Black membrane

A solution of brain lipids was brushed across a small hole in a 5-ml. polyethylene pH cup immersed in an electrolyte solution. As observed under low power magnification, the thick lipid film initially formed exhibited intense interference colors. Finally, after thinning, black spots of poor reflectivity suddenly appeared in the film. The black spots grew rapidly and evenutally extended to the limit of the opening (5, 10). The black membranes have a thickness ranging from 60-90 A. under the electron microscope. Optical and electrical capacitance measurements have also demonstrated that the membrane, when in the final black state, corresponds closely to a bimolecular leaflet structure. Hence, these membranous structures are known as bimolecular, black, or bilayer lipid membranes (abbreviated as BLM). The transverse electrical and transport properties of BLM have been studied usually by forming such a structure interposed between two aqueous phases (10, 17). 

Fig. 15a-c. Formation of a bimolecular lipid membrane26). a) colored membrane (lamella), b) blackening membrane, c) black membrane... 

First, samples are filtered over a black membrane filter (e.g. polyester or polycarbonate) with an appropriate pore size (i.e. 0.4 rm for bacteria and 0.8-2 (xm for eukaryotic cells). These screen filters are used because of their low background fluorescence and high contrast, which facilitates validation using the epifluorescence microscope (see below) (Brailsford 1996). Secondly, the retained cells are fluorescently stained using one or more physiological or taxonomic probes (see Section Fluorescent Stains for SPC). 

FIGURE 6.2 Amino acid side chain analog insertion profiles with explicit (red) and implicit (purple, black) membrane models. The explicit lipid profiles were calculated with the OPLS force field [87], implicit profiles were calculated with both CHARMM [92] and OPLS force fields [93]. Experimental water-cyclohexane transfer free energies [85] are indicated as red dots. 

Benz, Frohlich, and Lau r have used the charge-pulse relaxation technique to determine the rate constants of association (A r) and dissociation (A d) of the valino-mycin-Rb+ complex as well as the rate constants of translocation of the complex (A ms) and the free carrier (Ars) in experiments with a series of lipid bilayers ( black membranes ). For membranes made from glycerol mono-oleate dissolved in different n-alkanes (ranging from n-decane to n-hexadecane) and for solvent-free membranes of the same lipid, the translocation rate constants Ars and A ms were both found to differ by less than a factor of two (Table 1). On the other hand, fairly large changes of the rate constants were observed (Table 2) if the structure of the fatty acid residue... 

As a point of interest, it is possible to form very thin films or membranes in water, that is, to have the water-film-water system. Thus a solution of lipid can be stretched on an underwater wire frame and, on thinning, the film goes through a succession of interference colors and may end up as a black film of 60-90 A thickness [109]. The situation is reminiscent of soap films in air (see Section XIV-9) it also represents a potentially important modeling of biological membranes. A theoretical model has been discussed by Good [110]. 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

The nonquantitative detection of radioactive emission often is required for special experimental conditions. Autoradiography, which is the exposure of photographic film to radioactive emissions, is a commonly used technique for locating radiotracers on thin-layer chromatographs, electrophoresis gels, tissue mounted on sHdes, whole-body animal sHces, and specialized membranes (13). After exposure to the radiolabeled emitters, dark or black spots or bands appear as the film develops. This technique is especially useful for tritium detection but is also widely used for P, P, and 1. 

Figure 12.14 The three-dimensional structure of a photosynthetic reaction center of a purple bacterium was the first high-resolution structure to be obtained from a membrane-bound protein. The molecule contains four subunits L, M, H, and a cytochrome. Subunits L and M bind the photosynthetic pigments, and the cytochrome binds four heme groups. The L (yellow) and the M (red) subunits each have five transmembrane a helices A-E. The H subunit (green) has one such transmembrane helix, AH, and the cytochrome (blue) has none. Approximate membrane boundaries are shown. The photosynthetic pigments and the heme groups appear in black. (Adapted from L. Stryer, Biochemistry, 3rd ed. New York ...

Fig. 5.9. AFM image of a Lavsan filtration membrane. Image size 5 )xm x 5 )xm, depth scale 500 nm from black to white.

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. 

Figure 13.1 Black color comparison standards for the measurement of suspended/transported iron oxide (Fe304) content in condensate, based on filtering 1 liter samples through a 0.45 p, membrane filter.

The experimental setup is shown in Figure 9.23. The Pt-black catalyst film also served as the working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H2-02 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. The Pt catalyst-working electrode potential, Urhe (=Uwr)> was measured with respect to a reversible reference H2 electrode (RHE) via a Luggin capillary in contact with the Pt-free side of the Nafion membrane. 

The catalytic-electrocatalytic reactor consists of a membrane electrode assembly, such as Pt-black/Nafion/Pd/C sandwiched between sheets of porous carbon cloth, housed in a fuel cell assembly. 

Figure 41 -15. Two types of endocytosis. An endocytotic vesicle (V) forms as a result of invagination of a portion of the plasma membrane. Fluid-phase endocytosis (A) is random and nondirected. Receptor-mediated endocytosis (B) is selective and occurs in coated pits (CP) lined with the protein clathrin (the fuzzy material). Targeting is provided by receptors (black symbols) specific for a variety of molecules. This results in the formation of a coated vesicle (CV).

The actions of proteins isolated from sea anemones, or other coelenterates, involve mechanisms different from those described for saponins. Thus, hemolysins from sea anemone R macrodactylus are capable of forming ion channels directly in membranes (98). The basic protein from S. helianthus also forms channels in black-lipid membranes. These channels are permeable to cations and show rectification (99). This ability of S. helianthus toxin III to form channels depends upon the nature of the host lipid membrane (100). Cytolysin S. helianthus binds to sphingomyelin and this substance may well serve as the binding site in cell membranes (101-106). 

Green tea consists of a wealth of simple phenolics (monomers), whereas black tea provides more complex polyphenols (dimers and polymers). It was found that with lipids the simple compounds were more effective antioxidants, while under aqueous conditions, polymers tended to have more activity. Weisburger (2001) suggested that polymers formed from a 2-5 unit polymerisation state seemed to be optimal, probably because the monomer is metabolised and excreted too rapidly, whereas the higher 6-10 unit polymers may suffer from difficulty in penetrating cellular membranes and be poorly absorbed. 

---