Planar bilayers

This parameter corresponds to cylindrical packing shapes. Surfactants and amphiphiles falling in this range often produce planar bilayers and lamellar mesophases. Such cylindrical building blocks also contribute to many... 

As first shown by Hladky and Haydon 7,8), it is possible to observe the current due to a single transmembrane channel by using extensions of the planar lipid hilaver approach of Mueller and Rudin 9). The basic system is shown in Fig. 2 and is commonly referred to as the black lipid membrane (BLM) method. This is because, as the lipid in the hole between the two chambers thins, the areas that have become planar bilayers are seen as black. Additional terms are bilayer lipid membranes or planar lipid bilayer membranes. These lipid bilayer membranes, particularly those which are solvent free, have capacitances which are very close to those of biological membranes. 

Fig. 3. Step conductance changes across planar bilayer due to Gramicidin A (traces A and C), D Leu2 Gramicidin A (trace B) and N-acetyl desformyl Gramicidin A (trace D).

We have developed a series of models in which 8-12 monomers were packed in an antiparallel manner to form the channel. The kinetics of pore formation in liposomes (9) and the dependence of planar bilayer conductance on toxin... 

Nanosized supramolecular materials have received increasing attention during the last two decades. Their properties have been surveyed for their ability to form aggregates in the solution phase, which form channel-like arrays in the solid state, and which ultimately form single channels in planar bilayer membranes. These systems therefore illustrate, in general, the convergence of supramolecular selforganization and supramolecular function. 

Using a concentration jump as the perturbation, Sutherland et a/.(113) measured the kinetics of binding of fluorescein-labeled human IgG (present as an antigen in solution) to surface-immobilized sheep anti-human IgG. Two TIRF surfaces were used a planar slide and a fiber-optic cylinder. Also using a TIRF recovery after a concentration jump, Kalb et a/,(114) measured the slow ( 10 4 s 1) unbinding kinetics of anti-trinitrophenol (TNP) antibodies in solution and a TNP-derivatized lipid in a planar bilayer. 

Figure 4. (a) Planar bilayer membrane system for single-channel currents measurement. Soybean lecithin in n-decane was applied to a hole separating two aqueous chambers. Chambers were filled with metal chloride salt at pH 7.2. The voltage was applied to the outer cell with respect to the inner. The currents across the bilayer were recorded on a PCM recorder through a patch-clamp amplifier and a lowpass filter, (b) Typical records of current observed at -t-50.0 mV (symmetrical 0.5 M solution). Currents increase upward from the zero level shown by the dotted line in each panel. 

In view of the use of a similar repeating unit, tetramethylenecarboxyl, Seebach s approach should be included here. He incorporated oligomers and polymers of 3-hydroxybutanoic acid (3-HB) into planar bilayers and observed single-channel currents. It is known that poly(3-HB) forms lamellar crystallites with thickness in the range of 40 to 60 A when crystallized from dilute solutions. Therefore it is assumed that poly(3-HB) forms lamellar crystallites with a thickness of ca. 50 A. 

These hydrophobic crystallites fit properly into a planar bilayer by formation of H-bonds between the free end groups of the oligomers and the polar head groups of the phospholipids. In other words, a membrane contains islands of crystalline poly(3-HB) within the liquid crystalline phospholipid phase. A schematic representation is shown in Figure 8. Single-channel current fluctuations are assumed to... 

When this amphiphile was incorporated into planar bilayer membrane, interesting current behavior was observed the open-closed probabilities varied with the applied voltage. A typical record of the channel current is shown in Figure 22. At 50 mV, the channel is almost closed with only occasional opening. At 70 to 85 mV, the open frequency increases and its duration becomes significantly longer. At 100... 

Fig. 60. Transformation of vesicles to planar bilayers via spontaneous monolayer formation [394]...

Benz, R., Janko, K., and Langer, P., Pore formation by the matrix protein (porin) to Escherichia coli in planar bilayer membranes, Ann. N.Y. Acad. Set, 358, 13-24, 1980. 

Ion channels have been purified from several types of excitable cells. The proteins that make up the voltage-gated Na+ channels of brain neurons were first identified by labeling them with reactive derivatives of neurotoxins obtained from scorpions. Intact channels were purified from both brain and muscle after solubilization with detergents. When the purified proteins were incorporated into phospholipid vesicles or planar bilayer membranes, they were found to conduct Na+ across the membrane. The ion specificity of the reconstituted channels and the alterations of the conductance in response to changes in Aift or to various neurotoxins were similar to the properties of the original nerve or muscle membranes. In addition to scorpion toxins, a variety of other specific neurotoxins bind to the purified channels and inhibit their activities. These include tetrodotoxin (a poison obtained from... 

Bilayers are preferentially formed for Ns = 0.5...1. Lipids that form bilayers cannot pack into micellar or cylindrical structures because of their small head group area and because their alkyl chains are too bulky to fit into a micelle. For bilayer-forming lipids this requires that for the same head group area a a, and chain length Lc, the alkyl chains must have twice the volume. For this reason lipids with two alkyl chains are likely to form bilayers. Examples are double-chained phospholipids such as phophatidyl choline or phophatidyl ethanolamine. Lipids with surfactant parameters slightly below 1 tend to form flexible bilayers or vesicles. Lipids with Ns = 1 form real planar bilayers. At high lipid concentration this leads to a so-called lamellar phase. A lamellar phase consist of stacks of roughly parallel planar bilayers. In some cases more complex, bicontinuous structures are also formed. As indicated by the name, bicontinuous structures consist of two continuous phases. 

Another way to assess ion channel conductance is to use artificial phospholipid vesicles (liposomes) as cell models. These structures (described in more detail in the next chapter) are commonly used to transport vaccines, drugs, enzymes, or other substances to target cells or organs. The vesicles, which are several hundred nanometres in diameter, do not suffer from interference from residual natural ion channel peptides or ionophores, unlike purified natural cells. A liposome model was used to test the ion transport behaviour of the redox-active hydraphile 12.36. The compound transports Na+ and the process can also be monitored using 23Na NMR spectroscopy.26 The presence of the ferrocene-derived group in the central relay allows the ion transport to be redox-controlled - oxidation to ferrocinium completely prevents Na+ transport for electrostatic reasons. Some representative data from a planar bilayer measurement is shown for hydraphile 12.36 in Figure 12.16. 

Figure 12.16 Typical planar bilayer Na+ conductance traces for 10 pmol 12.36 at +50 mV (top trace) and -50 mV (bottom trace) in aqueous buffer using symmetrical KC1 conditions. The arrows at the left hand side of the traces indicates the current level of the closed state. Peaks indicate the opening of individual ion channels (reproduced by permission of The Royal Society of Chemistry).

Abel, E. Maguire, G. E. M. Meadows, E. S. Murillo, O. Jin, T. Gokel, G. W., (1997) Planar bilayer conductance and fluorescent studies confirm the function and location of a synthetic sodium-ion-conducting channel in a phospholipid bilayer membrane J. Am. Chem. Soc. 119, 9061-9062. 

E. Kalb, S. Frey, and L. K. Tamm, Formation of supported planar bilayers by fusion of vesicles to supported phospholipid monolayers, Biochim. Biophys. Acta 1103, 307-316 (1992). 

Tozeren, P. K.-L. Sung, L. A. Sung, M. L. Dustin, P. Y. Chan, T. A. Springer, and S. Chien, Micromanipulation of adhesion of a jurkat cell to a planar bilayer-membrane containing lymphocyte function-associated antigen 3 molecules, J. Cell. Biol. 116, 997-1066 (1992). 

Fig. 24 Magnetic susceptibility for BABI at 10,000 Oe external field. A-D represent fits to the experimental data (A) square planar AFM system with J/k = — 1.6 K (B) square planar bilayer AFM system with J2olk = —1.4 K and interlayer Jik = —1.3 K (C) AFM spin pairing with Jjk = —3.8 K C is same model as C, with Jik = — 2.4 K D is same model as B, with J2T>lk = — 1.2K and interlayer Jik = — 1.9K (from calorimetric analysis).

Fig. 27 Magnetic heat capacity for PhBABI for 7 < 100 K showing variation with external magnetic field (left) zero-field magnetic heat capacity showing fits (right) to ID AFM chain, 2D AFM square planar, 2D AFM square planar bilayer, singlet-triplet spin pairing (ST), and spin ladder models.

Enhanced photovoltage and photocurrent signals were observed by the authors of Refs. [183,184] with linked porphyrin-quinone molecules in planar bilayer lipid membranes (BLM) as compared with preparations containing the non-Iinked components. They interposed BLM between two aqueous compartments containing a secondary electron donor on one side and a secondary acceptor on the other side. The efficiency of PET increased when the P-L-Q molecules were oriented in the membrane. 

PET — photoindueed electron transfer BLM — planar bilayer lipid membranes P — porphyrin Car - carotene... 

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