Modified BLM

To a first approximation, the BLM can be considered to behave like a parallel plate capacitor immersed in a conducting electrolyte solution. In reality, even such a thin insulator as the modified BLM (designated by and R, in Fig. 108) could block the specific adsorption of some species from solution and/or modify the electrochemical behavior of the system. Similarly, System C may turn out to be a semiconductor(l)-insulator-semiconductor(2) (SIS ) rather than a semiconductor(l)-semiconductor(2) (SS ) junction. The obtained data, however, did not allow for an unambiguous distinction between these two alternative junctions we have chosen the simpler of the two [652], The equivalent circuit describing the working (Ew), the reference (Eg), and the counter (Ec) electrodes the resistance (Rm) and the capacitance (C of the BLM the resistance (R ) and capacitance (Ch) of the Helmholtz electrical double layer surrounding the BLM as well as the resistance of the electrolyte solution (RSO ) is shown in Fig. 108a [652],... 

Figure 9.1 A general scheme of ligand-receptor contact interaction. The operation of an electrochemical biosensor which is based on the properties of a modified BLM or s-BLM. L, ligand (substrate, antigen, hormone, ion or electron, donor or acceptor) R, receptor (enzyme, antibody, receptor site, carrier or channel, acceptor or donor).

Biosensors. Biosensors are essentially a special class of devices that transduce a bioreaction into an electrical signal. In terms of membrane research, the heart of a biosensor is a modified BLM that serves as a model... 

Fe(CN)g solution have been studied and it was found that the rate-limiting step of the whole photoinduced electron transfer depends on the applied potential and the redox concentration in solution. The sigmoidal-shaped steady-state photocurrent versus applied potential curve observed is different from the linear-shaped i(ph)/ curve of Cgo modified BLM reported by Bensasson et al. [68]. 

Finally, the fully assembled and modified BLM aglyeone is released from the BLM megasynthetase complex via a nucleophilic substitution of the RCO-S-PCP species by an amine (RNH2) catalyzed by Blmll and BlmXI [365, 367]. 

Agl AgCl,Cl (aq),left solutioni modified BLM right solution./ a-(aq),AgCl Ag (16)... 

It is worth stressing that a modified BLM in the above cell arrangement behaves as a bipolar electrode or two working electrodes connected in series. The two reference electrodes depicted in the setup merely facilitate the connection to the measuring instrument. The cell potential... 

In concluding this section, mention should be made concerning the dipoles of membrane constituents in the BLM. The dipole flip-flop could be important for the voltage-dependent conductance in BLM (e.g. alamethicin-modified BLM). Derzhanski et al. have derived an equation for the current as a function of frequencies. Pastushenko and Chizmadzhev have considered the energetic profile of dipole molecules in biomembranes in general, and in BLM in particular. This topic has been reviewed in detail by Shchipunov and Drachev, and by de Levie [3,15-24]. 

The pH-dependence of the effect of TFB and other uncouplers on the electrical conductivity of the BLM described above has been studied in detail by Sotnikov and Melnik i. They were interested in establishing the relationship between the pKa value of the modifier (e.g., DNP) and the conductivity of the BLM and found that the effectiveness of these compounds in lowering the conductivity follows their pK values. The conductivity for picric acid-modified BLM was about 10 Q cm. More recently, Wardak has investigated the effect of picric acid on the transport of and K+ and found that the salt of picric acid is highly permeable in the BLM. 

Bleomycin (BLM) was first isolated as a copper complex from a culture of Streptomyces verticillus. Since then numerous analogs have been prepared by modifying the conditions of fermentation. Bleomycins (114, bleomycin A2) are used clinically in combination cancer chemotherapy for the treatment of head and neck cancer, certain lymphomas, and testicular cancer (555). 

Since Upids are known to associate with DNA with high affinity, the adsorption of ssDNA at lipid membranes as a medium for DNA incorporation on a GC surface was extensively studied [60]. Exploiting DNA-Upid interactions, various approaches were designed for the incorporation of ssDNA [61] and dsDNA [62] at a modified bilayer lipid membrane (BLM) GC surface, such as (1) the formation of self-assembled BLMs over ssDNA previously adsorbed on GC, (2) the direct adsorption of ss- and dsDNA [62] into a previously BLM-modified GC and, (3) formation of a BLM with incorporated ssDNA at the GC surface using the monolayer folding technique [61]. 

Independent of the assumptions A to C the cation selectivity of the membranes in the equilibrium domain is therefore controlled by the ratio of the complex formation constants (6) and should therefore be identical for different types of neutral carrier membranes.18 Figure 2 indicates that there is indeed a close parallelism between the selectivities of solvent polymeric membranes (SPM) and bilayer lipid membranes (BLM) modified with valinomycin 1, nonactin 2, trinactin 5, and tetranac-tin 6 (see also Ref. 18). This is in good agreement with findings from Eisenman s45 and Lev s15 research groups. 

Fig. 2. Comparison of the selectivities of neutral-carrier-modified solvent polymeric- [43] and bilayer membranes. The permeability ratios PJP (at equilibrium" (Ref. 18) as far as available) fulfilled for the glyceryl dioleate BLM s are taken from Figs. 10 and 11 in Ref. 18. Values on the SPM s were obtained using 0.1 M solutions of the aqueous chlorides and membranes of the composition 33.1 wt.% polyvinyl chloride, 66.2 wt.% dioctyl adipate, 0.7 wt.% carrier. For the macrotetrolides I2 NH( for valinomycin IZ K. ...

Therefore, the authors have synthesized heptakis(carboxymethyl)-substituted a-cyclodextrin which was combined with hydrophobic ammonium to afford 9 (Figure 13), whereby the number of carboxylate-ammonium ion pairs was regulated to six. This compound was incorporated into BLM and subjected to singlechannel current measurements. The current-voltage plot gave a single straight line to afford a conductance level of 7.7 pS under several different experimental conditions. Then the modified cyclodextrin successfully controlled the ion channel structure and therefore the conductance. 

Due to their simplicity of construction, ease of modification, electrical methods of detection, fast response time and the fact that they are the principal structural component of all biomembranes, conventional bilayer lipid membrane (BLM) arises as an ideal system for biosensor technology [88] and they have been studied regarding the possibility of developing DNA biosensors consisting of a glassy carbon electrode-modified by a BLM with incorporated ssDNA [89]. 

Different other attractive way to modify the active electrode of amperometric sensors, besides deposition of polymers or electropolymerization, is formation of self-assembled structures, e.g., self-assembled monolayers (SAM) on solid supports 88 89 or bilayer lipid membranes (BLM) on various types of support.90 Both types of theses structures can either induce selectivity of sensor to particular analytes and... 

More recently, a polypyrrole-modified s-BLM has been found to be sensitive to hydrogen ions [80]. Concerning pH probes, they could be the basis for monitoring blood gases (oxygen, carbon dioxide) and related variables (pH, Na" ", K" ", Ca " ", Cl ) as well as metabolites (glucose, urea, lactate, creatinine) after suitable modification. It is envisioned that all these could be monitored simultaneously and/or sequentially in a small sample of undiluted whole blood or in plasma [78]. 

After the interface point has been reached, the paths for the two models differ. In the case of the vaporization plate efficiency (based on the modified Murphree efficiency model), a projection is made from the point (xjf, yj,) to the point (xji, yji) with the slope Kjt that satisfies Eq. (13-40). In the case of the Murphree plate efficiency, a projection is made from the point (xjf, yj,) to the point (x i, yft) along the tangent line, yf = mjXj + blm Since the equilibrium curve is seldom straight, an appropriate value of mi lying between the values at (xf, y ) and (xjj, yft) is selected. The values of yjt predicted by use of vaporization efficiencies are calculated by use of Eqs. (13-40), (13-47), and (13-51). 

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