Dynamic secondary membrane

For ultrafiltration, the macromolecular solutes and colloidal species usually have insignificant osmotic pressures. In this case, the concentration at the membrane surface (C ) can rise to the point of incipient gel precipitation, forming a dynamic secondary membrane on top of the primary structure (Figure 7). This secondary membrane can offer the major resistance to flow. 

Marcinkowsky et al. (16) were the first to use dynamic secondary membranes in reverse osmosis for rejection of salts. Giiell et al. (17) later investigated protein transmission and permeate fluxes in microfiltration of protein mixtures using yeast to form a predeposited secondary membrane, and they observed higher flux and protein transmission in the presence of the secondary layer. Kuberkar and Davis (18) also observed higher flux and transmission of BSA in the presence of a cake layer of yeast,... 

At hi solid concentration, greater than approximately 1% by wei t, a fouling layer of solids may form on the surfiice of the filter if the true pore opening is small enough or, alternatively, it may form a combined fouling and fiber layer within the membrane sur ce. Further filtration will then take place on the newly formed fouling layer and subsequent filtration behaviour may depend onfy on tiiis fouling layer. This has been called a dynamic or secondary membrane. 

Generally, the effectiveness of the separation is determined not by the membrane itself, but rather by the formation of a secondary or dynamic membrane caused by interactions of the solutes and particles with the membrane. The buildup of a gel layer on the surface of an ultrafiltration membrane owing to rejection of macromolecules can provide the primary separation characteristics of the membrane. Similarly, with colloidal suspensions, pore blocking and bridging of... 

The first experimental evidences that electron transfer from QA to P+ and from QA to Qb in reaction centers are controlled by the protein conformational dynamics, was obtained in the late 1970 s (Berg 1978a,b Likhtenshtein et al., 1979 a, b) This conclusion was confirmed in subsequent experimental studies in which molecular dynamics of RC and the photsynthetic membrane were determined with a whole set of physical labels. (Kotelnikov et al., 1983, Kochetkov et al., 1984 Parak et al., 1983). It was shown that the electron transfer from reduced primary acceptor QA to secondary acceptor Qb takes place only under conditions in which the labels record the mobility of the protein moiety in the membrane with the correlation frequency u0 > 107 s-1 (Fig. 3.16). 

ET) between the components of the photosynthetic chain. The emergence of an electron from the primary photosynthetic cell, e.g. the transport from the reduced primary acceptor Qa- to the secondary acceptor QB followed by the release of hydroquinone QbH2 was shown to take place only under conditions in which the labels record the mobility of the protein moiety in the membrane with oc > 107 s 1. The rate of another important process, the recombination of the primary product of the charge separation, i.e. reduced primary acceptor (Qa ) and oxidized primary donor, bacteriochlorophyl dimere (P4), fahs from 102 to 103 s 1 when dynamic processes with oc = 103 s 1 occur. 

Autoflltration The retention of any material at the surface of the membrane gives rise to the possibility of a secondary or a dynamic membrane being formed. This is a significant problem for fractionation by ultrafiltration because microsolutes are partially retained by almost all retained macrosolutes. The degree of retention is quite case-specific. As a rule of thumb, higher pressure and more polarization results in more autofiltration. Autofiltration is particularly problematic in attempts to fractionate macromolecules. 

Etzkorn M, Martell S, Andronesi OC, Seidel K, Engelhard M, Baldus M. Secondary structure, dynamics, and topology of a seven-helix receptor in native membranes, studied by solid-state NMR spectroscopy. Angew. Chem. Int. Ed. Engl. 2007 46 459-462. 

From these arguments, we may conclude that trehalose is the best compromise to maintain an artificial H-bond network that temporarily replaces that developed by H2O molecules and avoids both those catastrophic consequences, that is the formation upon freezing of crystalline ice in that part outside macromolecules that is in contact with liquid water, and the collapse of the structure of membranes or of secondary structures of proteins upon drying due to the escape of H2O molecules. This artificial H-bond network is much less flexible than that established by H2O molecules and considerably slows down the dynamics of H2O molecules (23, 28). It consequently does not allow normal activity. It does not allow life to proceed in the same way as in the H2O network of living conditions. It nevertheless avoids irreversible transformation of the structure of the macromolecule by hydric stress, thus allowing resumption of living activities by rehydration. The discussion that has appeared in the literature to decide which mechanism is the most important, the glass... 

We demonstrate here how the NMR approach is a very useful means to reveal the conformation and dynamics of biological macromolecule with reference to the conformation-dependent displacements of peaks, with illustrative examples from polysaccharides, structural and membrane proteins and biologically active peptides. It is emphasized here that careful examination of the displacements of or N chemical shifts can serve as an excellent probe when referred to an accumulated data base of reference samples of known secondary structure. 

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