Membrane defect formation

The most developed and widely used approach to electroporation and membrane rupture views pore formation as a result of large nonlinear fluctuations, rather than loss of stability for small (linear) fluctuations. This theory of electroporation has been intensively reviewed [68-70], and we will discuss it only briefly. The approach is similar to the theory of crystal defect formation or to the phenomenology of nucleation in first-order phase transitions. The idea of applying this approach to pore formation in bimolecular free films can be traced back to the work of Deryagin and Gutop [71]. 

Lipid membranes are quite deformable, allowing water and head groups into their interiors when perturbed. A "water defect" is shown in Figure 1C, where water and lipid head groups enter the hydrophobic interior of only one of the bilayer leaflets. Figure ID shows a "water pore," where both leaflets are perturbed. At the molecular level, pore and defect formation are directly related to specific lipid-lipid interactions. It is important to understand the free energy required for pore formation in membranes and the effect of lipid composition on the process. In Section 3 of this chapter, we review recent MD studies of the thermodynamics of pore formation. 

What is the function of the membrane skeleton There is a group of hereditary diseases including spherocytosis in which erythrocytes do not maintain their biconcave disc shape but become spherical or have other abnormal shapes and are extremely fragile.269 272 Causes of spherocytosis include defective formation of spectrin tetramers and defective association of spectrin with ankyrin or the band 4.1 protein.265 273 Thus, the principal functions of these proteins in erythrocytes may be to strengthen the membrane and to preserve the characteristic shape of erythrocytes during their 120-day lifetime in the bloodstream. In other cells the spectrins are able to interact with microtubules, which are absent from erythrocytes, and to microtubule-associated proteins of the cytoskeleton (Chapter 7, Section F).270 In nerve terminals a protein similar to erythrocyte protein 4.1 may be involved in transmitter release.274 The cytoskeleton is also actively involved in transmembrane signaling. 

Based on our observations, we generalize the fuel cell membrane degradation and failure mechanisms as the schematics in Fig. 23. So far, the evidence has shown that defects formation and growth play an important role both in chemical and in mechanical degradation processes. Drawing an analogy with material corrosion behavior,... 

Ab initio methods provide elegant solutions to the problem of simulating proton diffusion and conduction with the vehicular and Grotthuss mechanism. Modeling of water and representative Nation clusters has been readily performed. Notable findings include the formation of a defect structure in the ordered liquid water cluster. The activation energy for the defect formation is similar to that for conduction of proton in Nafion membrane. Classical MD methods can only account for physical diffusion of proton but can create very realistic model... 

Although ABO incompatibility and Rh-anti-body formation are the most common causes of hemolysis, other causes need to be considered, such as cephalohematoma formation during delivery with resultant increase in bilirubin production as the hematoma is resorbed hereditary spherocytosis, which is a red cell membrane defect that results in premature breakdown of the red cells and glucose 6-phosphate dehydrogenase deficiency, which is involved in maintaining adequate reduced glutathione levels in the red cell. Infection in the neonatal period is uncommon but still must be considered as a cause of jaundice. In particular, infections of the urinary tract lead frequently to jaundice as a preliminary symptom. The increase in infection associated with instrumentation in the premature infant is always a concern. 

Using the principle of ion pair formation between ammonium cations and the phosphate anions of lipids, Matile et al.33 prepared 8, an amphiphilic polyamine dendrimer. Rather than acting as a membrane channel, 8 was expected to form reversible membrane defects in the lipid bilayer. The steroid moiety was expected to act as the hydrophobic anchor for bilayer orientation and steric bulk was expected to prevent the polyamine penetrating the bilayer. Proton transport was assessed in unilamellar vesicles using the pH-fluorescence technique in which the external pH was increased to 7.8 relative to the internal pH at 7.4. The results demonstrated that 8 was almost as active as gramicidin, and maximal flux was achieved in ca. 20 s. 

Considering techniques that allow the imaging of lipid surfaces, scanning probe microscopes such as the atomic force microscope (AFM) (13, 23) have become very appealing. The AFM allows measurements of native lipid samples under physiologic-like conditions and while biological processes are at work. It is hence often used to determine lipid membrane stmctures, stmctural defects in membranes, domain formation, and even the behavior of lipid rafts with high nanometer-scale lateral resolution. 

In this chapter we have seen that application of porous ceramic coatings on porous substrates for preparing membrane supports is a complex process. Every step has to be carried out successfully to obtain substrates or membranes themselves which fulfil the requirements. We have seen that models of the coating processes are useful but still far from capable of describing the processes completely. We have seen that specific aspects such as prevention of defect formation still defy quantitative and sometimes also qualitative understanding. We have seen that sccding-up is not a trivial matter and that much work has to be performed to enable successful preparation of large surface areas. 

Extensive AFM work has been done by Banaszak Holl concerning defect formation in supported bilayer membranes by polyamidoamine (PAMAM) dendrimers. " While dendri-mers are outside the scope of this chapter, the same researchers have also examined similar interactions of bilayers with PEI 109,148 PAMAM dendrimers, PEI is shown to... 

Moreover, since also the mismatch of palladium and alumina thermal expansion coefficients is reduced, high temperature delamination and defects formation in the membrane are also reduced. It is also possible to control the palladium thickness (to about 5 pm) by increasing the hydrogen flux and so minimizing the cost of the composite membrane. 

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