible Michael reaction and then act as organic wires. There are, however, no reports on stable anion radicals of such chro-mophores in the literature. Claims of electron transport through vesicle membranes are very probably erroneous. It has been shown by reduction of an entrapped indigo dye that bixin derivatives in DPPC vesicle membranes favor the transport of borohydride and dithionite ions through the membrane rather... 

Figure 24. Biomesogenic structures a) (Bio)meso-gens displaying order-disorder distributions in CPK-presentation (left to right and top to bottom) hexa-n-alkanoyl-oxybenzene discoid - Chandrasekar s first non-rodlike liquid crystal [28 a, 51c] enantiomeric cholesteric estradiol- and estrone-derivatives [ 17 a, c, d, 26 f, 51 a, s, u] Reinitzer s cholesterolbenzoate [21, 22] - together with the acetate the foundation stones of liquid crystal history [21, 22] Kelker s MBBA -first liquid crystal fluid at ambient temperature [ 13 f, g] Gray s cyanobiphenyl nematics for electrooptic displays [25 a, 51 e] lyotropic lecithin membrane component [7 a, 14, 27 d, 52 a] and valinomycin-K -membrane carrier [7 a, 35] thermotropic cholesteryl-side-chain-modifiedpolysiloxanes with the combination of flexible main-chain and side-chain spacers [51 a, h] thermotropic azoxybenzene polymers with flexible main-chain spacers [51a] thermotropic cya-...

A simple apparatus has been designed for measuring the passive diffusion of drugs through an artificial lecithin membrane. It is claimed that this model system gives results similar to those obtained with natural Type 1 membranes (Misra, Hunger and Keberle, 1966). Other work with artificial membranes is outlined in Chapter 14. 

The orientational distribution functions reconstructed from these values (figure 3) show that -carotene in soybean lecithin membranes orients preferentially parallel to the lipid chains (/3 - 0 ), whereas in DGDG membranes a fraction of the -carotene molecules also orients parallel to the bilayer plane()9 - 90 ). 

The conduction properties of bilayer lecithin membranes in iodine-containing solutions have been examined from a potentiodynamic experimental approach. 

SELECTIVE DETERMINATION OF PHOSPHATE WITH PhoE PORIN-LECITHIN MEMBRANE ELECTRODE... 

The electrode system for the characterization of the PhoE porin-lecithin membrane is depicted in Figure 1. A basal plane pyrolytic graphite (BPG) electrode was coated with a PhoE porin-lecithin membrane. The electrode system consisted of the PhoE porin-lecithin membrane BPG electrode with a surface area of 0.19 cm, a counter electrode (platinum wire), and a reference electrode (saturated calomel electrode S.C.E.). Cyclic voltammograms were obtained with a potentiostat (Hokuto Denko,... 

Model HA301), a function generator (Hokuto Denko, Model HB104) and an X-Y recorder (Riken Denshi, F35). The measurement cell was of all-glass construction, approximately 10 ml in volume, incorporating a conventional three-electrode system. An anion-selective polymer membrane electrode was also coated with a PhoE porin-lecithin membrane. Potentiometric measurements were made with an electrometer (Hokuto Denko, Model HE-IOIA) in conjunction with a recorder (Riken Denshi, Model SP-J3C). 

Figure 1. Schematic diagram of the PhoE porin-lecithin membrane-BPG electrode system for measurement of phosphocompounds.

A PhoE porin-lecithin membrane-BPG electrode was prepared as follows n-decane containing 0.5% egg lecithin and 0.25% cholesterol was brushed on the BPG electrode and dried in air. The resulting lecithin membrane-BPG electrode was inserted in 10 ml, 50 mM Tris-HCL buffer (pH 7.0), and coated again with the n-decane solution containing lecithin and cholesterol. After the lecithin membrane turned black, extracted PhoE porin was added to the lecithin membrane-BPG electrode and Tris-HCl buffer solution system. The anion-selective polymer membrane electrode was an Ag/AgCl electrode (0.422 cm2) coated with a PVC membrane containing 6% methyltridodecyl ammonium chloride and 30% nitrophenyloctyl ether. A PhoE porin-lecithin membrane-anion selective membrane electrode was prepared in the same way as the PhoE porin-lecithin membrane-BPG electrode described above. 

The PhoE porin-lecithin membrane electrodes were inserted in the sample solution. Cyclic voltammetry was run in the range -1.0 to 1.0 V vs SCE. The potential was measured in combination with the saturated calomel electrode with an electrometer and displayed on a recorder. 

Figure 2 shows cyclic voltammograms of FMN at a lecithin membrane electrode (A) and a porin-lecithin membrane electrode (B). Cathodic waves appeared at -0.45 V and -0.57 V vs SCE at the porin-lecithin membrane electrode on the first scan in the negative direction. After scan reversal an oxidation peak was observed at -0.52 V. The peak potentials of FMN at the porin-lecithin membrane electrode were similar to those at a bare BPG electrode, although the peak currents were one-sixth to one-eighth their size at the latter electrode. No peak current was observed at the lecithin membrane electrode. The peak current of riboflavin was not obtained at either the lecithin or the porin-lecithin membrane electrode (Figure 3). 

Selectivity of PhoE porin-lecithin membrane-BPG electrode... 

The selectivity of the PhoE porin-lecithin membrane electrode is indicated in Table 1. The cyclic voltammograms were obtained at the PhoE membrane electrode for L-Cysteine, riboflavin, FMN, NADH and FADH2. Peak currents were obtained for FMN. L-cysteine and riboflavin, which have no phosphate, did not show a peak current. Peak currents were not obtained for NADH and FADH2. The molecular weights of NADH and FADH2 were more than 700. The PhoE porin-lecithin membrane is apparently permeable to phospho-compounds having molecular weights less than 700 daltons. 

Figure 4 shows the relationship between the peak current of FMN and the amount of PhoE porin in the lecithin membrane-BPG electrode. The peak... 

Figure 3. Cyclic voltammograms of riboflavin at a PhoE porin-lecithin membrane-BPG electrode and lecithin membrane electrode. Both voltammograms are similar. Scan rate was 10 mV/s. Riboflavin concentration was 60 pM. The experiments were performed in 50 mM Tris-HCl buffer (pH 7.0).

Table 1. Redox potentials of cyclic voltammograms of electroactive substrates observed at a PhoE porin-lecithin membrane BPG electrode. 

Porin-Lecithin Membrane-Anion Selective Electrode System... 

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