Carrier-ion complexes

METAL ION a UNCOMPLEXED CARRIER [Pg.296]

These models take into account the diffusion of the carrier and the metal ion-carrier complex in the emulsion globules. They must also account for the reversible reactions at the external and internal interfaces. 

Detailed information about the mechanism of the carrier complex formation can be obtained from kinetic measurements. Various relaxation techniques have been applied to both equihbrium and rate studies with biological alkali ion carrier complexes. These methods were described in detail in several review articles cf. ref. 31. 

In the case of carrier-mediated ion transport, the quantities designated is refer to the complex formed between the carrier and ion while the quantities designated s refer to the free carrier. This latter scheme is more complicated as in all phases, except in the interior of the membrane, the ion—carrier complex is in a steady-state relationship with the free carrier. In addition, further complications for carrier transport are introduced when some of the rate constants for the reactions are potentiaf-dependent as well as being concentration-dependent. Introduction of voltage dependency has been necessary to explain the 7/F relationships for some carrier systems. 

Membrane-soluble carrier molecules can bind an ion from aqueous solution to create a charged ion/carrier complex within the membrane. This soluble ion then moves down a concentration or electrical gradient to the opposite interface where the ion is released to that solution. Carrier transport is modeled in four kinetic steps (1) the carrier absorbs an ion from solution 1 (2) the ion/carrier complex moves down its gradient to the opposite interface ... 

Extensive theoretical and experimental work has previously been reported for supported liquid membrane systems (SLMS) as effective mimics of active transport of ions (Cussler et al., 1989 Kalachev et al., 1992 Thoresen and Fisher, i995 Stockton and Fisher, 1998). This was successfully demonstrated using di-(2-ethyl hexyl)-phosphoric acid as the mobile carrier dissolved in n-dodecane, supported in various inert hydrophobic microporous matrices (e.g., polypropylene), with copper and nickel ions as the transported species. The results showed that a pH differential between the aqueous feed and strip streams, separated by the SLMS, mimics the PMF required for the emulated active transport process that occurred. The model for transport in an SLMS is represented by a five-step resistance-in-series approach, as follows (1) diffusion of the ion through a hydrodynamic boundary layer (2) desolvation of the ion, where it expels the water molecules in its coordination sphere and enters the organic phase via ion exchange with the mobile carrier at the feed/membrane interface (3) diffusion of the ion-carrier complex across the SLMS to the strip/membrane interface (4) solvation of the ion as it enters... 

The lipoprotein membranes of the cell act as diffusion barriers they have a high resistance to passive penetration by ions. How then can we explain the rapid active movement of ions across such membranes One explanation is to postulate that the ions undergo reversible binding with some constituent of the membrane (this constituent is usually called the carrier). The ion is then visualised to pass across the thickness of the membrane, not as a free ion but as an ion-carrier complex (Fig. 7.3). This concept has the immediate attraction that it suggests an explanation of selectivity selective absorption would reflect the abundance in the membrane and chemical affinities of the carrier molecules. Ions which compete with one another would be ions capable of combination with the same carrier but the affinities of the carrier for the separate ions of the group... 

Quite recently, Okahara and his coworkers have extended their method to the formation of long chain N-alkylmonoazacrowns. It was expected that such compounds as N-decylmonoaza-18-crown-6 may be useful as new surfactants with complexing ability with metal salts, phase transfer catalysts and selective ion carriers . ... 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

It is now recognised that a wide range of organic molecules, collectively termed ionophores 185,186) or complexones 187), are able to facilitate ion (usually cation) transport. Two major mechanisms have been revealed for this process, namely the involvement of transmembrane ion carriers and transmembrane pores or channels (see Fig. 19). The majority of ionophores studied to date are natural antibiotics and their synthetic analogues which are, on a biological scale, comparatively small molecules lending themselves to study outside the biological system. In contrast far less is known about the molecular structures involved in normal transport processes. Such molecules are likely to be more complex or present in small amounts and may require... 

Solvent polymeric membranes, conventionally prepared from a polymer that is highly plasticized with lipophilic organic esters or ethers, are the scope of the present chapter. Such membranes commonly contain various constituents such as an ionophore (or ion carrier), a highly selective complexing agent, and ionic additives (ion exchangers and lipophilic salts). The variety and chemical versatility of the available membrane components allow one to tune the membrane properties, ensuring the desired analytical characteristics. 

This group of ISEs is based on the ion-selective character of the distribution equilibrium between water and the membrane phase. As was demonstrated in chapter 3, this ion-selectivity may be affected if an ion pair is formed in the membrane (section 3.2) and increased markedly if complexes are formed in the membrane between the test ion and special complexing agents, ion carriers or ionophores (section 3.3). 

A great number of ligands, such as the anions of ethylenediamine-NNN N -tetra-acetic acid (EDTA), described in detail by Schwarzen-bach and his school (29, 30), show a pronounced selectivity for alkaline earth and other metal cations (30). Because of the limited lipid solubility of these ligands and their complexes, such compounds are, however, not suited as ion carriers in lipophilic membranes (Fig. 2). The ability... 

Step 1 the P ion, after diffusing to the feed-SLM interface, reacts with QCl (Eq. 4) forming a solute-carrier complex, QP CL ions are simultaneously released into the feed solution (coupled transport). 

The mechanism by which cations are transported across a membrane is represented in Figure 18a. A cation-carrier complex is initially formed at the interface. This lipophilic species then diffuses across the membrane as an ion pair and dissociates at the other interface to water soluble ion pair and membrane-soluble carrier. The final step is back diffusion of the free carrier to the initial interface. The factors which influence transport rates and selectivity have been the subject of much research (79PAC979, B-81MI52102). 

These comparatively lipophilic ligands have been conceived as ion carriers for systems such as ion-selective electrodes, 29 therefore they do not have high stability constant values, but require fast complexation kinetics in order to achieve rapid equilibration. Nevertheless, it has been possible to recover crystalline species which often have present additional water molecules to help stabilize the crystal lattice. Coordinative participation of the carbonyl oxygen... 

The results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

Uncomplexed valinomycin has a more extended conformation than it does in the potassium complex.385,386 The conformational change results in the breaking of a pair of hydrogen bonds and formation of new hydrogen bonds as the molecule folds around the potassium ion. Valinomycin facilitates potassium transport in a passive manner. However, there are cyclic changes between two conformations as the carrier complexes with ions, diffuses across the membrane, and releases ions on the other side. Tire rate of transport is rapid, with each valinomycin molecule being able to carry 104 potassium ions per second across a membrane. Tlius, a very small amount of this ionophore is sufficient to alter the permeability and the conductance of a membrane. 

The resultant hydroxyl radicals are effective in initiating many chain reactions. The number of metal ions and complexes which are capable of activating hydrogen peroxide in this manner is quite large and is determined in part by the redox potentials of the activator. Related systems in which free radicals are generated by the intervention of suitable metallic catalysts include many in which oxygen is consumed in autoxidations. Cobalt(H) compounds which act as oxygen carriers can often activate radicals in such systems by reactions of the type ... 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

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