Asymmetric functionalization

The lipids and proteins of membranes are inserted into the bilayer with specific sidedness thus membranes are structurally and functionally asymmetric. Many membrane proteins contain covalently attached oligosaccharides. Plasma membrane glycoproteins are always oriented with the carbohydrate-bearing domain on the extracellular surface. 

Organic chemists have not had much use for prochirality, but it is an important concept for biochemists following the stereochemistry of bio-organic reactions. Almost all biochemical reactions are under the control of enzymes, which function asymmetrically even on symmetrical (but prochiral) molecules. Thus it has been found that only one of the two methylene groups of... 

Figure 13. Schematic of the plasma membrane with a structural and functional asymmetric orientation of the molecular constituents (adapted from Refs. 89, 91, and 98). Aqueous medium = exterior.

Membranes are structurally and functionally asymmetric. The outer and inner surfaces of all known biological membranes have different components and different enzymatic activities. A clear-cut example is the pump that regulates the concentration of Na+ and K+ ions in cells (Figure 12.34). This transport protein is located in the plasma membrane of nearly all cells in higher organisms. The Na+-K+ pump is oriented so that it pumps Na+ out of the cell and K+ into it. Furthermore, ATP must be on the inside of the cell to drive the pump. Ouabain, a specific inhibitor of the pump, is effective only if it is located outside. 

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mammalian plasma membranes. Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (transverse diffusion, or flip-flop) is usually very slow. Proteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a... 

Figure 16. Evolution of molecular and organismic chirality (top to bottom) l- and o-amino acid enantiomers [31] valinomycin K -ion carrier [35] as a highly sophisticated outcome of independent amphiphilic bichirality protein developments transitions from hetero- to homochirality via selection of enantiomers in building up suprachiral structures of RNAs and their self-replication [34] development of structural and functional asymmetric organismic bichirality on the basis of molecular homochirality [7 a, 17, 18, 31].

The solution procedure of the linearized Poisson-Boltzmann equation used above is not suited to include hard core effects of the ions, the most we can do is to give a size to the central ion, but that makes the pair distribution function asymmetric. To include the hard core effects in a symmetric way, we have to change the formalism. We notice, first, that Poisson s equation (1.8) relates the potential (r) to the charge distribution We can formally integrate this equation to yield ... 

Zhao, Y, Higashihara, T., Sugiyama, K., and Hirao, A. (2005) Synthesis of functionalized asymmetric star polymers containing conductive polyacetylene segments by living anionic polymerization. Journal the American Chemical Society, 127,14158-14159. 

The viscosity of an IL is heavily influenced by temperature [14]. Specifically, an increase in temperature leads to an increase in the Brownian motion of the constituent molecules of an IL [39], Okoturo et al. investigated the temperature dependence of IL viscosity and observed that ILs containing un-functionalized asymmetric cations typically obeyed the Arrhenius law [56]. On the other hand, the majority of ILs containing small, symmetric cations with low molar masses obeyed the Vogel-Tammann-Fulcher laws [56]. ILs that obeyed neither the Arrhenius law nor the Vogel-Tammann-Fulcher laws generally contained functionalized asymmetric cations with higher molar masses [56]. 

This division can also be viewed favorably from the perspective of functionality. Asymmetric materials possess certain unique properties, with the symmetry making interactions with other materials more selective and specific. An example is DNA, which is helical helicity is a special type of asymmetry where the pitch and diameter of the helix determine how the materials interact with their counterparts. Asymmetric catalysis follows the same principle. Furthermore, the interaction between hehcal nanomaterials and other materials can be tuned by using the surfactants on the helices, and manipulating this factor of symmetry will provide another handle to control the function of the materials. 

The exchange of ADP and ATP across the mitochondrial inner membrane, mediated by the adenine nucleotide translocase, is thought to be rate-limiting for oxidative phosphorylation (Heldt and Pfaff, 1969). This translocase functions asymmetrically, i.e., it favors a rapid entry of ADP dependent on the availablity of intramitochondrial ATP and results in an asymmetric distribution of ATP and ADP on either side of the membrane (Heldt et al., 1972, Slater et al., 1973). Thus the... 

If however a pH gradient appears in the membrane for example by a modification of the acidity on both faces of the membrane (Figure 15) then one of the enzymes will be activated and the other one inhibited on the face of the higher pH the inverse will happen on the side of the lower pH. We obtain a functionally asymmetric dissipative structure created by the pH gradient the active transport will start and always pump from the first enzyme towards the second enzyme in the sequence of the reactions, as with the permanent structures, but here the absolute direction is controlled by the direction of the pH gradient. An inversion of the pH gradient can reverse the pump the disappearance of this gradient will stop it. 

Uchiyama M, Miyoshi T, Kajihara Y et al (2002) Generation of functionalized asymmetric benzynes with TMP-zincates effects of ligands on selectivity and reactivity of zincates. J Am Chem Soc 124 8514-8515. doi 10.1021/Ja0202199... 

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