Membrane,synthetic transport

With synthetic membranes, carrier transport has been realized predominately in supported liquid membranes. Artificial membranes with synthetic transport channels are still far from any practical relevance. 

Transport Across Biological Membranes Membranes, Synthetic, Applications Solvent Extraction Surface Chemistry Wastewater Treatment and Water Reclamation... 

In this chapter, we will review methods to characterize the activity of synthetic transport systems in translocating molecules across otherwise impermeable membranes for beginners in the field. The following two sections briefly introduce methods with bulk membranes (U-tube experiments) and planar or black lipid membranes (BLMs), and close with a more comprehensive overview of methods involving liposomes, in particular large unilamellar vesicles (LUVs). The fourth section will then focus on how to apply... 

Failure to detect activity may be traced back to a very high selectivity of the synthetic transport system. For example, a highly selective K+ carrier would not transport a proton or hydroxide ion which would be necessary to detect its activity by the HPTS assay (see Section 4.4 for a modified version of the HPTS assay). Even less intuitively, the activity of a proton transporter may not fully be detected by the HPTS assay, because proton transport will Stop owing to the opposing membrane potential that has built up (Section 4.5). The use of the HPTS assay to detect ion selectivity will be described later on (Section 4.5),... 

Because of their advanced level of development, high sensitivity, and broad applicability, fluorescence spectroscopy with labeled LUVs and planar bilayer conductance experiments are the two techniques of choice to study synthetic transport systems. The broad applicability of the former also includes ion carriers, but it is extremely difficult to differentiate a carrier from a channel or pore mechanism by LUV experiments. However, the breadth and depth accessible with fluorogenic vesicles in a reliable user-friendly manner are unmatched by any other technique. Planar bilayer conductance experiments are restricted to ion channels and pores and are commonly accepted as substantial evidence for their existence. Exflemely informative, these fragile single-molecule experiments can be very difficult to execute and interpret. Another example for alternative techniques to analyze synthetic transport systems in LUVs is ion-selective electrodes. Conductance experiments in supported lipid bilayer membranes may be mentioned as well. Although these methods are less frequently used, they may be added to the repertoire of the supramolecular chemist. 

The overall activity of a transporter is influenced by numerous parameters, which include buffer and membrane composition, membrane polarization, and osmotic stress, to name only a few. The comparison of the intrinsic activity of different transporters on an absolute scale is nearly impossible for this reason. This is not further problematic because absolute activities are probably the least interesting aspect of synthetic transport systems and arguably deserve little priority. What really matters is responsiveness to specific chemical or physical stimuh. This includes sensitivity toward membrane composition, membrane potential, pH, anions, cations, molecular recognition, molecular transformation (catalysis), or light. These stimuli-responsive, multifunctional, or smart transport systems are attractive for use in biological, medicinal, and materials sciences. Standard techniques to identify such unique characteristics rather than absolute activities or mechanistic details are outlined in this section. 

Membrane filtration has been used in the laboratory for over a century. The earliest membranes were homogeneous stmctures of purified coUagen or 2ein. The first synthetic membranes were nitrocellulose (collodion) cast from ether in the 1850s. By the early 1900s, standard graded nitrocellulose membranes were commercially available (1). Their utihty was limited to laboratory research because of low transport rates and susceptibiUty to internal plugging. They did, however, serve a useflil role in the separation and purification of coUoids, proteins, blood sera, enzymes, toxins, bacteria, and vimses (2). 

S. Hwang and K. Kammermeyer, Membranes in Separations,]ohn Wdey Sons, Inc., New York, 1975 good study of membrane transport phenomenon. R. E. Kesting, Synthetic Polymeric Membranes, McGraw-HiU, New York, 1971 good bibhographies. 

Commercially available membranes are usually reinforced with woven, synthetic fabrics to improve the mechanical properties. Several hundred thousand square meters of IX membranes are now produced aimuaHy, and the mechanical and electrochemical properties are varied by the manufacturers to suit the proposed appHcations. The electrochemical properties of most importance for ED are (/) the electrical resistance per unit area of membrane (2) the ion transport number, related to current efficiency (2) the electrical water transport, related to process efficiency and (4) the back-diffusion, also related to process efficiency. 

Table 4. Amounts of cation transported by the synthetic ionophores through chloroform liquid membrane after 2 days...

In this review, recent development of active transport of ions accross the liquid membranes using the synthetic ionophores such as crown ethers and other acyclic ligands, which selectively complex with cations based on the ion-dipole interaction, was surveyed,... 

---