Interface membrane-water

Lipid bilayer (Section 26 4) Arrangement of two layers of phospholipids that constitutes cell membranes The polar termini are located at the inner and outer membrane-water interfaces and the lipophilic hydrocarbon tails cluster on the inside... 

Polyphosphate, often with sodium chloride. This is a very low-tech approach, relying primarily on the threshold mechanism of polyphosphate to prevent calcium carbonate deposition at the membrane-water interface. Products based on this simple technology are subject to many limitations and probably are inappropriate to most industrial RO situations. 

Antonenko et al. [540] considered pH gradients forming in the UWL under bulk solution iso-pH conditions. They elegantly expanded on the buffer effect model and made it more general by considering multicomponent buffer mixtures. Direct measurements of the pH gradients (using wire-coated micro-pH electrodes) near the membrane-water interface were described. 

Matzke, M. and Matzke, A.J.M. (2003). RNA extends its reach. Science, 301, 1060-1061 Mulkidjanian, A.Y., Cherepanar, D.A., Heberle, J. and Junge, W. (2005). Proton transfer dynamics at membrane/water interface and the mechanism of biological energy conversion. Biochem. (Moscow), 70, 251-256... 

First of all it is seen that the SCF results are free of any noise, whereas there is plenty of noise in the MD profiles (note, however, that the density profiles on both halves of the bilayer are in this case not averaged the close resemblance between the profiles on both halves thus indicates that the membranes are well equilibrated). Apart from this, inspection of Figure 18 shows a remarkable resemblance between the two set of predictions. Many details are in semi-quantitative agreement. Moreover, many of the features of membranes composed of SOPC resemble those of DMPC discussed above. For example, the width of the membrane-water interface is about 1 nm, i.e. the size of just two to three water molecules. This width is consistent with the scaling arguments mentioned at the beginning of this chapter. A more accurate comparison... 

In the literature, one can find many more interesting MD studies concerning lipid bilayers with additives. In particular, a wealth of MD simulations of such systems is in the field of anaesthetics (for a review see [142]). Many anaesthetics tend to accumulate at the membrane/water interface, implying that their potencies are not related to their ability to cross the membrane. Instead, it seems to be more likely that their functioning is via binding to membrane receptors. Generally, they have an effect opposite to that of cholesterol, i.e. they increase the membrane fluidity and permeability. 

R. Homan and M. Eisenberg, A fluorescence quenching technique for the measurement of paramagnetic ion concentrations at the membrane/water interface. Intrinsic and X537A-mediated cobalt fluxes across lipid bilayer membranes, Biochim. Biophys. Acta 812, 485—4-92 (1985). 

The conflicting results are obtained at solving the question if H+ ions are mixed with aqueous medium or, vice versa, are transported by the membrane-water interface (or inside the membrane) to the nearest A/ZH consumer, what the accurate value of proton potential decrease on H+-ATP-synthase molecule is, and if unmixed layers are present at the interface, and what the membrane profile complexity is [22],... 

Recombination constants kr of a pair of primary products of PET at the inner membrane // water interface were also measured for the system of Fig. 4d using flash photolysis in the absence of EDTA and Fe(CN). The values kt, kr and q for a series of membrane-bound viologens are listed in Table 2. Taking into account that 0 amounts to 15%-20%, it is easy to calculate (using Eq. (32)) that even in the absence of an irreversible electron donor, the value of <5 for PET across the membrane can achieve several percent. 

Photoinduced Charge Separation and Recombination at Membrane // Water Interface... 

As follows from the data from Sect. 2, the primary photochemical stage in the majority of the membrane systems studied is the redox quenching of the excited photosensitizer by an electron acceptor or donor leading to electron transfer across the membrane // water interface. For electron transfer to occur from the membrane-embedded photosensitizer to the water soluble acceptor, it is necessary for the former to be located sufficiently close to the membrane surface, though the direct contact of the photosensitizer with the aqueous phase is not obligatory. For example, Tsuchida et al. [147] have shown that electron transfer to MV2 + from photoexcited Zn-porphyrin inserted into the lecithin membrane, is observed only until the distance from the porphyrin ring to the membrane surface does not exceed about 12 A. 

Important factors controlling the formation of the radical-ion products of the PET, their escape from the geminate pair, as well as recombination at the membrane // water interface, are the electrostatic and hydrophobic interactions of the reagents with the membrane. 

The 1-2 order of magnitude decrease of the rate constant for the recombination of PET products, in comparison with the homogeneous solution, was also observed by Matsuo and co-workers [167, 168]. In their studies one of these products was hydrophilic and thus located in the aqueous phase, while the other was hydrophobic and thus immersed in the membrane. Such a decrease of the rate is, apparently, a common feature of the reactions providing electron transfer across the membrane // water interface between the reagents with substantially different hydro-phobidty. 

The lipid membrane is not only anisotropic, but also inhomogeneous in the transmembrane direction. It contains relatively hydrophilic polar layers adjacent to the membrane // water interfaces, which include the polar heads of lipid molecules, and highly hydrophobic non-polar central core, containing the hydrocarbon chains. 

For the transmembrane transfer of ions containing hydrophobic substituents the model was proposed that takes into account the variations of dielectric properties across the membrane. According to this model [194-200] the lipophilic ions are adsorbed at the minima of the potential energy near to the membrane // water interface (see Fig. 6b). The transfer of the ions across the membrane is considered to be monomolecular reaction of the ion s surmounting of the hydrophobic barrier in the center of the membrane with the first order rate constant k,. 

Unfortunately, the experimental data concerning the distances at which electron exchange reactions in the membranes take place are very scarce. Tsuchida et al. have shown [147], that even when the photoexcited Zn porphyrin embedded in the membrane cannot approach the membrane // water interface closer than 12 A, the electron transfer is still possible to MV2+ located in the water phase outside the membrane. However, when the distance of the closest approach of these reactants is increased up to 17 A, the electron transfer is totally stopped. Examples of electron transfer proceeding presumably via electron tunneling across molecular layers about 20 A thick, which separate electron donor and acceptor molecules, can be found in papers by Mobius [230, 231] and Kuhn [232, 233]. Note, that in... 

Fig. 8. The application of vesicles for photocatalytic water decomposition in sacrificial systems (a) — dihydrogen evolution in the vesicle cavity. Pt metal catalyst is anchored to the inner membrane // water interface (b) — dioxygen evolution in the bulk solution. Manganese oxide catalyst is anchored to the outer membrane // water interface of the vesicle...

---