Hexadecane membrane

C. and D. Monoolein/hexadecane membranes, 100 mV applied potential 1 M RbCl and 23 °C. Note that, during the time period of an average Gramicidin A channel (above), the N-acetyl desformyl Gramicidin A channel turns on and off many times and that the conductance step is smaller. The N-acetyl methyl replacing the formyl proton also crowds and destabilizes the head to head junction and results in less favorable lateral coordination of the cation at the junction. Reproduced with permission from Ref.111... 

C. Histogram of single channel conductances of Gramicidin A in glyceryl monoolein/hexadecane membranes for 1 M KC1, 103 mV applied potential and 23 °C. Reproduced with permission from Ref. 13>... 

A similar system has been reported based on polycarbonate filters coated with hexadecane, also called hexadecane membranes (HDM) [124, 125]. Thus, this... 

Composition of the PAM PA membrane varies from a purely organic solvent membrane to a purely phospholipid membrane. At the first international conference of PAM PA in 2002 (www.pampa2002.com/), it was agreed that these variations would be notated as initials or a short adjective at the head of PAMPA (e.g., BM-PAMPA for biomimetic PAMPA). The original PAMPA (Egg-PAMPA) [47], hexadecane membrane PAMPA (HDM-PAMPA) [48], BM-PAMPA [49], double sink PAMPA (DS-PAMPA) and blood-brain barrier PAMPA (BBB-PAMPA) [51] are reviewed elsewhere [3]. 

Chen et al. reduced the volume of organic solvent (hexadecane) down to 1 j,L per well (usually 4—5 j,L/well) [63]. Phospholipid/hexane solution was applied on both side of the thin hexadecane membrane (40 p,g phospholipid). After the evaporation of hexane, phospholipid/hexadecane/phospholipid tri-layer was formed (according to the original article). With the 2% phospholipid/dodecane membrane (4 p,L/ well) and DS-PAMPA, permeability of some compounds such as caffeine and antipyrine were underestimated, whereas tri-layer PAMPA gave an appropriate estimate. 

First of all, the capacitance of lecithin-hexadecane membrane is about 0.62 yF/cm. This value is smaller than the capacitance of biological membranes, i.e., 1 yF/cm2. The difference is perhaps partially due to the absence of proteins in artificial membranes. In addition, it is known that the presence of solvents decreases the values of membrane capacitance. For example, membranes formed by the Montal-Mueller Method (21), which are believed to be free of solvents, have a capacitance of 0.7 yF/cm2 (22). Thus, the capacitance of bilayer membranes shown in this figure may be in error by about 0.1 yF/cm2 because of the presence of solvent molecules. However, it is more important to note that membrane capacitance is independent of frequency, which provides unequivocal evidence that there is no relaxation process in lipid membranes in this frequency range. Coster and Smith (23) reported that they observed a frequency dispersion of membrane capacitance of artificial layers at very... 

There are a number of useful in vitro measures of permeability that can be used to assess how well a given molecule will be absorbed. One of the most simple is the permeation of molecules through artificial (hexadecane) membranes (referred to as PAMPA studies). These measure transcellular permeation through bulk diffusion in systems that avoid the complexity of active transport. With these types of assays, compounds can be ranked on the basis of lipid permeation alone this can be a useful gauge of ability to penetrate the gut intestinal wall. 

TABLE 9.2 Rates of Permeation Through Artificial Hexadecane Membrane (PAMPA)... 

Recently, Matsson et al. compared artificial membranes (hexadecane-membranes HDM) with 2 different cell monolayer models [i.e., the rat fetal duodenal cell line (2/4/Al) and the commonly used Caco-2 cell line] with respect to their potential for predicting the fraction absorbed in man they also successfully built a computer-aided prediction model of drug permeability using the same compound data set (Matsson et al., 2005). The three methods describe the importance of different pathways ... 

Faller and Wohnsland [18, 19] developed the PAMPA assay using phospholipid-free hexadecane, supported on 10 pm-thick polycarbonate filters, and were able to demonstrate interesting predictions. Their PAMPA method appeared to be a satisfactory substitute for obtaining alkane/water partition coefficients, which are usually very difficult to measure directly, due to the poor solubility of drug molecules in alkanes. Apparently, membrane retention was not measured. 

Figure 27. Relationship between water-hexadecane partition coefficients and membrane permeabilities for a broad selection of solutes. (Data collected by Walter and Gutknecht [124]. Reproduced with permission from the American Chemical Society)...

Wohnsland and Faller ([175] performed measurements using a thin (9-10 //in) supported, phospholipid-free hexadecane layer. To validate their model, they used 32 well-characterized chemically diverse compounds. The permeability values obtained with their model could be correlated with known human absorption values if the maximum permeability obtained at different pH was taken into account. However, several disadvantages are related to this method. For hydrophilic drugs, hexadecane by itself has an increased barrier function in comparison with membranes. In addition, the hexadecane layers are not very stable, which makes this assay difficult to apply as a routine screening method. The advantage of this PAMPA setup is that it appears to be a satisfactory substitute for obtaining alkane-water partition coefficients, which are usually very difficult to measure directly, due to the poor solubility of drug molecules in alkanes. 

Finally, one additional observation should be of interest to this group, namely the use of stable interference-colored membranes (ca. 2000 A thick) to enable thicknesses to be studied intermediate between that of the above bilayer (60 A) and Simon s bulk electrodes. This is illustrated in Fig. 3 where the steady-state conductance of stable interference-colored membranes made from GMO/hexadecane (bottom) could be compared with that in black bilayers of GMO/decane (filled circles). At the highest carrier concentration the interference-colored membrane was caused to go black with an applied voltage, giving the 500x increase in conductance indicated by the open circle, which is nicely in agreement with that of the GMO/decane bilayers. 

Guang Hui Ma et al. [83] prepared microcapsules with narrow size distribution, in which hexadecane (HD) was used as the oily core and poly(styrene-co-dimethyla-mino-ethyl metahcrylate) [P(st-DMAEMA] as the wall. The emulsion was first prepared using SPG membranes and a subsequent suspension polymerization process was performed to complete the microcapsule formation. Experimental and simulated results confirmed that high monomer conversion, high HD fraction, and addition of DMAEMA hydrophilic monomer were three main factors for the complete encapsulation of HD. The droplets were polymerized at 70 °C and the obtained microcapsules have a diameter ranging from 6 to 10 pm, six times larger than the membrane pore size of 1.4 p.m. 

The frequency response of various chemical constituents of nerve membrane was studied. Biological membranes in general consist of lipids and proteins. Firstly, impedance characteristics of artificial lipid bilayer membranes are examined using lecithin-hexadecane preparations. It was observed that the capacitance of plain lipid membranes was independent of frequency between 100 Hz and 20 KHz. Moreover, application of external voltages has no effect up to 200 mV. Secondly, membrane capacitance and conductance of nerve axon were investigated. There are three components in nerve membranes, i.e., conductance, capaci-... 

Span surfactant niosomes have been dispersed in oil-in-water emulsions to yield a vesicle in a water-in-oil system, v/w/o, using the same surfactant that was used to make niosomes [152]. The release of CF from these systems followed the trend v/w/odifference between the v/w/o and w/o formulations was minimal. The release of CF encapsulated within these niosomes was influenced by the emulsion oil following the trend, isopropyl myristate>octane>hexadecane and by the nature of the surfactant, following the trend span 20>span 40>span 60. Span 80 v/w/o systems had a rather faster release rate due to the unsaturation in the oleyl alkyl chain, which leads to the formation of a more leaky membrane. 

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