Anion , carrier-facilitated membrane transport

One of the most important categories of ion selective chemical sensors is based on what are called liquid membranes. This term was flrst used in 196U to describe a matrix that is not water soluble it contains either anionic or cationic sites (liquid ion exchangers), which can selectivity facilitate the exchange of inorganic ions. In order to study the active carrier-mediated ion transport through these liquid membranes, a cell such as the one shown in Figure 3.4.4 has been employed. 

Behr and Lehn (14) first demonstrated carrier-facilitated transport of amino acids through "liquid membranes" composed of a toluene layer floating on top of two isolated aqueous solutions. The carriers used were the quaternary ammonium salt Aliquat 336 (trioctylmethylammonium chloride) and the alkylated arylsulfonic acid dinonylnapthalenesulfonic acid. Amino acids were transported in the form of anions or cations, respectively, with the above carriers. The process is an ion-exchange process, as the carrier must exchange an ionized atom or molecule each time it forms a new ion pair (Figure 2). The net result of this type of carrier-facilitated transport process is that a solute ion is transported into the LM while an equal number of counterions are transported out of the LM. 

It has been shown that carrier-facilitated transport of amino acid ions plays a central role of development of LM enzyme systems. Better anion carriers and new cation carriers are needed to exploit enzymatic processes where amino acids (or derivatives) and their products must transport readily through the LM. Facile transport with little or no enzyme deactivation is required. Design of new carriers tailored specifically for LM processes will help pave the way for the industrial development of liquid membrane enzyme reactors. 

Lamb, J. D. Christensen, J. J. Izatt, S. R. Bedke, K. Astin, M. S. Izatt, R. M. "Effects of Salt Concentration and Anion on the Rate of Carrier-Facilitated Transport of Metal Cations through Bulk Liquid Membranes Containing Crown Ethers" J. Am. Chem. Soc., 1980 102 (10), p. 3399. 

At least seven carriers control entry through the mitochondrial membrane. One carrier facilitates entry of succinate, D- and L-malate, malonate, and m o-tartrate anions, but not tartrate, maleate, or fumarate. Another mediates the entry of citrate, m-aconitate, wocitrate, and D- or L-tartrate, but not fumarate or maleate. A third carrier transports adenosine nucleotides. Also phosphate anions can enter mitochondria whereas other inorganic anions cannot (Chappell, 1966). 

J.D.Lamb, J.J.Christensen, S.R.Izatt, K.Bedke, M.S.Astin, R.M.Izatt, Effects of Salt Concentration and Anion on the Rate of Carrier Facilitated Transport of Metal Cations Through Bulk Liquid Membrane Containing Crown Ethers, J.Am.Chem.Soc., 102, 3399 (1980) H.Tsukube, Effects of Cation on Transport Efficiency and Selectivity of Amino Acid Derivative Anions, Bull.Chem.Soc.Jpn.,... 

The family of OATPs consists of membrane carriers that mediate the transport of anionic molecules, although not exclusively, and more recently, transport of nonanionic molecules has been observed. OATPs are located in the small intestines on luminal membranes of enterocytes, where they mediate the uptake of drugs. In the liver, OATPs facilitate the uptake of... 

Example 9.14 Nonisothermal facilitated transport An approximate analysis of facilitated transport based on the nonequilibrium thermodynamics approach is reported (Selegny et al., 1997) for the nonisothermal facilitated transport of boric acid by borate ions as carriers in anion exchange membranes within a reasonable range of chemical potential and temperature differences. A simple arrangement consists of a two-compartment system separated by a membrane. The compartments are maintained at different temperatures T] and T2, and the solutions in these compartments contain equal substrate concentrations. The resulting temperature gradient may induce the flow of the substrate besides the heat flow across the membrane. The direction of mass flow is controlled by the temperature gradient. 

The carrier protein facilitating Pj and phosphate ester transport is of particular interest in leaves in connection with carbon processing - i.e., the synthesis, transport and degradation of carbohydrate, all of which occur in the cytosol [51]. This metabolite carrier, called the phosphate translocator, is a polypeptide with a molecular mass of 29 kDa and is a major component of the inner envelope membrane [52,53]. The phosphate translocator mediates the counter-transport of 3-PGA, DHAP and Pj. The rate of Pj transport alone is three orders of magnitude lower than with simultaneous DHAP or 3-PGA counter-transport [54]. Consequently operation of the phosphate translocator keeps the total amount of esterified phosphate and Pj constant inside the chloroplast. Significantly, the carrier is specific for the divalent anion of phosphate. 

A recent approach has been to use ion-exchange membranes as a support for the carrier. This support has the advantage that the carrier cannot easily be forced out or washed out of the membrane since the carrier is retained by strong electrostatic forces. This approach could provide a longer useful operating life. Both anion and cation exchange membranes were used by Leblanc et al. (21) to prepare facilitated transport membranes selective for COg and ethylene. Kajima et al. ( ) also used an ion-exchange membrane for copper extraction. 

The folate receptor facilitates the cellular uptake of folate and 5-methyltetrahydrofolate via receptor-mediated endocytosis at caveolae (caveolae are plasma membrane invaginations distinct from the classical clathrin-coated pits) (6). It has been hypothesized that the folate receptor is functionally coupled to an anion transporter to mediate cytosolic folate delivery by a process defined as potocytosis (6). More recent studies suggest that folate receptor endocytosis also occurs at clathrin-coated pits (7). Studies by Low and coworkers at Purdue University have shown that folate conjugates are also taken up by the folate receptor (8-10), but not by the reduced folate carrier. Figure 2 illustrates an endocytic pathway of the type envisioned for folate conjugates. Unfortunately, the subcellular transport pathway of the folate conjugates has been only partially characterized and may well be affected by the properties of the molecule attached to folate. 

Gardner, J. S., Peterson, Q. R, Walker, J. O., Jensen, B. D., Adhikary, B., Harrison, R. G., and Lamb, J. D. 2006. Anion transport through polymer inclusion membranes facilitated by transition metal containing carriers. Jourrml of Membrane Science 277 165-176. 

Both cations and anions can be easily removed via facilitated transport because a wide range of carriers is available. Among the numerous cations that can be recovered by liquid membranes, the following may be mentioned copper (Cu2+), mercury (Hg2+), nickel (Ni ), cadmium (Cd ), zinc (Zn2+) and lead (Pb2+). 

Optimization of the flux is investigated through the variation of external parameters which affect transport, such as the membrane polarity, viscosity, anion used and operating temperature (see the section on Optimization of the Flux External Influences on Transport). The influence on the transport kinetics has been investigated as well. Until recently, the transport of salts has only been facilitated through the use of a cation carrier. The cation is complexed by the carrier while the anion accompanies the complex as a free ion, hence a lipophilic anion is used to accompany the complex. 

The action of phosphonic acid substituted calix[4]arenes (701)-(704) on solvent-containing planar bilayer membranes made of cholesterol and egg phosphatidylcholine (egg PC) or synthetic 18-carbon-tail phospholipid DOPC have been investigated in a voltage-clamp mode. A steady-state voltage-dependent transmembrane current has been achieved only after addition of the compound (702) from the side of the membrane the positive potential has been applied to. This current exhibited anion selectivity passing more chloride at negative potentials applied from the side of the membrane to which calix[4]arene (702) has been introduced. The kinetics and temperature-dependence determined for calix[4]arene (702)-mediated ionic transport suggested a carrier mode of facilitated diffusion. 

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