Membrane macromolecular structures

Among the main difficulties with electron crystallography are (1) sample damage from the electron beam (a 0.1-A wave carries a lot of energy), (2) low contrast between the solvent and the object under study, and (3) weak diffraction from the necessarily very thin arrays that can be studied by this method. Despite these obstacles, cryoscopic methods (Chapter 3, Section V) and image processing techniques have made electron crystallography a powerful probe of macromolecular structure, especially for membrane proteins, many of which resist crystallization. 

Polymers for membrane preparation can be classified into natural and synthetic ones. Polysaccharides and rubbers are important examples of natural membrane materials, but only cellulose derivatives are still used in large scale for technical membranes. By far the majority of current membranes are made from synthetic polymers (which, however, originally had been developed for many other engineering applications). Macromolecular structure is crucial for membrane barrier and other properties main factors include the chemical structure of the chain segments, molar mass (chain length), chain flexibility as well as intra- and intermolecular interactions. 

Much as liquid water is essential for life, frozen water, ice, is frequently lethal, especially if ice formation occurs within the cell. Upon formation of ice, loss of liquid water may impair or preclude the four basic water-related functions listed above. In particular, the structures and the activities of macromolecules and membranes may be severely damaged. In fact, the harmful effects of ice formation are due to a suite of physical and chemical effects. Physical damage from ice crystals that form within a cell can lead to rupture of membranes and the consequent dissipation of concentration gradients between the cell and external fluids or between membrane-bounded compartments within the cell. Ice formation in the extracellular fluids also can lead to damage to membranes as well as to lethal dehydration of the cell, as water moves down its concentration gradient from the intracellular space to the now depleted pool of liquid water in the extracellular space. Dehydration of the cell not only deprives it of water, but also leads to harmful and perhaps lethal increases in the concentrations of inorganic ions, which remain behind in the cell. Because the activities and structures of nucleic acids and proteins are affected by the concentrations of ions in their milieu, dehydration is expected to lead to perturbation of macromolecular structure and metabolic activity. It should come as no surprise, therefore, that with rare exceptions such as the fat body cells of certain cold-tolerant insects (Lee et al., 1993b Salt, 1962), ice formation within cells is lethal. 

Dendritic macromolecules exhibit compact globular structures which lead to their low viscosity in the melt or in solution. Furthermore, dendritic macromolecules are characterized by a very large number of available functional groups, which lead to unprecedented freedom for changing/tuning/tailoring the properties of these multivalent scaffolds via complete or partial derivatization with other chemical moieties. All these features have contributed to multidisciplinary applications of these unique macromolecular structures in recent years 6, 7). The development of efficient synthetic routes in recent years has given rise to a virtually unlimited supply of commercially available dendritic polymers, at very affordable price. The transport properties of hyperbranched and dendritic polymers have recently attracted attention as potentially new barrier and membrane materials 8-9). 

The objectives of investigators using iodination of proteins have been (a) to study the structure function relationships in proteins, (b) to provide a label for proteins in order to investigate their metabolic fate, (c) to provide a method of increasing the sensitivity for assay procedures of proteins such as in radioimmunoassays, and (d) more recently, as a tool for the investigation of the arrangement of proteins in macromolecular structures such as membranes. A number of reviews dealing with various aspects of peroxidase-catalyzed iodination have appeared. 

The selectivity of the reaction also makes the peroxidase-catalyzed io-dination a very good tool for the study of the position of proteins within macromolecular structure such as membranes, ribosomes, and micellular polypeptides. Its use in this way is based on the fact that it is a high molecular weight protein and therefore does not readily penetrate these macromolecular structures. - " If the experimental conditions are correct, it catalyzes the halogenation selectively with those groups on the protein with which the enzyme has access. Thus, when the enzyme has access to proteins on only one side of the macromolecular structure such as the cell membrane, only the accessible proteins will be labeled with iodine. The labeled polypeptide of the macromolecular structure can then be isolated and identified. This provides a general method that can be applied to all macromolecular structures. 

Special proteins, called apoLipoproteins, are required for handling and traruv port of lipid droplets. These proteins are synthesized on the ER and enter the lumen of the ER, where they are assembled into large macromolecular structures. The relevant proteins include apolipoprotein A apo A) and apo lipoprotein B (apo B), Apo A and apo B combine with lipid droplets to form structures called chylomicrons, microscopic particles with large cores of lipid coated with a thin shell of protein. The chylomicrons are transferred to secretory vesicles, which migrate through the cytoplasm to the basal membrane of the cell. Here the vesicles fuse with the membrane, resulhng in the expulsion of chylomicrons from the cell. (If the vesicles fused with the apical membrane of the enterocyte, the effect would be a futile transfer of the dietary lipids back to the lumen of the small intestine.)... 

In contrast to the polymeric materials for RO and NF membranes, for which the macromolecular structures have much to do with their permeation properties such as salt rejection characteristics, the choice of membrane material for UF does not depend on the material s influence on the permeation properties. Membrane permeation properties are largely governed by the pore sizes and the pore size distributions of UF membranes. Rather, the thermal, chemical, mechanical, and biological stability is considered of greater importance. 

Black JA, Waxman SG, Sims TJ, Gilmore SA 1986 Effects of delayed myelination by oligodendrocytes and Schwann cells on the macromolecular structure of axonal membrane in rat spinal cord. J Neurocytol 15 745-761... 

Receptor A receptor can be envisioned as a macromolecular structure such as a protein being an integral part of the complex molecular structure of the cellular membrane in which it is anchored or associated with. The recognition... 

Figure 3.1 Macromolecular structures in biological molecules. Most biological macromolecules are composed of a limited number of building blocks joined by covalent bonds, as shown for deoxyribonucleic acid (A) and proteins (B). Membranes (C) are composed of a large diversity of molecules sharing similar physical properties that allow for noncovalent self-association.

Peppas, N. and D. Meadows, Macromolecular structure and solute diffusion in membranes An overview of recent theories. Journal of Membrane Science, 1983, 16, 361-377. 

Attempts to quantify the fouling propensity of a feed water such as the Silt Density Index (SDI) and Modified Fouling Index (MFI) have met with limited success [128] due to the complex interactions between membrane and foulant. Fouling by natural organic matter depends critically on solution pH, ionic strength, and the presence of divalent cations due to changes in macromolecular structure [129]. Techniques for monitoring biofilm and scale formation are summarized in the literature [130]. 

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